Adding upstream version 9.0.3.

Signed-off-by: Daniel Baumann <daniel@debian.org>

17 files changed, 1658 insertions, 0 deletions

diff --git a/services/auth/source/oauth2/assert_interface_test.go b/services/auth/source/oauth2/assert_interface_test.go
new file mode 100644
index 0000000..56fe0e4
--- /dev/null
+++ b/services/auth/source/oauth2/assert_interface_test.go
@@ -0,0 +1,22 @@
+// Copyright 2021 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package oauth2_test
+
+import (
+	auth_model "code.gitea.io/gitea/models/auth"
+	"code.gitea.io/gitea/services/auth"
+	"code.gitea.io/gitea/services/auth/source/oauth2"
+)
+
+// This test file exists to assert that our Source exposes the interfaces that we expect
+// It tightly binds the interfaces and implementation without breaking go import cycles
+
+type sourceInterface interface {
+	auth_model.Config
+	auth_model.SourceSettable
+	auth_model.RegisterableSource
+	auth.PasswordAuthenticator
+}
+
+var _ (sourceInterface) = &oauth2.Source{}
diff --git a/services/auth/source/oauth2/init.go b/services/auth/source/oauth2/init.go
new file mode 100644
index 0000000..5c25681
--- /dev/null
+++ b/services/auth/source/oauth2/init.go
@@ -0,0 +1,86 @@
+// Copyright 2021 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package oauth2
+
+import (
+	"context"
+	"encoding/gob"
+	"net/http"
+	"sync"
+
+	"code.gitea.io/gitea/models/auth"
+	"code.gitea.io/gitea/models/db"
+	"code.gitea.io/gitea/modules/log"
+	"code.gitea.io/gitea/modules/optional"
+	"code.gitea.io/gitea/modules/setting"
+
+	"github.com/google/uuid"
+	"github.com/gorilla/sessions"
+	"github.com/markbates/goth/gothic"
+)
+
+var gothRWMutex = sync.RWMutex{}
+
+// UsersStoreKey is the key for the store
+const UsersStoreKey = "gitea-oauth2-sessions"
+
+// ProviderHeaderKey is the HTTP header key
+const ProviderHeaderKey = "gitea-oauth2-provider"
+
+// Init initializes the oauth source
+func Init(ctx context.Context) error {
+	if err := InitSigningKey(); err != nil {
+		return err
+	}
+
+	// Lock our mutex
+	gothRWMutex.Lock()
+
+	gob.Register(&sessions.Session{})
+
+	gothic.Store = &SessionsStore{
+		maxLength: int64(setting.OAuth2.MaxTokenLength),
+	}
+
+	gothic.SetState = func(req *http.Request) string {
+		return uuid.New().String()
+	}
+
+	gothic.GetProviderName = func(req *http.Request) (string, error) {
+		return req.Header.Get(ProviderHeaderKey), nil
+	}
+
+	// Unlock our mutex
+	gothRWMutex.Unlock()
+
+	return initOAuth2Sources(ctx)
+}
+
+// ResetOAuth2 clears existing OAuth2 providers and loads them from DB
+func ResetOAuth2(ctx context.Context) error {
+	ClearProviders()
+	return initOAuth2Sources(ctx)
+}
+
+// initOAuth2Sources is used to load and register all active OAuth2 providers
+func initOAuth2Sources(ctx context.Context) error {
+	authSources, err := db.Find[auth.Source](ctx, auth.FindSourcesOptions{
+		IsActive:  optional.Some(true),
+		LoginType: auth.OAuth2,
+	})
+	if err != nil {
+		return err
+	}
+	for _, source := range authSources {
+		oauth2Source, ok := source.Cfg.(*Source)
+		if !ok {
+			continue
+		}
+		err := oauth2Source.RegisterSource()
+		if err != nil {
+			log.Critical("Unable to register source: %s due to Error: %v.", source.Name, err)
+		}
+	}
+	return nil
+}
diff --git a/services/auth/source/oauth2/jwtsigningkey.go b/services/auth/source/oauth2/jwtsigningkey.go
new file mode 100644
index 0000000..92adfc4
--- /dev/null
+++ b/services/auth/source/oauth2/jwtsigningkey.go
@@ -0,0 +1,422 @@
+// Copyright 2021 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package oauth2
+
+import (
+	"crypto/ecdsa"
+	"crypto/ed25519"
+	"crypto/elliptic"
+	"crypto/rand"
+	"crypto/rsa"
+	"crypto/x509"
+	"encoding/base64"
+	"encoding/pem"
+	"fmt"
+	"math/big"
+	"os"
+	"path/filepath"
+	"strings"
+
+	"code.gitea.io/gitea/modules/log"
+	"code.gitea.io/gitea/modules/setting"
+	"code.gitea.io/gitea/modules/util"
+
+	"github.com/golang-jwt/jwt/v5"
+)
+
+// ErrInvalidAlgorithmType represents an invalid algorithm error.
+type ErrInvalidAlgorithmType struct {
+	Algorithm string
+}
+
+func (err ErrInvalidAlgorithmType) Error() string {
+	return fmt.Sprintf("JWT signing algorithm is not supported: %s", err.Algorithm)
+}
+
+// JWTSigningKey represents a algorithm/key pair to sign JWTs
+type JWTSigningKey interface {
+	IsSymmetric() bool
+	SigningMethod() jwt.SigningMethod
+	SignKey() any
+	VerifyKey() any
+	ToJWK() (map[string]string, error)
+	PreProcessToken(*jwt.Token)
+}
+
+type hmacSigningKey struct {
+	signingMethod jwt.SigningMethod
+	secret        []byte
+}
+
+func (key hmacSigningKey) IsSymmetric() bool {
+	return true
+}
+
+func (key hmacSigningKey) SigningMethod() jwt.SigningMethod {
+	return key.signingMethod
+}
+
+func (key hmacSigningKey) SignKey() any {
+	return key.secret
+}
+
+func (key hmacSigningKey) VerifyKey() any {
+	return key.secret
+}
+
+func (key hmacSigningKey) ToJWK() (map[string]string, error) {
+	return map[string]string{
+		"kty": "oct",
+		"alg": key.SigningMethod().Alg(),
+	}, nil
+}
+
+func (key hmacSigningKey) PreProcessToken(*jwt.Token) {}
+
+type rsaSingingKey struct {
+	signingMethod jwt.SigningMethod
+	key           *rsa.PrivateKey
+	id            string
+}
+
+func newRSASingingKey(signingMethod jwt.SigningMethod, key *rsa.PrivateKey) (rsaSingingKey, error) {
+	kid, err := util.CreatePublicKeyFingerprint(key.Public().(*rsa.PublicKey))
+	if err != nil {
+		return rsaSingingKey{}, err
+	}
+
+	return rsaSingingKey{
+		signingMethod,
+		key,
+		base64.RawURLEncoding.EncodeToString(kid),
+	}, nil
+}
+
+func (key rsaSingingKey) IsSymmetric() bool {
+	return false
+}
+
+func (key rsaSingingKey) SigningMethod() jwt.SigningMethod {
+	return key.signingMethod
+}
+
+func (key rsaSingingKey) SignKey() any {
+	return key.key
+}
+
+func (key rsaSingingKey) VerifyKey() any {
+	return key.key.Public()
+}
+
+func (key rsaSingingKey) ToJWK() (map[string]string, error) {
+	pubKey := key.key.Public().(*rsa.PublicKey)
+
+	return map[string]string{
+		"kty": "RSA",
+		"alg": key.SigningMethod().Alg(),
+		"kid": key.id,
+		"e":   base64.RawURLEncoding.EncodeToString(big.NewInt(int64(pubKey.E)).Bytes()),
+		"n":   base64.RawURLEncoding.EncodeToString(pubKey.N.Bytes()),
+	}, nil
+}
+
+func (key rsaSingingKey) PreProcessToken(token *jwt.Token) {
+	token.Header["kid"] = key.id
+}
+
+type eddsaSigningKey struct {
+	signingMethod jwt.SigningMethod
+	key           ed25519.PrivateKey
+	id            string
+}
+
+func newEdDSASingingKey(signingMethod jwt.SigningMethod, key ed25519.PrivateKey) (eddsaSigningKey, error) {
+	kid, err := util.CreatePublicKeyFingerprint(key.Public().(ed25519.PublicKey))
+	if err != nil {
+		return eddsaSigningKey{}, err
+	}
+
+	return eddsaSigningKey{
+		signingMethod,
+		key,
+		base64.RawURLEncoding.EncodeToString(kid),
+	}, nil
+}
+
+func (key eddsaSigningKey) IsSymmetric() bool {
+	return false
+}
+
+func (key eddsaSigningKey) SigningMethod() jwt.SigningMethod {
+	return key.signingMethod
+}
+
+func (key eddsaSigningKey) SignKey() any {
+	return key.key
+}
+
+func (key eddsaSigningKey) VerifyKey() any {
+	return key.key.Public()
+}
+
+func (key eddsaSigningKey) ToJWK() (map[string]string, error) {
+	pubKey := key.key.Public().(ed25519.PublicKey)
+
+	return map[string]string{
+		"alg": key.SigningMethod().Alg(),
+		"kid": key.id,
+		"kty": "OKP",
+		"crv": "Ed25519",
+		"x":   base64.RawURLEncoding.EncodeToString(pubKey),
+	}, nil
+}
+
+func (key eddsaSigningKey) PreProcessToken(token *jwt.Token) {
+	token.Header["kid"] = key.id
+}
+
+type ecdsaSingingKey struct {
+	signingMethod jwt.SigningMethod
+	key           *ecdsa.PrivateKey
+	id            string
+}
+
+func newECDSASingingKey(signingMethod jwt.SigningMethod, key *ecdsa.PrivateKey) (ecdsaSingingKey, error) {
+	kid, err := util.CreatePublicKeyFingerprint(key.Public().(*ecdsa.PublicKey))
+	if err != nil {
+		return ecdsaSingingKey{}, err
+	}
+
+	return ecdsaSingingKey{
+		signingMethod,
+		key,
+		base64.RawURLEncoding.EncodeToString(kid),
+	}, nil
+}
+
+func (key ecdsaSingingKey) IsSymmetric() bool {
+	return false
+}
+
+func (key ecdsaSingingKey) SigningMethod() jwt.SigningMethod {
+	return key.signingMethod
+}
+
+func (key ecdsaSingingKey) SignKey() any {
+	return key.key
+}
+
+func (key ecdsaSingingKey) VerifyKey() any {
+	return key.key.Public()
+}
+
+func (key ecdsaSingingKey) ToJWK() (map[string]string, error) {
+	pubKey := key.key.Public().(*ecdsa.PublicKey)
+
+	return map[string]string{
+		"kty": "EC",
+		"alg": key.SigningMethod().Alg(),
+		"kid": key.id,
+		"crv": pubKey.Params().Name,
+		"x":   base64.RawURLEncoding.EncodeToString(pubKey.X.Bytes()),
+		"y":   base64.RawURLEncoding.EncodeToString(pubKey.Y.Bytes()),
+	}, nil
+}
+
+func (key ecdsaSingingKey) PreProcessToken(token *jwt.Token) {
+	token.Header["kid"] = key.id
+}
+
+// CreateJWTSigningKey creates a signing key from an algorithm / key pair.
+func CreateJWTSigningKey(algorithm string, key any) (JWTSigningKey, error) {
+	var signingMethod jwt.SigningMethod
+	switch algorithm {
+	case "HS256":
+		signingMethod = jwt.SigningMethodHS256
+	case "HS384":
+		signingMethod = jwt.SigningMethodHS384
+	case "HS512":
+		signingMethod = jwt.SigningMethodHS512
+
+	case "RS256":
+		signingMethod = jwt.SigningMethodRS256
+	case "RS384":
+		signingMethod = jwt.SigningMethodRS384
+	case "RS512":
+		signingMethod = jwt.SigningMethodRS512
+
+	case "ES256":
+		signingMethod = jwt.SigningMethodES256
+	case "ES384":
+		signingMethod = jwt.SigningMethodES384
+	case "ES512":
+		signingMethod = jwt.SigningMethodES512
+	case "EdDSA":
+		signingMethod = jwt.SigningMethodEdDSA
+	default:
+		return nil, ErrInvalidAlgorithmType{algorithm}
+	}
+
+	switch signingMethod.(type) {
+	case *jwt.SigningMethodEd25519:
+		privateKey, ok := key.(ed25519.PrivateKey)
+		if !ok {
+			return nil, jwt.ErrInvalidKeyType
+		}
+		return newEdDSASingingKey(signingMethod, privateKey)
+	case *jwt.SigningMethodECDSA:
+		privateKey, ok := key.(*ecdsa.PrivateKey)
+		if !ok {
+			return nil, jwt.ErrInvalidKeyType
+		}
+		return newECDSASingingKey(signingMethod, privateKey)
+	case *jwt.SigningMethodRSA:
+		privateKey, ok := key.(*rsa.PrivateKey)
+		if !ok {
+			return nil, jwt.ErrInvalidKeyType
+		}
+		return newRSASingingKey(signingMethod, privateKey)
+	default:
+		secret, ok := key.([]byte)
+		if !ok {
+			return nil, jwt.ErrInvalidKeyType
+		}
+		return hmacSigningKey{signingMethod, secret}, nil
+	}
+}
+
+// DefaultSigningKey is the default signing key for JWTs.
+var DefaultSigningKey JWTSigningKey
+
+// InitSigningKey creates the default signing key from settings or creates a random key.
+func InitSigningKey() error {
+	var err error
+	var key any
+
+	switch setting.OAuth2.JWTSigningAlgorithm {
+	case "HS256":
+		fallthrough
+	case "HS384":
+		fallthrough
+	case "HS512":
+		key = setting.GetGeneralTokenSigningSecret()
+	case "RS256":
+		fallthrough
+	case "RS384":
+		fallthrough
+	case "RS512":
+		fallthrough
+	case "ES256":
+		fallthrough
+	case "ES384":
+		fallthrough
+	case "ES512":
+		fallthrough
+	case "EdDSA":
+		key, err = loadOrCreateAsymmetricKey()
+	default:
+		return ErrInvalidAlgorithmType{setting.OAuth2.JWTSigningAlgorithm}
+	}
+
+	if err != nil {
+		return fmt.Errorf("Error while loading or creating JWT key: %w", err)
+	}
+
+	signingKey, err := CreateJWTSigningKey(setting.OAuth2.JWTSigningAlgorithm, key)
+	if err != nil {
+		return err
+	}
+
+	DefaultSigningKey = signingKey
+
+	return nil
+}
+
+// loadOrCreateAsymmetricKey checks if the configured private key exists.
+// If it does not exist a new random key gets generated and saved on the configured path.
+func loadOrCreateAsymmetricKey() (any, error) {
+	keyPath := setting.OAuth2.JWTSigningPrivateKeyFile
+
+	isExist, err := util.IsExist(keyPath)
+	if err != nil {
+		log.Fatal("Unable to check if %s exists. Error: %v", keyPath, err)
+	}
+	if !isExist {
+		err := func() error {
+			key, err := func() (any, error) {
+				switch {
+				case strings.HasPrefix(setting.OAuth2.JWTSigningAlgorithm, "RS"):
+					var bits int
+					switch setting.OAuth2.JWTSigningAlgorithm {
+					case "RS256":
+						bits = 2048
+					case "RS384":
+						bits = 3072
+					case "RS512":
+						bits = 4096
+					}
+					return rsa.GenerateKey(rand.Reader, bits)
+				case setting.OAuth2.JWTSigningAlgorithm == "EdDSA":
+					_, pk, err := ed25519.GenerateKey(rand.Reader)
+					return pk, err
+				default:
+					var curve elliptic.Curve
+					switch setting.OAuth2.JWTSigningAlgorithm {
+					case "ES256":
+						curve = elliptic.P256()
+					case "ES384":
+						curve = elliptic.P384()
+					case "ES512":
+						curve = elliptic.P521()
+					}
+					return ecdsa.GenerateKey(curve, rand.Reader)
+				}
+			}()
+			if err != nil {
+				return err
+			}
+
+			bytes, err := x509.MarshalPKCS8PrivateKey(key)
+			if err != nil {
+				return err
+			}
+
+			privateKeyPEM := &pem.Block{Type: "PRIVATE KEY", Bytes: bytes}
+
+			if err := os.MkdirAll(filepath.Dir(keyPath), os.ModePerm); err != nil {
+				return err
+			}
+
+			f, err := os.OpenFile(keyPath, os.O_RDWR|os.O_CREATE|os.O_TRUNC, 0o600)
+			if err != nil {
+				return err
+			}
+			defer func() {
+				if err = f.Close(); err != nil {
+					log.Error("Close: %v", err)
+				}
+			}()
+
+			return pem.Encode(f, privateKeyPEM)
+		}()
+		if err != nil {
+			log.Fatal("Error generating private key: %v", err)
+			return nil, err
+		}
+	}
+
+	bytes, err := os.ReadFile(keyPath)
+	if err != nil {
+		return nil, err
+	}
+
+	block, _ := pem.Decode(bytes)
+	if block == nil {
+		return nil, fmt.Errorf("no valid PEM data found in %s", keyPath)
+	} else if block.Type != "PRIVATE KEY" {
+		return nil, fmt.Errorf("expected PRIVATE KEY, got %s in %s", block.Type, keyPath)
+	}
+
+	return x509.ParsePKCS8PrivateKey(block.Bytes)
+}
diff --git a/services/auth/source/oauth2/jwtsigningkey_test.go b/services/auth/source/oauth2/jwtsigningkey_test.go
new file mode 100644
index 0000000..4db538b
--- /dev/null
+++ b/services/auth/source/oauth2/jwtsigningkey_test.go
@@ -0,0 +1,116 @@
+// Copyright 2024 The Forgejo Authors. All rights reserved.
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+package oauth2
+
+import (
+	"crypto/ecdsa"
+	"crypto/ed25519"
+	"crypto/rsa"
+	"crypto/x509"
+	"encoding/pem"
+	"os"
+	"path/filepath"
+	"testing"
+
+	"code.gitea.io/gitea/modules/setting"
+	"code.gitea.io/gitea/modules/test"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestLoadOrCreateAsymmetricKey(t *testing.T) {
+	loadKey := func(t *testing.T) any {
+		t.Helper()
+		loadOrCreateAsymmetricKey()
+
+		fileContent, err := os.ReadFile(setting.OAuth2.JWTSigningPrivateKeyFile)
+		require.NoError(t, err)
+
+		block, _ := pem.Decode(fileContent)
+		assert.NotNil(t, block)
+		assert.EqualValues(t, "PRIVATE KEY", block.Type)
+
+		parsedKey, err := x509.ParsePKCS8PrivateKey(block.Bytes)
+		require.NoError(t, err)
+
+		return parsedKey
+	}
+	t.Run("RSA-2048", func(t *testing.T) {
+		defer test.MockVariableValue(&setting.OAuth2.JWTSigningPrivateKeyFile, filepath.Join(t.TempDir(), "jwt-rsa-2048.priv"))()
+		defer test.MockVariableValue(&setting.OAuth2.JWTSigningAlgorithm, "RS256")()
+
+		parsedKey := loadKey(t)
+
+		rsaPrivateKey := parsedKey.(*rsa.PrivateKey)
+		assert.EqualValues(t, 2048, rsaPrivateKey.N.BitLen())
+
+		t.Run("Load key with differ specified algorithm", func(t *testing.T) {
+			defer test.MockVariableValue(&setting.OAuth2.JWTSigningAlgorithm, "EdDSA")()
+
+			parsedKey := loadKey(t)
+			rsaPrivateKey := parsedKey.(*rsa.PrivateKey)
+			assert.EqualValues(t, 2048, rsaPrivateKey.N.BitLen())
+		})
+	})
+
+	t.Run("RSA-3072", func(t *testing.T) {
+		defer test.MockVariableValue(&setting.OAuth2.JWTSigningPrivateKeyFile, filepath.Join(t.TempDir(), "jwt-rsa-3072.priv"))()
+		defer test.MockVariableValue(&setting.OAuth2.JWTSigningAlgorithm, "RS384")()
+
+		parsedKey := loadKey(t)
+
+		rsaPrivateKey := parsedKey.(*rsa.PrivateKey)
+		assert.EqualValues(t, 3072, rsaPrivateKey.N.BitLen())
+	})
+
+	t.Run("RSA-4096", func(t *testing.T) {
+		defer test.MockVariableValue(&setting.OAuth2.JWTSigningPrivateKeyFile, filepath.Join(t.TempDir(), "jwt-rsa-4096.priv"))()
+		defer test.MockVariableValue(&setting.OAuth2.JWTSigningAlgorithm, "RS512")()
+
+		parsedKey := loadKey(t)
+
+		rsaPrivateKey := parsedKey.(*rsa.PrivateKey)
+		assert.EqualValues(t, 4096, rsaPrivateKey.N.BitLen())
+	})
+
+	t.Run("ECDSA-256", func(t *testing.T) {
+		defer test.MockVariableValue(&setting.OAuth2.JWTSigningPrivateKeyFile, filepath.Join(t.TempDir(), "jwt-ecdsa-256.priv"))()
+		defer test.MockVariableValue(&setting.OAuth2.JWTSigningAlgorithm, "ES256")()
+
+		parsedKey := loadKey(t)
+
+		ecdsaPrivateKey := parsedKey.(*ecdsa.PrivateKey)
+		assert.EqualValues(t, 256, ecdsaPrivateKey.Params().BitSize)
+	})
+
+	t.Run("ECDSA-384", func(t *testing.T) {
+		defer test.MockVariableValue(&setting.OAuth2.JWTSigningPrivateKeyFile, filepath.Join(t.TempDir(), "jwt-ecdsa-384.priv"))()
+		defer test.MockVariableValue(&setting.OAuth2.JWTSigningAlgorithm, "ES384")()
+
+		parsedKey := loadKey(t)
+
+		ecdsaPrivateKey := parsedKey.(*ecdsa.PrivateKey)
+		assert.EqualValues(t, 384, ecdsaPrivateKey.Params().BitSize)
+	})
+
+	t.Run("ECDSA-512", func(t *testing.T) {
+		defer test.MockVariableValue(&setting.OAuth2.JWTSigningPrivateKeyFile, filepath.Join(t.TempDir(), "jwt-ecdsa-512.priv"))()
+		defer test.MockVariableValue(&setting.OAuth2.JWTSigningAlgorithm, "ES512")()
+
+		parsedKey := loadKey(t)
+
+		ecdsaPrivateKey := parsedKey.(*ecdsa.PrivateKey)
+		assert.EqualValues(t, 521, ecdsaPrivateKey.Params().BitSize)
+	})
+
+	t.Run("EdDSA", func(t *testing.T) {
+		defer test.MockVariableValue(&setting.OAuth2.JWTSigningPrivateKeyFile, filepath.Join(t.TempDir(), "jwt-eddsa.priv"))()
+		defer test.MockVariableValue(&setting.OAuth2.JWTSigningAlgorithm, "EdDSA")()
+
+		parsedKey := loadKey(t)
+
+		assert.NotNil(t, parsedKey.(ed25519.PrivateKey))
+	})
+}
diff --git a/services/auth/source/oauth2/providers.go b/services/auth/source/oauth2/providers.go
new file mode 100644
index 0000000..f2c1bb4
--- /dev/null
+++ b/services/auth/source/oauth2/providers.go
@@ -0,0 +1,190 @@
+// Copyright 2021 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package oauth2
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"html"
+	"html/template"
+	"net/url"
+	"sort"
+
+	"code.gitea.io/gitea/models/auth"
+	"code.gitea.io/gitea/models/db"
+	"code.gitea.io/gitea/modules/log"
+	"code.gitea.io/gitea/modules/optional"
+	"code.gitea.io/gitea/modules/setting"
+
+	"github.com/markbates/goth"
+)
+
+// Provider is an interface for describing a single OAuth2 provider
+type Provider interface {
+	Name() string
+	DisplayName() string
+	IconHTML(size int) template.HTML
+	CustomURLSettings() *CustomURLSettings
+}
+
+// GothProviderCreator provides a function to create a goth.Provider
+type GothProviderCreator interface {
+	CreateGothProvider(providerName, callbackURL string, source *Source) (goth.Provider, error)
+}
+
+// GothProvider is an interface for describing a single OAuth2 provider
+type GothProvider interface {
+	Provider
+	GothProviderCreator
+}
+
+// AuthSourceProvider provides a provider for an AuthSource. Multiple auth sources could use the same registered GothProvider
+// So each auth source should have its own DisplayName and IconHTML for display.
+// The Name is the GothProvider's name, to help to find the GothProvider to sign in.
+// The DisplayName is the auth source config's name, site admin set it on the admin page, the IconURL can also be set there.
+type AuthSourceProvider struct {
+	GothProvider
+	sourceName, iconURL string
+}
+
+func (p *AuthSourceProvider) Name() string {
+	return p.GothProvider.Name()
+}
+
+func (p *AuthSourceProvider) DisplayName() string {
+	return p.sourceName
+}
+
+func (p *AuthSourceProvider) IconHTML(size int) template.HTML {
+	if p.iconURL != "" {
+		img := fmt.Sprintf(`<img class="tw-object-contain tw-mr-2" width="%d" height="%d" src="%s" alt="%s">`,
+			size,
+			size,
+			html.EscapeString(p.iconURL), html.EscapeString(p.DisplayName()),
+		)
+		return template.HTML(img)
+	}
+	return p.GothProvider.IconHTML(size)
+}
+
+// Providers contains the map of registered OAuth2 providers in Gitea (based on goth)
+// key is used to map the OAuth2Provider with the goth provider type (also in AuthSource.OAuth2Config.Provider)
+// value is used to store display data
+var gothProviders = map[string]GothProvider{}
+
+// RegisterGothProvider registers a GothProvider
+func RegisterGothProvider(provider GothProvider) {
+	if _, has := gothProviders[provider.Name()]; has {
+		log.Fatal("Duplicate oauth2provider type provided: %s", provider.Name())
+	}
+	gothProviders[provider.Name()] = provider
+}
+
+// GetSupportedOAuth2Providers returns the map of unconfigured OAuth2 providers
+// key is used as technical name (like in the callbackURL)
+// values to display
+func GetSupportedOAuth2Providers() []Provider {
+	providers := make([]Provider, 0, len(gothProviders))
+
+	for _, provider := range gothProviders {
+		providers = append(providers, provider)
+	}
+	sort.Slice(providers, func(i, j int) bool {
+		return providers[i].Name() < providers[j].Name()
+	})
+	return providers
+}
+
+func CreateProviderFromSource(source *auth.Source) (Provider, error) {
+	oauth2Cfg, ok := source.Cfg.(*Source)
+	if !ok {
+		return nil, fmt.Errorf("invalid OAuth2 source config: %v", oauth2Cfg)
+	}
+	gothProv := gothProviders[oauth2Cfg.Provider]
+	return &AuthSourceProvider{GothProvider: gothProv, sourceName: source.Name, iconURL: oauth2Cfg.IconURL}, nil
+}
+
+// GetOAuth2Providers returns the list of configured OAuth2 providers
+func GetOAuth2Providers(ctx context.Context, isActive optional.Option[bool]) ([]Provider, error) {
+	authSources, err := db.Find[auth.Source](ctx, auth.FindSourcesOptions{
+		IsActive:  isActive,
+		LoginType: auth.OAuth2,
+	})
+	if err != nil {
+		return nil, err
+	}
+
+	providers := make([]Provider, 0, len(authSources))
+	for _, source := range authSources {
+		provider, err := CreateProviderFromSource(source)
+		if err != nil {
+			return nil, err
+		}
+		providers = append(providers, provider)
+	}
+
+	sort.Slice(providers, func(i, j int) bool {
+		return providers[i].Name() < providers[j].Name()
+	})
+
+	return providers, nil
+}
+
+// RegisterProviderWithGothic register a OAuth2 provider in goth lib
+func RegisterProviderWithGothic(providerName string, source *Source) error {
+	provider, err := createProvider(providerName, source)
+
+	if err == nil && provider != nil {
+		gothRWMutex.Lock()
+		defer gothRWMutex.Unlock()
+
+		goth.UseProviders(provider)
+	}
+
+	return err
+}
+
+// RemoveProviderFromGothic removes the given OAuth2 provider from the goth lib
+func RemoveProviderFromGothic(providerName string) {
+	gothRWMutex.Lock()
+	defer gothRWMutex.Unlock()
+
+	delete(goth.GetProviders(), providerName)
+}
+
+// ClearProviders clears all OAuth2 providers from the goth lib
+func ClearProviders() {
+	gothRWMutex.Lock()
+	defer gothRWMutex.Unlock()
+
+	goth.ClearProviders()
+}
+
+var ErrAuthSourceNotActivated = errors.New("auth source is not activated")
+
+// used to create different types of goth providers
+func createProvider(providerName string, source *Source) (goth.Provider, error) {
+	callbackURL := setting.AppURL + "user/oauth2/" + url.PathEscape(providerName) + "/callback"
+
+	var provider goth.Provider
+	var err error
+
+	p, ok := gothProviders[source.Provider]
+	if !ok {
+		return nil, ErrAuthSourceNotActivated
+	}
+
+	provider, err = p.CreateGothProvider(providerName, callbackURL, source)
+	if err != nil {
+		return provider, err
+	}
+
+	// always set the name if provider is created so we can support multiple setups of 1 provider
+	if provider != nil {
+		provider.SetName(providerName)
+	}
+
+	return provider, err
+}
diff --git a/services/auth/source/oauth2/providers_base.go b/services/auth/source/oauth2/providers_base.go
new file mode 100644
index 0000000..9d4ab10
--- /dev/null
+++ b/services/auth/source/oauth2/providers_base.go
@@ -0,0 +1,51 @@
+// Copyright 2021 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package oauth2
+
+import (
+	"html/template"
+
+	"code.gitea.io/gitea/modules/log"
+	"code.gitea.io/gitea/modules/svg"
+)
+
+// BaseProvider represents a common base for Provider
+type BaseProvider struct {
+	name        string
+	displayName string
+}
+
+// Name provides the technical name for this provider
+func (b *BaseProvider) Name() string {
+	return b.name
+}
+
+// DisplayName returns the friendly name for this provider
+func (b *BaseProvider) DisplayName() string {
+	return b.displayName
+}
+
+// IconHTML returns icon HTML for this provider
+func (b *BaseProvider) IconHTML(size int) template.HTML {
+	svgName := "gitea-" + b.name
+	switch b.name {
+	case "gplus":
+		svgName = "gitea-google"
+	case "github":
+		svgName = "octicon-mark-github"
+	}
+	svgHTML := svg.RenderHTML(svgName, size, "tw-mr-2")
+	if svgHTML == "" {
+		log.Error("No SVG icon for oauth2 provider %q", b.name)
+		svgHTML = svg.RenderHTML("gitea-openid", size, "tw-mr-2")
+	}
+	return svgHTML
+}
+
+// CustomURLSettings returns the custom url settings for this provider
+func (b *BaseProvider) CustomURLSettings() *CustomURLSettings {
+	return nil
+}
+
+var _ Provider = &BaseProvider{}
diff --git a/services/auth/source/oauth2/providers_custom.go b/services/auth/source/oauth2/providers_custom.go
new file mode 100644
index 0000000..65cf538
--- /dev/null
+++ b/services/auth/source/oauth2/providers_custom.go
@@ -0,0 +1,123 @@
+// Copyright 2021 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package oauth2
+
+import (
+	"code.gitea.io/gitea/modules/setting"
+
+	"github.com/markbates/goth"
+	"github.com/markbates/goth/providers/azureadv2"
+	"github.com/markbates/goth/providers/gitea"
+	"github.com/markbates/goth/providers/github"
+	"github.com/markbates/goth/providers/gitlab"
+	"github.com/markbates/goth/providers/mastodon"
+	"github.com/markbates/goth/providers/nextcloud"
+)
+
+// CustomProviderNewFn creates a goth.Provider using a custom url mapping
+type CustomProviderNewFn func(clientID, secret, callbackURL string, custom *CustomURLMapping, scopes []string) (goth.Provider, error)
+
+// CustomProvider is a GothProvider that has CustomURL features
+type CustomProvider struct {
+	BaseProvider
+	customURLSettings *CustomURLSettings
+	newFn             CustomProviderNewFn
+}
+
+// CustomURLSettings returns the CustomURLSettings for this provider
+func (c *CustomProvider) CustomURLSettings() *CustomURLSettings {
+	return c.customURLSettings
+}
+
+// CreateGothProvider creates a GothProvider from this Provider
+func (c *CustomProvider) CreateGothProvider(providerName, callbackURL string, source *Source) (goth.Provider, error) {
+	custom := c.customURLSettings.OverrideWith(source.CustomURLMapping)
+
+	return c.newFn(source.ClientID, source.ClientSecret, callbackURL, custom, source.Scopes)
+}
+
+// NewCustomProvider is a constructor function for custom providers
+func NewCustomProvider(name, displayName string, customURLSetting *CustomURLSettings, newFn CustomProviderNewFn) *CustomProvider {
+	return &CustomProvider{
+		BaseProvider: BaseProvider{
+			name:        name,
+			displayName: displayName,
+		},
+		customURLSettings: customURLSetting,
+		newFn:             newFn,
+	}
+}
+
+var _ GothProvider = &CustomProvider{}
+
+func init() {
+	RegisterGothProvider(NewCustomProvider(
+		"github", "GitHub", &CustomURLSettings{
+			TokenURL:   availableAttribute(github.TokenURL),
+			AuthURL:    availableAttribute(github.AuthURL),
+			ProfileURL: availableAttribute(github.ProfileURL),
+			EmailURL:   availableAttribute(github.EmailURL),
+		},
+		func(clientID, secret, callbackURL string, custom *CustomURLMapping, scopes []string) (goth.Provider, error) {
+			if setting.OAuth2Client.EnableAutoRegistration {
+				scopes = append(scopes, "user:email")
+			}
+			return github.NewCustomisedURL(clientID, secret, callbackURL, custom.AuthURL, custom.TokenURL, custom.ProfileURL, custom.EmailURL, scopes...), nil
+		}))
+
+	RegisterGothProvider(NewCustomProvider(
+		"gitlab", "GitLab", &CustomURLSettings{
+			AuthURL:    availableAttribute(gitlab.AuthURL),
+			TokenURL:   availableAttribute(gitlab.TokenURL),
+			ProfileURL: availableAttribute(gitlab.ProfileURL),
+		}, func(clientID, secret, callbackURL string, custom *CustomURLMapping, scopes []string) (goth.Provider, error) {
+			scopes = append(scopes, "read_user")
+			return gitlab.NewCustomisedURL(clientID, secret, callbackURL, custom.AuthURL, custom.TokenURL, custom.ProfileURL, scopes...), nil
+		}))
+
+	RegisterGothProvider(NewCustomProvider(
+		"gitea", "Gitea", &CustomURLSettings{
+			TokenURL:   requiredAttribute(gitea.TokenURL),
+			AuthURL:    requiredAttribute(gitea.AuthURL),
+			ProfileURL: requiredAttribute(gitea.ProfileURL),
+		},
+		func(clientID, secret, callbackURL string, custom *CustomURLMapping, scopes []string) (goth.Provider, error) {
+			return gitea.NewCustomisedURL(clientID, secret, callbackURL, custom.AuthURL, custom.TokenURL, custom.ProfileURL, scopes...), nil
+		}))
+
+	RegisterGothProvider(NewCustomProvider(
+		"nextcloud", "Nextcloud", &CustomURLSettings{
+			TokenURL:   requiredAttribute(nextcloud.TokenURL),
+			AuthURL:    requiredAttribute(nextcloud.AuthURL),
+			ProfileURL: requiredAttribute(nextcloud.ProfileURL),
+		},
+		func(clientID, secret, callbackURL string, custom *CustomURLMapping, scopes []string) (goth.Provider, error) {
+			return nextcloud.NewCustomisedURL(clientID, secret, callbackURL, custom.AuthURL, custom.TokenURL, custom.ProfileURL, scopes...), nil
+		}))
+
+	RegisterGothProvider(NewCustomProvider(
+		"mastodon", "Mastodon", &CustomURLSettings{
+			AuthURL: requiredAttribute(mastodon.InstanceURL),
+		},
+		func(clientID, secret, callbackURL string, custom *CustomURLMapping, scopes []string) (goth.Provider, error) {
+			return mastodon.NewCustomisedURL(clientID, secret, callbackURL, custom.AuthURL, scopes...), nil
+		}))
+
+	RegisterGothProvider(NewCustomProvider(
+		"azureadv2", "Azure AD v2", &CustomURLSettings{
+			Tenant: requiredAttribute("organizations"),
+		},
+		func(clientID, secret, callbackURL string, custom *CustomURLMapping, scopes []string) (goth.Provider, error) {
+			azureScopes := make([]azureadv2.ScopeType, len(scopes))
+			for i, scope := range scopes {
+				azureScopes[i] = azureadv2.ScopeType(scope)
+			}
+
+			return azureadv2.New(clientID, secret, callbackURL, azureadv2.ProviderOptions{
+				Tenant: azureadv2.TenantType(custom.Tenant),
+				Scopes: azureScopes,
+			}), nil
+		},
+	))
+}
diff --git a/services/auth/source/oauth2/providers_openid.go b/services/auth/source/oauth2/providers_openid.go
new file mode 100644
index 0000000..285876d
--- /dev/null
+++ b/services/auth/source/oauth2/providers_openid.go
@@ -0,0 +1,58 @@
+// Copyright 2021 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package oauth2
+
+import (
+	"html/template"
+
+	"code.gitea.io/gitea/modules/log"
+	"code.gitea.io/gitea/modules/setting"
+	"code.gitea.io/gitea/modules/svg"
+
+	"github.com/markbates/goth"
+	"github.com/markbates/goth/providers/openidConnect"
+)
+
+// OpenIDProvider is a GothProvider for OpenID
+type OpenIDProvider struct{}
+
+// Name provides the technical name for this provider
+func (o *OpenIDProvider) Name() string {
+	return "openidConnect"
+}
+
+// DisplayName returns the friendly name for this provider
+func (o *OpenIDProvider) DisplayName() string {
+	return "OpenID Connect"
+}
+
+// IconHTML returns icon HTML for this provider
+func (o *OpenIDProvider) IconHTML(size int) template.HTML {
+	return svg.RenderHTML("gitea-openid", size, "tw-mr-2")
+}
+
+// CreateGothProvider creates a GothProvider from this Provider
+func (o *OpenIDProvider) CreateGothProvider(providerName, callbackURL string, source *Source) (goth.Provider, error) {
+	scopes := setting.OAuth2Client.OpenIDConnectScopes
+	if len(scopes) == 0 {
+		scopes = append(scopes, source.Scopes...)
+	}
+
+	provider, err := openidConnect.New(source.ClientID, source.ClientSecret, callbackURL, source.OpenIDConnectAutoDiscoveryURL, scopes...)
+	if err != nil {
+		log.Warn("Failed to create OpenID Connect Provider with name '%s' with url '%s': %v", providerName, source.OpenIDConnectAutoDiscoveryURL, err)
+	}
+	return provider, err
+}
+
+// CustomURLSettings returns the custom url settings for this provider
+func (o *OpenIDProvider) CustomURLSettings() *CustomURLSettings {
+	return nil
+}
+
+var _ GothProvider = &OpenIDProvider{}
+
+func init() {
+	RegisterGothProvider(&OpenIDProvider{})
+}
diff --git a/services/auth/source/oauth2/providers_simple.go b/services/auth/source/oauth2/providers_simple.go
new file mode 100644
index 0000000..e95323a
--- /dev/null
+++ b/services/auth/source/oauth2/providers_simple.go
@@ -0,0 +1,109 @@
+// Copyright 2021 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package oauth2
+
+import (
+	"code.gitea.io/gitea/modules/setting"
+
+	"github.com/markbates/goth"
+	"github.com/markbates/goth/providers/azuread"
+	"github.com/markbates/goth/providers/bitbucket"
+	"github.com/markbates/goth/providers/discord"
+	"github.com/markbates/goth/providers/dropbox"
+	"github.com/markbates/goth/providers/facebook"
+	"github.com/markbates/goth/providers/google"
+	"github.com/markbates/goth/providers/microsoftonline"
+	"github.com/markbates/goth/providers/twitter"
+	"github.com/markbates/goth/providers/yandex"
+)
+
+// SimpleProviderNewFn create goth.Providers without custom url features
+type SimpleProviderNewFn func(clientKey, secret, callbackURL string, scopes ...string) goth.Provider
+
+// SimpleProvider is a GothProvider which does not have custom url features
+type SimpleProvider struct {
+	BaseProvider
+	scopes []string
+	newFn  SimpleProviderNewFn
+}
+
+// CreateGothProvider creates a GothProvider from this Provider
+func (c *SimpleProvider) CreateGothProvider(providerName, callbackURL string, source *Source) (goth.Provider, error) {
+	scopes := make([]string, len(c.scopes)+len(source.Scopes))
+	copy(scopes, c.scopes)
+	copy(scopes[len(c.scopes):], source.Scopes)
+	return c.newFn(source.ClientID, source.ClientSecret, callbackURL, scopes...), nil
+}
+
+// NewSimpleProvider is a constructor function for simple providers
+func NewSimpleProvider(name, displayName string, scopes []string, newFn SimpleProviderNewFn) *SimpleProvider {
+	return &SimpleProvider{
+		BaseProvider: BaseProvider{
+			name:        name,
+			displayName: displayName,
+		},
+		scopes: scopes,
+		newFn:  newFn,
+	}
+}
+
+var _ GothProvider = &SimpleProvider{}
+
+func init() {
+	RegisterGothProvider(
+		NewSimpleProvider("bitbucket", "Bitbucket", []string{"account"},
+			func(clientKey, secret, callbackURL string, scopes ...string) goth.Provider {
+				return bitbucket.New(clientKey, secret, callbackURL, scopes...)
+			}))
+
+	RegisterGothProvider(
+		NewSimpleProvider("dropbox", "Dropbox", nil,
+			func(clientKey, secret, callbackURL string, scopes ...string) goth.Provider {
+				return dropbox.New(clientKey, secret, callbackURL, scopes...)
+			}))
+
+	RegisterGothProvider(NewSimpleProvider("facebook", "Facebook", nil,
+		func(clientKey, secret, callbackURL string, scopes ...string) goth.Provider {
+			return facebook.New(clientKey, secret, callbackURL, scopes...)
+		}))
+
+	// named gplus due to legacy gplus -> google migration (Google killed Google+). This ensures old connections still work
+	RegisterGothProvider(NewSimpleProvider("gplus", "Google", []string{"email"},
+		func(clientKey, secret, callbackURL string, scopes ...string) goth.Provider {
+			if setting.OAuth2Client.UpdateAvatar || setting.OAuth2Client.EnableAutoRegistration {
+				scopes = append(scopes, "profile")
+			}
+			return google.New(clientKey, secret, callbackURL, scopes...)
+		}))
+
+	RegisterGothProvider(NewSimpleProvider("twitter", "Twitter", nil,
+		func(clientKey, secret, callbackURL string, scopes ...string) goth.Provider {
+			return twitter.New(clientKey, secret, callbackURL)
+		}))
+
+	RegisterGothProvider(NewSimpleProvider("discord", "Discord", []string{discord.ScopeIdentify, discord.ScopeEmail},
+		func(clientKey, secret, callbackURL string, scopes ...string) goth.Provider {
+			return discord.New(clientKey, secret, callbackURL, scopes...)
+		}))
+
+	// See https://tech.yandex.com/passport/doc/dg/reference/response-docpage/
+	RegisterGothProvider(NewSimpleProvider("yandex", "Yandex", []string{"login:email", "login:info", "login:avatar"},
+		func(clientKey, secret, callbackURL string, scopes ...string) goth.Provider {
+			return yandex.New(clientKey, secret, callbackURL, scopes...)
+		}))
+
+	RegisterGothProvider(NewSimpleProvider(
+		"azuread", "Azure AD", nil,
+		func(clientID, secret, callbackURL string, scopes ...string) goth.Provider {
+			return azuread.New(clientID, secret, callbackURL, nil, scopes...)
+		},
+	))
+
+	RegisterGothProvider(NewSimpleProvider(
+		"microsoftonline", "Microsoft Online", nil,
+		func(clientID, secret, callbackURL string, scopes ...string) goth.Provider {
+			return microsoftonline.New(clientID, secret, callbackURL, scopes...)
+		},
+	))
+}
diff --git a/services/auth/source/oauth2/source.go b/services/auth/source/oauth2/source.go
new file mode 100644
index 0000000..675005e
--- /dev/null
+++ b/services/auth/source/oauth2/source.go
@@ -0,0 +1,51 @@
+// Copyright 2021 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package oauth2
+
+import (
+	"code.gitea.io/gitea/models/auth"
+	"code.gitea.io/gitea/modules/json"
+)
+
+// Source holds configuration for the OAuth2 login source.
+type Source struct {
+	Provider                      string
+	ClientID                      string
+	ClientSecret                  string
+	OpenIDConnectAutoDiscoveryURL string
+	CustomURLMapping              *CustomURLMapping
+	IconURL                       string
+
+	Scopes              []string
+	RequiredClaimName   string
+	RequiredClaimValue  string
+	GroupClaimName      string
+	AdminGroup          string
+	GroupTeamMap        string
+	GroupTeamMapRemoval bool
+	RestrictedGroup     string
+	SkipLocalTwoFA      bool `json:",omitempty"`
+
+	// reference to the authSource
+	authSource *auth.Source
+}
+
+// FromDB fills up an OAuth2Config from serialized format.
+func (source *Source) FromDB(bs []byte) error {
+	return json.UnmarshalHandleDoubleEncode(bs, &source)
+}
+
+// ToDB exports an SMTPConfig to a serialized format.
+func (source *Source) ToDB() ([]byte, error) {
+	return json.Marshal(source)
+}
+
+// SetAuthSource sets the related AuthSource
+func (source *Source) SetAuthSource(authSource *auth.Source) {
+	source.authSource = authSource
+}
+
+func init() {
+	auth.RegisterTypeConfig(auth.OAuth2, &Source{})
+}
diff --git a/services/auth/source/oauth2/source_authenticate.go b/services/auth/source/oauth2/source_authenticate.go
new file mode 100644
index 0000000..bbda35d
--- /dev/null
+++ b/services/auth/source/oauth2/source_authenticate.go
@@ -0,0 +1,19 @@
+// Copyright 2021 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package oauth2
+
+import (
+	"context"
+
+	user_model "code.gitea.io/gitea/models/user"
+	"code.gitea.io/gitea/services/auth/source/db"
+)
+
+// Authenticate falls back to the db authenticator
+func (source *Source) Authenticate(ctx context.Context, user *user_model.User, login, password string) (*user_model.User, error) {
+	return db.Authenticate(ctx, user, login, password)
+}
+
+// NB: Oauth2 does not implement LocalTwoFASkipper for password authentication
+// as its password authentication drops to db authentication
diff --git a/services/auth/source/oauth2/source_callout.go b/services/auth/source/oauth2/source_callout.go
new file mode 100644
index 0000000..f95a80f
--- /dev/null
+++ b/services/auth/source/oauth2/source_callout.go
@@ -0,0 +1,68 @@
+// Copyright 2021 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package oauth2
+
+import (
+	"net/http"
+	"net/url"
+
+	"github.com/markbates/goth"
+	"github.com/markbates/goth/gothic"
+)
+
+// Callout redirects request/response pair to authenticate against the provider
+func (source *Source) Callout(request *http.Request, response http.ResponseWriter, codeChallengeS256 string) error {
+	// not sure if goth is thread safe (?) when using multiple providers
+	request.Header.Set(ProviderHeaderKey, source.authSource.Name)
+
+	var querySuffix string
+	if codeChallengeS256 != "" {
+		querySuffix = "&" + url.Values{
+			"code_challenge_method": []string{"S256"},
+			"code_challenge":        []string{codeChallengeS256},
+		}.Encode()
+	}
+
+	// don't use the default gothic begin handler to prevent issues when some error occurs
+	// normally the gothic library will write some custom stuff to the response instead of our own nice error page
+	// gothic.BeginAuthHandler(response, request)
+
+	gothRWMutex.RLock()
+	defer gothRWMutex.RUnlock()
+
+	url, err := gothic.GetAuthURL(response, request)
+	if err == nil {
+		// hacky way to set the code_challenge, but no better way until
+		// https://github.com/markbates/goth/issues/516 is resolved
+		http.Redirect(response, request, url+querySuffix, http.StatusTemporaryRedirect)
+	}
+	return err
+}
+
+// Callback handles OAuth callback, resolve to a goth user and send back to original url
+// this will trigger a new authentication request, but because we save it in the session we can use that
+func (source *Source) Callback(request *http.Request, response http.ResponseWriter, codeVerifier string) (goth.User, error) {
+	// not sure if goth is thread safe (?) when using multiple providers
+	request.Header.Set(ProviderHeaderKey, source.authSource.Name)
+
+	if codeVerifier != "" {
+		// hacky way to set the code_verifier...
+		// Will be picked up inside CompleteUserAuth: params := req.URL.Query()
+		// https://github.com/markbates/goth/pull/474/files
+		request = request.Clone(request.Context())
+		q := request.URL.Query()
+		q.Add("code_verifier", codeVerifier)
+		request.URL.RawQuery = q.Encode()
+	}
+
+	gothRWMutex.RLock()
+	defer gothRWMutex.RUnlock()
+
+	user, err := gothic.CompleteUserAuth(response, request)
+	if err != nil {
+		return user, err
+	}
+
+	return user, nil
+}
diff --git a/services/auth/source/oauth2/source_name.go b/services/auth/source/oauth2/source_name.go
new file mode 100644
index 0000000..eee789e
--- /dev/null
+++ b/services/auth/source/oauth2/source_name.go
@@ -0,0 +1,18 @@
+// Copyright 2021 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package oauth2
+
+// Name returns the provider name of this source
+func (source *Source) Name() string {
+	return source.Provider
+}
+
+// DisplayName returns the display name of this source
+func (source *Source) DisplayName() string {
+	provider, has := gothProviders[source.Provider]
+	if !has {
+		return source.Provider
+	}
+	return provider.DisplayName()
+}
diff --git a/services/auth/source/oauth2/source_register.go b/services/auth/source/oauth2/source_register.go
new file mode 100644
index 0000000..82a36ac
--- /dev/null
+++ b/services/auth/source/oauth2/source_register.go
@@ -0,0 +1,50 @@
+// Copyright 2021 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package oauth2
+
+import (
+	"fmt"
+)
+
+// RegisterSource causes an OAuth2 configuration to be registered
+func (source *Source) RegisterSource() error {
+	err := RegisterProviderWithGothic(source.authSource.Name, source)
+	return wrapOpenIDConnectInitializeError(err, source.authSource.Name, source)
+}
+
+// UnregisterSource causes an OAuth2 configuration to be unregistered
+func (source *Source) UnregisterSource() error {
+	RemoveProviderFromGothic(source.authSource.Name)
+	return nil
+}
+
+// ErrOpenIDConnectInitialize represents a "OpenIDConnectInitialize" kind of error.
+type ErrOpenIDConnectInitialize struct {
+	OpenIDConnectAutoDiscoveryURL string
+	ProviderName                  string
+	Cause                         error
+}
+
+// IsErrOpenIDConnectInitialize checks if an error is a ExternalLoginUserAlreadyExist.
+func IsErrOpenIDConnectInitialize(err error) bool {
+	_, ok := err.(ErrOpenIDConnectInitialize)
+	return ok
+}
+
+func (err ErrOpenIDConnectInitialize) Error() string {
+	return fmt.Sprintf("Failed to initialize OpenID Connect Provider with name '%s' with url '%s': %v", err.ProviderName, err.OpenIDConnectAutoDiscoveryURL, err.Cause)
+}
+
+func (err ErrOpenIDConnectInitialize) Unwrap() error {
+	return err.Cause
+}
+
+// wrapOpenIDConnectInitializeError is used to wrap the error but this cannot be done in modules/auth/oauth2
+// inside oauth2: import cycle not allowed models -> modules/auth/oauth2 -> models
+func wrapOpenIDConnectInitializeError(err error, providerName string, source *Source) error {
+	if err != nil && source.Provider == "openidConnect" {
+		err = ErrOpenIDConnectInitialize{ProviderName: providerName, OpenIDConnectAutoDiscoveryURL: source.OpenIDConnectAutoDiscoveryURL, Cause: err}
+	}
+	return err
+}
diff --git a/services/auth/source/oauth2/store.go b/services/auth/source/oauth2/store.go
new file mode 100644
index 0000000..e031653
--- /dev/null
+++ b/services/auth/source/oauth2/store.go
@@ -0,0 +1,98 @@
+// Copyright 2021 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package oauth2
+
+import (
+	"encoding/gob"
+	"fmt"
+	"net/http"
+
+	"code.gitea.io/gitea/modules/log"
+	session_module "code.gitea.io/gitea/modules/session"
+
+	chiSession "code.forgejo.org/go-chi/session"
+	"github.com/gorilla/sessions"
+)
+
+// SessionsStore creates a gothic store from our session
+type SessionsStore struct {
+	maxLength int64
+}
+
+// Get should return a cached session.
+func (st *SessionsStore) Get(r *http.Request, name string) (*sessions.Session, error) {
+	return st.getOrNew(r, name, false)
+}
+
+// New should create and return a new session.
+//
+// Note that New should never return a nil session, even in the case of
+// an error if using the Registry infrastructure to cache the session.
+func (st *SessionsStore) New(r *http.Request, name string) (*sessions.Session, error) {
+	return st.getOrNew(r, name, true)
+}
+
+// getOrNew gets the session from the chi-session if it exists. Override permits the overriding of an unexpected object.
+func (st *SessionsStore) getOrNew(r *http.Request, name string, override bool) (*sessions.Session, error) {
+	chiStore := chiSession.GetSession(r)
+
+	session := sessions.NewSession(st, name)
+
+	rawData := chiStore.Get(name)
+	if rawData != nil {
+		oldSession, ok := rawData.(*sessions.Session)
+		if ok {
+			session.ID = oldSession.ID
+			session.IsNew = oldSession.IsNew
+			session.Options = oldSession.Options
+			session.Values = oldSession.Values
+
+			return session, nil
+		} else if !override {
+			log.Error("Unexpected object in session at name: %s: %v", name, rawData)
+			return nil, fmt.Errorf("unexpected object in session at name: %s", name)
+		}
+	}
+
+	session.IsNew = override
+	session.ID = chiStore.ID() // Simply copy the session id from the chi store
+
+	return session, chiStore.Set(name, session)
+}
+
+// Save should persist session to the underlying store implementation.
+func (st *SessionsStore) Save(r *http.Request, w http.ResponseWriter, session *sessions.Session) error {
+	chiStore := chiSession.GetSession(r)
+
+	if session.IsNew {
+		_, _ = session_module.RegenerateSession(w, r)
+		session.IsNew = false
+	}
+
+	if err := chiStore.Set(session.Name(), session); err != nil {
+		return err
+	}
+
+	if st.maxLength > 0 {
+		sizeWriter := &sizeWriter{}
+
+		_ = gob.NewEncoder(sizeWriter).Encode(session)
+		if sizeWriter.size > st.maxLength {
+			return fmt.Errorf("encode session: Data too long: %d > %d", sizeWriter.size, st.maxLength)
+		}
+	}
+
+	return chiStore.Release()
+}
+
+type sizeWriter struct {
+	size int64
+}
+
+func (s *sizeWriter) Write(data []byte) (int, error) {
+	s.size += int64(len(data))
+	return len(data), nil
+}
+
+var _ (sessions.Store) = &SessionsStore{}
diff --git a/services/auth/source/oauth2/token.go b/services/auth/source/oauth2/token.go
new file mode 100644
index 0000000..3405619
--- /dev/null
+++ b/services/auth/source/oauth2/token.go
@@ -0,0 +1,100 @@
+// Copyright 2021 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package oauth2
+
+import (
+	"fmt"
+	"time"
+
+	"code.gitea.io/gitea/modules/timeutil"
+
+	"github.com/golang-jwt/jwt/v5"
+)
+
+// ___________     __
+// \__    ___/___ |  | __ ____   ____
+//   |    | /  _ \|  |/ // __ \ /    \
+//   |    |(  <_> )    <\  ___/|   |  \
+//   |____| \____/|__|_ \\___  >___|  /
+//                     \/    \/     \/
+
+// Token represents an Oauth grant
+
+// TokenType represents the type of token for an oauth application
+type TokenType int
+
+const (
+	// TypeAccessToken is a token with short lifetime to access the api
+	TypeAccessToken TokenType = 0
+	// TypeRefreshToken is token with long lifetime to refresh access tokens obtained by the client
+	TypeRefreshToken = iota
+)
+
+// Token represents a JWT token used to authenticate a client
+type Token struct {
+	GrantID int64     `json:"gnt"`
+	Type    TokenType `json:"tt"`
+	Counter int64     `json:"cnt,omitempty"`
+	jwt.RegisteredClaims
+}
+
+// ParseToken parses a signed jwt string
+func ParseToken(jwtToken string, signingKey JWTSigningKey) (*Token, error) {
+	parsedToken, err := jwt.ParseWithClaims(jwtToken, &Token{}, func(token *jwt.Token) (any, error) {
+		if token.Method == nil || token.Method.Alg() != signingKey.SigningMethod().Alg() {
+			return nil, fmt.Errorf("unexpected signing algo: %v", token.Header["alg"])
+		}
+		return signingKey.VerifyKey(), nil
+	})
+	if err != nil {
+		return nil, err
+	}
+	if !parsedToken.Valid {
+		return nil, fmt.Errorf("invalid token")
+	}
+	var token *Token
+	var ok bool
+	if token, ok = parsedToken.Claims.(*Token); !ok || !parsedToken.Valid {
+		return nil, fmt.Errorf("invalid token")
+	}
+	return token, nil
+}
+
+// SignToken signs the token with the JWT secret
+func (token *Token) SignToken(signingKey JWTSigningKey) (string, error) {
+	token.IssuedAt = jwt.NewNumericDate(time.Now())
+	jwtToken := jwt.NewWithClaims(signingKey.SigningMethod(), token)
+	signingKey.PreProcessToken(jwtToken)
+	return jwtToken.SignedString(signingKey.SignKey())
+}
+
+// OIDCToken represents an OpenID Connect id_token
+type OIDCToken struct {
+	jwt.RegisteredClaims
+	Nonce string `json:"nonce,omitempty"`
+
+	// Scope profile
+	Name              string             `json:"name,omitempty"`
+	PreferredUsername string             `json:"preferred_username,omitempty"`
+	Profile           string             `json:"profile,omitempty"`
+	Picture           string             `json:"picture,omitempty"`
+	Website           string             `json:"website,omitempty"`
+	Locale            string             `json:"locale,omitempty"`
+	UpdatedAt         timeutil.TimeStamp `json:"updated_at,omitempty"`
+
+	// Scope email
+	Email         string `json:"email,omitempty"`
+	EmailVerified bool   `json:"email_verified,omitempty"`
+
+	// Groups are generated by organization and team names
+	Groups []string `json:"groups,omitempty"`
+}
+
+// SignToken signs an id_token with the (symmetric) client secret key
+func (token *OIDCToken) SignToken(signingKey JWTSigningKey) (string, error) {
+	token.IssuedAt = jwt.NewNumericDate(time.Now())
+	jwtToken := jwt.NewWithClaims(signingKey.SigningMethod(), token)
+	signingKey.PreProcessToken(jwtToken)
+	return jwtToken.SignedString(signingKey.SignKey())
+}
diff --git a/services/auth/source/oauth2/urlmapping.go b/services/auth/source/oauth2/urlmapping.go
new file mode 100644
index 0000000..d0442d5
--- /dev/null
+++ b/services/auth/source/oauth2/urlmapping.go
@@ -0,0 +1,77 @@
+// Copyright 2021 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package oauth2
+
+// CustomURLMapping describes the urls values to use when customizing OAuth2 provider URLs
+type CustomURLMapping struct {
+	AuthURL    string `json:",omitempty"`
+	TokenURL   string `json:",omitempty"`
+	ProfileURL string `json:",omitempty"`
+	EmailURL   string `json:",omitempty"`
+	Tenant     string `json:",omitempty"`
+}
+
+// CustomURLSettings describes the urls values and availability to use when customizing OAuth2 provider URLs
+type CustomURLSettings struct {
+	AuthURL    Attribute `json:",omitempty"`
+	TokenURL   Attribute `json:",omitempty"`
+	ProfileURL Attribute `json:",omitempty"`
+	EmailURL   Attribute `json:",omitempty"`
+	Tenant     Attribute `json:",omitempty"`
+}
+
+// Attribute describes the availability, and required status for a custom url configuration
+type Attribute struct {
+	Value     string
+	Available bool
+	Required  bool
+}
+
+func availableAttribute(value string) Attribute {
+	return Attribute{Value: value, Available: true}
+}
+
+func requiredAttribute(value string) Attribute {
+	return Attribute{Value: value, Available: true, Required: true}
+}
+
+// Required is true if any attribute is required
+func (c *CustomURLSettings) Required() bool {
+	if c == nil {
+		return false
+	}
+	if c.AuthURL.Required || c.EmailURL.Required || c.ProfileURL.Required || c.TokenURL.Required || c.Tenant.Required {
+		return true
+	}
+	return false
+}
+
+// OverrideWith copies the current customURLMapping and overrides it with values from the provided mapping
+func (c *CustomURLSettings) OverrideWith(override *CustomURLMapping) *CustomURLMapping {
+	custom := &CustomURLMapping{
+		AuthURL:    c.AuthURL.Value,
+		TokenURL:   c.TokenURL.Value,
+		ProfileURL: c.ProfileURL.Value,
+		EmailURL:   c.EmailURL.Value,
+		Tenant:     c.Tenant.Value,
+	}
+	if override != nil {
+		if len(override.AuthURL) > 0 && c.AuthURL.Available {
+			custom.AuthURL = override.AuthURL
+		}
+		if len(override.TokenURL) > 0 && c.TokenURL.Available {
+			custom.TokenURL = override.TokenURL
+		}
+		if len(override.ProfileURL) > 0 && c.ProfileURL.Available {
+			custom.ProfileURL = override.ProfileURL
+		}
+		if len(override.EmailURL) > 0 && c.EmailURL.Available {
+			custom.EmailURL = override.EmailURL
+		}
+		if len(override.Tenant) > 0 && c.Tenant.Available {
+			custom.Tenant = override.Tenant
+		}
+	}
+	return custom
+}