diff --git a/models/perm/access/actions_repo_permission_test.go b/models/perm/access/actions_repo_permission_test.go
index aee097cf93e..9fdd8ae7f43 100644
--- a/models/perm/access/actions_repo_permission_test.go
+++ b/models/perm/access/actions_repo_permission_test.go
@@ -95,6 +95,40 @@ func TestGetActionsUserRepoPermission(t *testing.T) {
 		assert.False(t, perm.CanRead(unit.TypeCode))
 	})
 
+	t.Run("CollaborativeOwner_ForkPR_Denied", func(t *testing.T) {
+		// Target repo15 trusts repo2's owner as a collaborative owner.
+		repo15ActionsUnit := repo15.MustGetUnit(ctx, unit.TypeActions)
+		repo15ActionsUnit.ActionsConfig().AddCollaborativeOwner(owner2.ID)
+		require.NoError(t, repo_model.UpdateRepoUnitConfig(ctx, repo15ActionsUnit))
+
+		// Owner cross-repo policy does not allow repo15, so the only branch that
+		// could grant access is the collaborative-owner one.
+		require.NoError(t, actions_model.SetOwnerActionsConfig(ctx, owner2.ID, actions_model.OwnerActionsConfig{}))
+
+		task53 := unittest.AssertExistsAndLoadBean(t, &actions_model.ActionTask{ID: 53})
+
+		// Non-fork task is legitimately granted code-read via collaborative owner.
+		task53.IsForkPullRequest = false
+		require.NoError(t, actions_model.UpdateTask(ctx, task53, "is_fork_pull_request"))
+		perm, err := GetActionsUserRepoPermission(ctx, repo15, actionsUser, task53.ID)
+		require.NoError(t, err)
+		assert.True(t, perm.CanRead(unit.TypeCode))
+
+		// Fork PR must NOT be able to read a third private repo through the
+		// collaborative-owner branch.
+		task53.IsForkPullRequest = true
+		require.NoError(t, actions_model.UpdateTask(ctx, task53, "is_fork_pull_request"))
+		perm, err = GetActionsUserRepoPermission(ctx, repo15, actionsUser, task53.ID)
+		require.NoError(t, err)
+		assert.False(t, perm.CanRead(unit.TypeCode))
+
+		// Restore state for subsequent subtests.
+		task53.IsForkPullRequest = false
+		require.NoError(t, actions_model.UpdateTask(ctx, task53, "is_fork_pull_request"))
+		repo15ActionsUnit.ActionsConfig().RemoveCollaborativeOwner(owner2.ID)
+		require.NoError(t, repo_model.UpdateRepoUnitConfig(ctx, repo15ActionsUnit))
+	})
+
 	t.Run("Inheritance_And_Clamping", func(t *testing.T) {
 		task53 := unittest.AssertExistsAndLoadBean(t, &actions_model.ActionTask{ID: 53})
 		task53.IsForkPullRequest = false
diff --git a/models/perm/access/repo_permission.go b/models/perm/access/repo_permission.go
index 2d53b0699f8..69d54b9020e 100644
--- a/models/perm/access/repo_permission.go
+++ b/models/perm/access/repo_permission.go
@@ -369,7 +369,9 @@ func GetActionsUserRepoPermission(ctx context.Context, repo *repo_model.Reposito
 	// 2. The Actions Bot user has been explicitly granted access and repository is private
 	// 3. The repository is public (handled by botPerm above)
 
-	if taskRepo.IsPrivate {
+	// Fork PRs are never allowed cross-repo access to other private repositories,
+	// matching the discriminator enforced by checkSameOwnerCrossRepoAccess above.
+	if taskRepo.IsPrivate && !task.IsForkPullRequest {
 		actionsUnit := repo.MustGetUnit(ctx, unit.TypeActions)
 		if actionsUnit.ActionsConfig().IsCollaborativeOwner(taskRepo.OwnerID) {
 			return maxPerm, nil