fix OIDC introspection authentication (go-gitea#31632)

See discussion on go-gitea#31561 for some background.

The introspect endpoint was using the OIDC token itself for
authentication. This fixes it to use basic authentication with the
client ID and secret instead:

* Applications with a valid client ID and secret should be able to
  successfully introspect an invalid token, receiving a 200 response
  with JSON data that indicates the token is invalid
* Requests with an invalid client ID and secret should not be able
  to introspect, even if the token itself is valid

Unlike go-gitea#31561 (which just future-proofed the current behavior against
future changes to `DISABLE_QUERY_AUTH_TOKEN`), this is a potential
compatibility break (some introspection requests without valid client
IDs that would previously succeed will now fail). Affected deployments
must begin sending a valid HTTP basic authentication header with their
introspection requests, with the username set to a valid client ID and
the password set to the corresponding client secret.

modules/base/tool.go

@@ -48,13 +48,10 @@ func BasicAuthDecode(encoded string) (string, string, error) {
 		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

@@ -41,6 +41,9 @@ func TestBasicAuthDecode(t *testing.T) {
 
 	_, _, err = BasicAuthDecode("invalid")
 	assert.Error(t, err)
+
+	_, _, err = BasicAuthDecode("YWxpY2U=") // "alice", no colon
+	assert.Error(t, err)
 }
 
 func TestVerifyTimeLimitCode(t *testing.T) {

routers/web/auth/oauth.go

@@ -5,7 +5,6 @@ package auth
 
 import (
 	go_context "context"
-	"encoding/base64"
 	"errors"
 	"fmt"
 	"html"
@@ -326,10 +325,29 @@ func getOAuthGroupsForUser(ctx go_context.Context, user *user_model.User) ([]str
 	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 && 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
 	}
@@ -639,40 +657,32 @@ func AccessTokenOAuth(ctx *context.Context) {
 	// 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 && authType == "Basic" {
+			clientID, clientSecret, err := base.BasicAuthDecode(authData)
 			if err != nil {
 				handleAccessTokenError(ctx, AccessTokenError{
 					ErrorCode:        AccessTokenErrorCodeInvalidRequest,
 					ErrorDescription: "cannot parse basic auth header",
 				})
 				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

@@ -419,3 +419,59 @@ func TestRefreshTokenInvalidation(t *testing.T) {
 	assert.Equal(t, "unauthorized_client", string(parsedError.ErrorCode))
 	assert.Equal(t, "token was already used", parsedError.ErrorDescription)
 }
+
+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)
+
+	assert.NoError(t, json.Unmarshal(resp.Body.Bytes(), parsed))
+	assert.True(t, len(parsed.AccessToken) > 10)
+	assert.True(t, len(parsed.RefreshToken) > 10)
+
+	// successful request with a valid client_id/client_secret and a valid token
+	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)
+	type introspectResponse struct {
+		Active bool   `json:"active"`
+		Scope  string `json:"scope,omitempty"`
+	}
+	introspectParsed := new(introspectResponse)
+	assert.NoError(t, json.Unmarshal(resp.Body.Bytes(), introspectParsed))
+	assert.True(t, introspectParsed.Active)
+
+	// successful request with a valid client_id/client_secret, but an invalid token
+	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)
+	assert.NoError(t, json.Unmarshal(resp.Body.Bytes(), introspectParsed))
+	assert.False(t, introspectParsed.Active)
+
+	// unsuccessful request with an invalid client_id/client_secret
+	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")
+}