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

// 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:

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:

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):

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.

References

• GHSA-fj8v-hjwv-qm88
• go-gitea/gitea#38214
• go-gitea/gitea@1d43b73
• https://github.com/go-gitea/gitea/releases/tag/v1.27.0