GHSA-fj8v-hjwv-qm88

### Summary

`GetActionsUserRepoPermission` (`models/perm/access/repo_permission.go`) decides whether an Actions task token may access a target repo. Its cross-repo branches each enforce a fork-PR discriminator — **except the collaborative-owner branch**, which is missing the `!task.IsForkPullRequest` guard that its sibling has. As a result, when a private repo **B** lists owner **A** as a collaborative owner, an **attacker-controlled fork pull-request** workflow whose base repo is owned by A is granted code-read on B — i.e. the fork's YAML can clone a third private repository it has no rights to.

### Details

```go // models/perm/access/repo_permission.go (v1.26.2), in GetActionsUserRepoPermission if checkSameOwnerCrossRepoAccess(ctx, taskRepo, repo, task.IsForkPullRequest) { // passes isForkPR -> denies forks return maxPerm, nil } ... if taskRepo.IsPrivate { // <-- NO IsForkPullRequest check here actionsUnit := repo.MustGetUnit(ctx, unit.TypeActions) if actionsUnit.ActionsConfig().IsCollaborativeOwner(taskRepo.OwnerID) { return maxPerm, nil // grants code-read to target repo B } } ```

The sibling same-owner path correctly denies fork PRs:

```go func checkSameOwnerCrossRepoAccess(ctx, taskRepo, targetRepo, isForkPR bool) bool { if isForkPR { return false // Fork PRs are never allowed cross-repo access to other private repositories. } ... } ```

`taskRepo` = the repo whose workflow is running (the PR's base repo A); `repo` = the target being cloned (B). `IsCollaborativeOwner(taskRepo.OwnerID)` asks "does target B's Actions config trust A's owner for cross-repo read?" When B trusts ownerA, the branch returns `maxPerm` (code-read) **even when `task.IsForkPullRequest` is true** — i.e. when the executing YAML is the fork's, not A's.

Every sibling enforces the fork-PR discriminator; except for this branch: `checkSameOwnerCrossRepoAccess` denies forks; `ComputeTaskTokenPermissions` (`models/actions/token_permissions.go`) only clamps the token *ceiling* to read-only for fork/cross-repo (its own comment notes the access *decision* is in `GetActionsUserRepoPermission`, so it does not neutralize the gap — it just makes the leak read-only); secrets (`models/secret/secret.go`) and the approval gate (`services/actions/notifier_helper.go`) both correctly key on `IsForkPullRequest`.

**Reachability** — the runner clones target repo B over git-HTTP with the task token: `routers/web/repo/githttp.go` → `GetDoerRepoPermission(ctx, repoB, ActionsUser)` → `GetActionsUserRepoPermission(ctx, repoB, actionsUser, taskID)` with `IsForkPullRequest == true` → collaborative-owner branch returns code-read → `p.CanAccess(Read, code)` passes → private clone of B succeeds. (`CheckRepoScopedToken` in githttp is a no-op for the Actions token.)

### PoC

Setup: private base repo A (`usera/repoA`), private third repo B (`userb/repoB`) with a planted `SECRET.txt`, B's Actions config trusting `usera` as a collaborative owner, and a genuine running fork-PR task token (`token_hash` computed with Gitea's own `HashToken`) presented as HTTP Basic. Requesting `GET /userb/repoB.git/info/refs?service=git-upload-pack`:

| Condition (same fork-PR token) | HTTP | Meaning | |---|---|---| | anonymous (no token) | 401 | auth required | | token, A **public**, B trusts A | 404 | branch gated on `taskRepo.IsPrivate` ⇒ A public skips it | | token, A private, B has **no** collab-owner config | 404 | no trust ⇒ denied | | **token, A private, B trusts A (collab-owner)** | **200** | **`git clone` of private B succeeds** | | config removed / restored | 404 / 200 | deterministic |

In the 200 case, `git clone` of private repo B succeeded and yielded its `SECRET.txt` — the full source of a third private repo the fork-PR author has no rights to.

### Impact

Read-only confidentiality breach: discloses the full source of a *third* private repository (B) to an untrusted external fork-PR author. Read-only, not write/RCE.

Preconditions (honest): 1. B is deliberately configured with a collaborative owner — but that is exactly the feature's intended use, so realistic for any deployment using it. 2. The fork PR's base repo A is itself private (the branch is gated on `taskRepo.IsPrivate`). Forking a private A already requires read on A, so this is a normal internal-contributor situation, not a weakening — the escalation is "read A (granted) → read a *different* private repo B (never granted)." 3. The fork-PR workflow must actually run — most realistically via an attacker who had one earlier PR approved (the "approved before" path in `ifNeedApproval`), after which fork PRs auto-run.

### Suggested remediation

Add the same fork-PR guard the sibling path has (one line):

```go if taskRepo.IsPrivate && !task.IsForkPullRequest { actionsUnit := repo.MustGetUnit(ctx, unit.TypeActions) if actionsUnit.ActionsConfig().IsCollaborativeOwner(taskRepo.OwnerID) { return maxPerm, nil } } ```

This flips `Vuln_ForkPR_LeaksThirdPrivateRepo` to PASS, keeps `Control_NonFork_Allowed` PASS (legitimate collaborative-owner sharing still works), and leaves the existing `TestGetActionsUserRepoPermission` suite all green.