cherry-pick OIDC changes from gitea (#4724)

These are the three conflicted changes from #4716:

* https://github.com/go-gitea/gitea/pull/31632
* https://github.com/go-gitea/gitea/pull/31688
* https://github.com/go-gitea/gitea/pull/31706

cc @earl-warren; as per discussion on https://github.com/go-gitea/gitea/pull/31632 this involves a small compatibility break (OIDC introspection requests now require a valid client ID and secret, instead of a valid OIDC token)

## Checklist

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### Tests

- I added test coverage for Go changes...
- [ ] in their respective `*_test.go` for unit tests.
- [x] in the `tests/integration` directory if it involves interactions with a live Forgejo server.

### Documentation

- [ ] I created a pull request [to the documentation](https://codeberg.org/forgejo/docs) to explain to Forgejo users how to use this change.
- [ ] I did not document these changes and I do not expect someone else to do it.

### Release notes

- [ ] I do not want this change to show in the release notes.
- [ ] I want the title to show in the release notes with a link to this pull request.
- [ ] I want the content of the `release-notes/<pull request number>.md` to be be used for the release notes instead of the title.

<!--start release-notes-assistant-->

## Draft release notes
<!--URL:https://codeberg.org/forgejo/forgejo-->
- Breaking features
- [PR](https://codeberg.org/forgejo/forgejo/pulls/4724): <!--number 4724 --><!--line 0 --><!--description T0lEQyBpbnRlZ3JhdGlvbnMgdGhhdCBQT1NUIHRvIGAvbG9naW4vb2F1dGgvaW50cm9zcGVjdGAgd2l0aG91dCBzZW5kaW5nIEhUVFAgYmFzaWMgYXV0aGVudGljYXRpb24gd2lsbCBub3cgZmFpbCB3aXRoIGEgNDAxIEhUVFAgVW5hdXRob3JpemVkIGVycm9yLiBUbyBmaXggdGhlIGVycm9yLCB0aGUgY2xpZW50IG11c3QgYmVnaW4gc2VuZGluZyBIVFRQIGJhc2ljIGF1dGhlbnRpY2F0aW9uIHdpdGggYSB2YWxpZCBjbGllbnQgSUQgYW5kIHNlY3JldC4gVGhpcyBlbmRwb2ludCB3YXMgcHJldmlvdXNseSBhdXRoZW50aWNhdGVkIHZpYSB0aGUgaW50cm9zcGVjdGlvbiB0b2tlbiBpdHNlbGYsIHdoaWNoIGlzIGxlc3Mgc2VjdXJlLg==-->OIDC integrations that POST to `/login/oauth/introspect` without sending HTTP basic authentication will now fail with a 401 HTTP Unauthorized error. To fix the error, the client must begin sending HTTP basic authentication with a valid client ID and secret. This endpoint was previously authenticated via the introspection token itself, which is less secure.<!--description-->
<!--end release-notes-assistant-->

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/4724
Reviewed-by: Earl Warren <[e-mail]>
Co-authored-by: Shivaram Lingamneni <[e-mail]>
Co-committed-by: Shivaram Lingamneni <[e-mail]>

modules/base/tool.go

···
 		return "", "", err
 	}
 
-	auth := strings.SplitN(string(s), ":", 2)
-
-	if len(auth) != 2 {
-		return "", "", errors.New("invalid basic authentication")
+	if username, password, ok := strings.Cut(string(s), ":"); ok {
+		return username, password, nil
 	}
-
-	return auth[0], auth[1], nil
+	return "", "", errors.New("invalid basic authentication")
 }
 
 // VerifyTimeLimitCode verify time limit code

modules/base/tool_test.go

···
 
 	_, _, err = BasicAuthDecode("invalid")
 	require.Error(t, err)
+
+	_, _, err = BasicAuthDecode("YWxpY2U=") // "alice", no colon
+	require.Error(t, err)
 }
 
 func TestVerifyTimeLimitCode(t *testing.T) {

release-notes/4724.md

···
+OIDC integrations that POST to `/login/oauth/introspect` without sending HTTP basic authentication will now fail with a 401 HTTP Unauthorized error. To fix the error, the client must begin sending HTTP basic authentication with a valid client ID and secret. This endpoint was previously authenticated via the introspection token itself, which is less secure.

routers/web/auth/oauth.go

···
 	return groups, nil
 }
 
+func parseBasicAuth(ctx *context.Context) (username, password string, err error) {
+	authHeader := ctx.Req.Header.Get("Authorization")
+	if authType, authData, ok := strings.Cut(authHeader, " "); ok && strings.EqualFold(authType, "Basic") {
+		return base.BasicAuthDecode(authData)
+	}
+	return "", "", errors.New("invalid basic authentication")
+}
+
 // IntrospectOAuth introspects an oauth token
 func IntrospectOAuth(ctx *context.Context) {
-	if ctx.Doer == nil {
-		ctx.Resp.Header().Set("WWW-Authenticate", `Bearer realm=""`)
+	clientIDValid := false
+	if clientID, clientSecret, err := parseBasicAuth(ctx); err == nil {
+		app, err := auth.GetOAuth2ApplicationByClientID(ctx, clientID)
+		if err != nil && !auth.IsErrOauthClientIDInvalid(err) {
+			// this is likely a database error; log it and respond without details
+			log.Error("Error retrieving client_id: %v", err)
+			ctx.Error(http.StatusInternalServerError)
+			return
+		}
+		clientIDValid = err == nil && app.ValidateClientSecret([]byte(clientSecret))
+	}
+	if !clientIDValid {
+		ctx.Resp.Header().Set("WWW-Authenticate", `Basic realm=""`)
 		ctx.PlainText(http.StatusUnauthorized, "no valid authorization")
 		return
 	}
 
 	var response struct {
-		Active bool   `json:"active"`
-		Scope  string `json:"scope,omitempty"`
+		Active   bool   `json:"active"`
+		Scope    string `json:"scope,omitempty"`
+		Username string `json:"username,omitempty"`
 		jwt.RegisteredClaims
 	}
 
···
 				response.Issuer = setting.AppURL
 				response.Audience = []string{app.ClientID}
 				response.Subject = fmt.Sprint(grant.UserID)
+			}
+			if user, err := user_model.GetUserByID(ctx, grant.UserID); err == nil {
+				response.Username = user.Name
 			}
 		}
 	}
···
 	// if there is no ClientID or ClientSecret in the request body, fill these fields by the Authorization header and ensure the provided field matches the Authorization header
 	if form.ClientID == "" || form.ClientSecret == "" {
 		authHeader := ctx.Req.Header.Get("Authorization")
-		authContent := strings.SplitN(authHeader, " ", 2)
-		if len(authContent) == 2 && authContent[0] == "Basic" {
-			payload, err := base64.StdEncoding.DecodeString(authContent[1])
+		if authType, authData, ok := strings.Cut(authHeader, " "); ok && strings.EqualFold(authType, "Basic") {
+			clientID, clientSecret, err := base.BasicAuthDecode(authData)
 			if err != nil {
 				handleAccessTokenError(ctx, AccessTokenError{
 					ErrorCode:        AccessTokenErrorCodeInvalidRequest,
···
 				})
 				return
 			}
-			pair := strings.SplitN(string(payload), ":", 2)
-			if len(pair) != 2 {
-				handleAccessTokenError(ctx, AccessTokenError{
-					ErrorCode:        AccessTokenErrorCodeInvalidRequest,
-					ErrorDescription: "cannot parse basic auth header",
-				})
-				return
-			}
-			if form.ClientID != "" && form.ClientID != pair[0] {
+			// validate that any fields present in the form match the Basic auth header
+			if form.ClientID != "" && form.ClientID != clientID {
 				handleAccessTokenError(ctx, AccessTokenError{
 					ErrorCode:        AccessTokenErrorCodeInvalidRequest,
 					ErrorDescription: "client_id in request body inconsistent with Authorization header",
 				})
 				return
 			}
-			form.ClientID = pair[0]
-			if form.ClientSecret != "" && form.ClientSecret != pair[1] {
+			form.ClientID = clientID
+			if form.ClientSecret != "" && form.ClientSecret != clientSecret {
 				handleAccessTokenError(ctx, AccessTokenError{
 					ErrorCode:        AccessTokenErrorCodeInvalidRequest,
 					ErrorDescription: "client_secret in request body inconsistent with Authorization header",
 				})
 				return
 			}
-			form.ClientSecret = pair[1]
+			form.ClientSecret = clientSecret
 		}
 	}
 

tests/integration/oauth_test.go

···
 	resp = ctx.MakeRequest(t, req, http.StatusSeeOther)
 	assert.Contains(t, test.RedirectURL(resp), "error=access_denied&error_description=the+request+is+denied")
 }
+
+func TestOAuthIntrospection(t *testing.T) {
+	defer tests.PrepareTestEnv(t)()
+	req := NewRequestWithValues(t, "POST", "/login/oauth/access_token", map[string]string{
+		"grant_type":    "authorization_code",
+		"client_id":     "da7da3ba-9a13-4167-856f-3899de0b0138",
+		"client_secret": "4MK8Na6R55smdCY0WuCCumZ6hjRPnGY5saWVRHHjJiA=",
+		"redirect_uri":  "a",
+		"code":          "authcode",
+		"code_verifier": "N1Zo9-8Rfwhkt68r1r29ty8YwIraXR8eh_1Qwxg7yQXsonBt",
+	})
+	resp := MakeRequest(t, req, http.StatusOK)
+	type response struct {
+		AccessToken  string `json:"access_token"`
+		TokenType    string `json:"token_type"`
+		ExpiresIn    int64  `json:"expires_in"`
+		RefreshToken string `json:"refresh_token"`
+	}
+	parsed := new(response)
+
+	DecodeJSON(t, resp, parsed)
+	assert.Greater(t, len(parsed.AccessToken), 10)
+	assert.Greater(t, len(parsed.RefreshToken), 10)
+
+	type introspectResponse struct {
+		Active   bool   `json:"active"`
+		Scope    string `json:"scope,omitempty"`
+		Username string `json:"username"`
+	}
+
+	// successful request with a valid client_id/client_secret and a valid token
+	t.Run("successful request with valid token", func(t *testing.T) {
+		req := NewRequestWithValues(t, "POST", "/login/oauth/introspect", map[string]string{
+			"token": parsed.AccessToken,
+		})
+		req.Header.Add("Authorization", "Basic ZGE3ZGEzYmEtOWExMy00MTY3LTg1NmYtMzg5OWRlMGIwMTM4OjRNSzhOYTZSNTVzbWRDWTBXdUNDdW1aNmhqUlBuR1k1c2FXVlJISGpKaUE9")
+		resp := MakeRequest(t, req, http.StatusOK)
+
+		introspectParsed := new(introspectResponse)
+		DecodeJSON(t, resp, introspectParsed)
+		assert.True(t, introspectParsed.Active)
+		assert.Equal(t, "user1", introspectParsed.Username)
+	})
+
+	// successful request with a valid client_id/client_secret, but an invalid token
+	t.Run("successful request with invalid token", func(t *testing.T) {
+		req := NewRequestWithValues(t, "POST", "/login/oauth/introspect", map[string]string{
+			"token": "xyzzy",
+		})
+		req.Header.Add("Authorization", "Basic ZGE3ZGEzYmEtOWExMy00MTY3LTg1NmYtMzg5OWRlMGIwMTM4OjRNSzhOYTZSNTVzbWRDWTBXdUNDdW1aNmhqUlBuR1k1c2FXVlJISGpKaUE9")
+		resp := MakeRequest(t, req, http.StatusOK)
+		introspectParsed := new(introspectResponse)
+		DecodeJSON(t, resp, introspectParsed)
+		assert.False(t, introspectParsed.Active)
+	})
+
+	// unsuccessful request with an invalid client_id/client_secret
+	t.Run("unsuccessful request due to invalid basic auth", func(t *testing.T) {
+		req := NewRequestWithValues(t, "POST", "/login/oauth/introspect", map[string]string{
+			"token": parsed.AccessToken,
+		})
+		req.Header.Add("Authorization", "Basic ZGE3ZGEzYmEtOWExMy00MTY3LTg1NmYtMzg5OWRlMGIwMTM4OjRNSzhOYTZSNTVzbWRDWTBXdUNDdW1aNmhqUlBuR1k1c2FXVlJISGpK")
+		resp := MakeRequest(t, req, http.StatusUnauthorized)
+		assert.Contains(t, resp.Body.String(), "no valid authorization")
+	})
+}