Minimal OpenID Connect implementation (#14139)

This is "minimal" in the sense that only the Authorization Code Flow
from OpenID Connect Core is implemented.  No discovery, no configuration
endpoint, and no user scope management.

OpenID Connect is an extension to the (already implemented) OAuth 2.0
protocol, and essentially an `id_token` JWT is added to the access token
endpoint response when using the Authorization Code Flow.  I also added
support for the "nonce" field since it is required to be used in the
id_token if the client decides to include it in its initial request.

In order to enable this extension an OAuth 2.0 scope containing
"openid" is needed. Other OAuth 2.0 requests should not be impacted by
this change.

This minimal implementation is enough to enable single sign-on (SSO)
for other sites, e.g. by using something like `mod_auth_openidc` to
only allow access to a CI server if a user has logged into Gitea.

Fixes: #1310

Co-authored-by: 6543 <6543@obermui.de>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: zeripath <art27@cantab.net>

1 files changed, 49 insertions, 4 deletions

diff --git a/routers/user/oauth.go b/routers/user/oauth.go
index 12665e94db..dda1268f8a 100644
--- a/routers/user/oauth.go
+++ b/routers/user/oauth.go
@@ -107,9 +107,10 @@ type AccessTokenResponse struct {
 	TokenType    TokenType `json:"token_type"`
 	ExpiresIn    int64     `json:"expires_in"`
 	RefreshToken string    `json:"refresh_token"`
+	IDToken      string    `json:"id_token,omitempty"`
 }
 
-func newAccessTokenResponse(grant *models.OAuth2Grant) (*AccessTokenResponse, *AccessTokenError) {
+func newAccessTokenResponse(grant *models.OAuth2Grant, clientSecret string) (*AccessTokenResponse, *AccessTokenError) {
 	if setting.OAuth2.InvalidateRefreshTokens {
 		if err := grant.IncreaseCounter(); err != nil {
 			return nil, &AccessTokenError{
@@ -153,11 +154,40 @@ func newAccessTokenResponse(grant *models.OAuth2Grant) (*AccessTokenResponse, *A
 		}
 	}
 
+	// generate OpenID Connect id_token
+	signedIDToken := ""
+	if grant.ScopeContains("openid") {
+		app, err := models.GetOAuth2ApplicationByID(grant.ApplicationID)
+		if err != nil {
+			return nil, &AccessTokenError{
+				ErrorCode:        AccessTokenErrorCodeInvalidRequest,
+				ErrorDescription: "cannot find application",
+			}
+		}
+		idToken := &models.OIDCToken{
+			StandardClaims: jwt.StandardClaims{
+				ExpiresAt: expirationDate.AsTime().Unix(),
+				Issuer:    setting.AppURL,
+				Audience:  app.ClientID,
+				Subject:   fmt.Sprint(grant.UserID),
+			},
+			Nonce: grant.Nonce,
+		}
+		signedIDToken, err = idToken.SignToken(clientSecret)
+		if err != nil {
+			return nil, &AccessTokenError{
+				ErrorCode:        AccessTokenErrorCodeInvalidRequest,
+				ErrorDescription: "cannot sign token",
+			}
+		}
+	}
+
 	return &AccessTokenResponse{
 		AccessToken:  signedAccessToken,
 		TokenType:    TokenTypeBearer,
 		ExpiresIn:    setting.OAuth2.AccessTokenExpirationTime,
 		RefreshToken: signedRefreshToken,
+		IDToken:      signedIDToken,
 	}, nil
 }
 
@@ -264,6 +294,13 @@ func AuthorizeOAuth(ctx *context.Context, form auth.AuthorizationForm) {
 			handleServerError(ctx, form.State, form.RedirectURI)
 			return
 		}
+		// Update nonce to reflect the new session
+		if len(form.Nonce) > 0 {
+			err := grant.SetNonce(form.Nonce)
+			if err != nil {
+				log.Error("Unable to update nonce: %v", err)
+			}
+		}
 		ctx.Redirect(redirect.String(), 302)
 		return
 	}
@@ -272,6 +309,8 @@ func AuthorizeOAuth(ctx *context.Context, form auth.AuthorizationForm) {
 	ctx.Data["Application"] = app
 	ctx.Data["RedirectURI"] = form.RedirectURI
 	ctx.Data["State"] = form.State
+	ctx.Data["Scope"] = form.Scope
+	ctx.Data["Nonce"] = form.Nonce
 	ctx.Data["ApplicationUserLink"] = "<a href=\"" + html.EscapeString(setting.AppURL) + html.EscapeString(url.PathEscape(app.User.LowerName)) + "\">@" + html.EscapeString(app.User.Name) + "</a>"
 	ctx.Data["ApplicationRedirectDomainHTML"] = "<strong>" + html.EscapeString(form.RedirectURI) + "</strong>"
 	// TODO document SESSION <=> FORM
@@ -313,7 +352,7 @@ func GrantApplicationOAuth(ctx *context.Context, form auth.GrantApplicationForm)
 		ctx.ServerError("GetOAuth2ApplicationByClientID", err)
 		return
 	}
-	grant, err := app.CreateGrant(ctx.User.ID)
+	grant, err := app.CreateGrant(ctx.User.ID, form.Scope)
 	if err != nil {
 		handleAuthorizeError(ctx, AuthorizeError{
 			State:            form.State,
@@ -322,6 +361,12 @@ func GrantApplicationOAuth(ctx *context.Context, form auth.GrantApplicationForm)
 		}, form.RedirectURI)
 		return
 	}
+	if len(form.Nonce) > 0 {
+		err := grant.SetNonce(form.Nonce)
+		if err != nil {
+			log.Error("Unable to update nonce: %v", err)
+		}
+	}
 
 	var codeChallenge, codeChallengeMethod string
 	codeChallenge, _ = ctx.Session.Get("CodeChallenge").(string)
@@ -409,7 +454,7 @@ func handleRefreshToken(ctx *context.Context, form auth.AccessTokenForm) {
 		log.Warn("A client tried to use a refresh token for grant_id = %d was used twice!", grant.ID)
 		return
 	}
-	accessToken, tokenErr := newAccessTokenResponse(grant)
+	accessToken, tokenErr := newAccessTokenResponse(grant, form.ClientSecret)
 	if tokenErr != nil {
 		handleAccessTokenError(ctx, *tokenErr)
 		return
@@ -471,7 +516,7 @@ func handleAuthorizationCode(ctx *context.Context, form auth.AccessTokenForm) {
 			ErrorDescription: "cannot proceed your request",
 		})
 	}
-	resp, tokenErr := newAccessTokenResponse(authorizationCode.Grant)
+	resp, tokenErr := newAccessTokenResponse(authorizationCode.Grant, form.ClientSecret)
 	if tokenErr != nil {
 		handleAccessTokenError(ctx, *tokenErr)
 		return