fix(oauth): restrict introspection to the token's client (#38042)

Bind OAuth token introspection responses to the authenticated client.
Return an inactive response when the token grant belongs to a different
OAuth application to avoid leaking token metadata across clients.

Add integration coverage for cross-client introspection attempts against
both access tokens and refresh tokens.

Assisted-by: GPT-5.4

tests/integration/oauth_test.go

@@ -14,6 +14,7 @@ import (
 	"net/http"
 	"net/http/httptest"
 	"net/url"
+	"strconv"
 	"strings"
 	"testing"
 
@@ -33,6 +34,7 @@ import (
 	"gitea.dev/tests"
 
 	"github.com/PuerkitoBio/goquery"
+	jwt "github.com/golang-jwt/jwt/v5"
 	"github.com/markbates/goth"
 	"github.com/markbates/goth/gothic"
 	"github.com/stretchr/testify/assert"
@@ -122,6 +124,7 @@ func TestOAuth2(t *testing.T) {
 		t.Run("RefreshTokenInvalidation", testRefreshTokenInvalidation)
 		t.Run("RefreshTokenCrossClientUsage", testRefreshTokenCrossClientUsage)
 		t.Run("OAuthIntrospection", testOAuthIntrospection)
+		t.Run("OAuthIntrospectionCrossClientIsolation", testOAuthIntrospectionCrossClientIsolation)
 		t.Run("OAuthGrantScopesReadUserFailRepos", testOAuthGrantScopesReadUserFailRepos)
 		t.Run("OAuthGrantScopesBasicRespectsWriteUser", testOAuthGrantScopesBasicRespectsWriteUser)
 		t.Run("OAuthGrantScopesReadRepositoryFailOrganization", testOAuthGrantScopesReadRepositoryFailOrganization)
@@ -705,6 +708,79 @@ func testOAuthIntrospection(t *testing.T) {
 	assert.Contains(t, resp.Body.String(), "no valid authorization")
 }
 
+func testOAuthIntrospectionCrossClientIsolation(t *testing.T) {
+	resourceOwner := unittest.AssertExistsAndLoadBean(t, &user_model.User{ID: 2})
+	clientA := createOAuthTestApplication(t, "user1", "introspection-primary-client", []string{"https://primary.example/oauth/callback"})
+	clientB := createOAuthTestApplication(t, "user2", "introspection-secondary-client", []string{"https://secondary.example/oauth/callback"})
+	code, verifier := issueOAuthAuthorizationCode(t, resourceOwner, clientA, clientA.RedirectURIs[0], "openid profile")
+
+	req := NewRequestWithValues(t, "POST", "/login/oauth/access_token", map[string]string{
+		"grant_type":    "authorization_code",
+		"client_id":     clientA.ClientID,
+		"client_secret": clientA.ClientSecret,
+		"redirect_uri":  clientA.RedirectURIs[0],
+		"code":          code,
+		"code_verifier": verifier,
+	})
+	resp := MakeRequest(t, req, http.StatusOK)
+	type tokenResponse struct {
+		AccessToken  string `json:"access_token"`
+		RefreshToken string `json:"refresh_token"`
+	}
+	tokenParsed := new(tokenResponse)
+	require.NoError(t, json.Unmarshal(resp.Body.Bytes(), tokenParsed))
+	require.NotEmpty(t, tokenParsed.AccessToken)
+	require.NotEmpty(t, tokenParsed.RefreshToken)
+
+	type introspectResponse struct {
+		Active   bool   `json:"active"`
+		Scope    string `json:"scope,omitempty"`
+		Username string `json:"username,omitempty"`
+		jwt.RegisteredClaims
+	}
+
+	assertBlockedIntrospection := func(token string) {
+		t.Helper()
+
+		req = NewRequestWithValues(t, "POST", "/login/oauth/introspect", map[string]string{
+			"token": token,
+		})
+		req.SetBasicAuth(clientB.ClientID, clientB.ClientSecret)
+		resp = MakeRequest(t, req, http.StatusOK)
+
+		blocked := new(introspectResponse)
+		require.NoError(t, json.Unmarshal(resp.Body.Bytes(), blocked))
+		assert.False(t, blocked.Active)
+		assert.Empty(t, blocked.Scope)
+		assert.Empty(t, blocked.Username)
+		assert.Empty(t, blocked.Subject)
+		assert.Empty(t, blocked.Audience)
+	}
+
+	assertAllowedIntrospection := func(token string) {
+		t.Helper()
+
+		req = NewRequestWithValues(t, "POST", "/login/oauth/introspect", map[string]string{
+			"token": token,
+		})
+		req.SetBasicAuth(clientA.ClientID, clientA.ClientSecret)
+		resp = MakeRequest(t, req, http.StatusOK)
+
+		allowed := new(introspectResponse)
+		require.NoError(t, json.Unmarshal(resp.Body.Bytes(), allowed))
+		assert.True(t, allowed.Active)
+		assert.Equal(t, "openid profile", allowed.Scope)
+		assert.Equal(t, resourceOwner.Name, allowed.Username)
+		assert.Equal(t, strconv.FormatInt(resourceOwner.ID, 10), allowed.Subject)
+		assert.Equal(t, jwt.ClaimStrings{clientA.ClientID}, allowed.Audience)
+	}
+
+	assertBlockedIntrospection(tokenParsed.AccessToken)
+	assertAllowedIntrospection(tokenParsed.AccessToken)
+	assertBlockedIntrospection(tokenParsed.RefreshToken)
+	assertAllowedIntrospection(tokenParsed.RefreshToken)
+}
+
 func testOAuthGrantScopesReadUserFailRepos(t *testing.T) {
 	user := unittest.AssertExistsAndLoadBean(t, &user_model.User{ID: 2})
 	accessToken := issueOAuthAccessTokenForScope(t, user, "openid read:user")