Gitea: Cross-repository label-ID enumeration oracle via unscoped DeleteIssueLabel API

The API endpoint DELETE /repos/{owner}/{repo}/issues/{index}/labels/{id} loads the label by ID with a global,
unscoped lookup and never verifies the label belongs to the URL's repository (or its owning organization). Because
the response status differs by whether the label ID exists anywhere on the instance (204) versus not (422), an
authenticated user can use the endpoint as a cross-repository label-ID existence / enumeration oracle, including
for labels in repositories and organizations they cannot access.

• The leaked information is minimal (existence/count of label IDs instance-wide); no label name, color, or owning
  repository is disclosed, and no cross-repository write occurs.

• Affected: through 1.26.3 (latest at time of report).
• Patched: none yet.

DeleteIssueLabel resolves the label with a global loader and never checks its scope:

 1// routers/api/v1/repo/issue_label.go  (DeleteIssueLabel)
 2label, err := issues_model.GetLabelByID(ctx, ctx.PathParamInt64("id"))   // global, unscoped

GetLabelByID (models/issues/label.go) is e.ID(labelID).Get(l) with no repo_id / org_id filter. The
handler never verifies label.RepoID == ctx.Repo.Repository.ID (nor the org-label equivalent), and the downstream
issue_service.RemoveLabel (services/issue/label.go) only re-checks the doer's write permission on the issue's
own repository — never that the label belongs to it.

Every sibling label handler is correctly scoped — GetLabel / EditLabel / DeleteLabel (repo and org) use
GetLabelInRepoByID / GetLabelInOrgByID and return 404 for a foreign ID. DeleteIssueLabel is the only outlier.

Why it is only an oracle: deleteIssueLabel (models/issues/issue_label.go) deletes the issue_label row keyed
by (issue.ID, label.ID). For a foreign label, no such row exists → the function returns early before any
mutation or comment creation. So there is no cross-repo write and no leak of the label's name. But the HTTP status
differs:

• label ID exists anywhere on the instance (incl. private repos/orgs) → 204 No Content
• label ID does not exist → 422 (ErrLabelNotExist)

Label IDs are sequential auto-increment, so this enumerates the instance-wide label population and probes existence
of specific IDs across tenant boundaries.

Verified end-to-end on a build of the v1.26.3 tag.

• alice (private repo alice/secret) creates a label → internal id 1.
• Attacker bob (separate user; public repo bob/pub with issue #1; no access to alice/secret) holds a token
  with write:issue on his own repo.

 1bob DELETE /api/v1/repos/bob/pub/issues/1/labels/1          -> HTTP 204   (alice's PRIVATE label id exists)
 2bob DELETE /api/v1/repos/bob/pub/issues/1/labels/99999999   -> HTTP 422   (no such label)

Differing only by the label ID: 204 vs 422 distinguishes "label ID exists" from "does not exist." bob has zero rights to alice/secret but can still learn label id 1 exists. (Alice's label is untouched — no write.)

Reproduction steps:

1. Create two users alice, bob. As alice, create a private repo and a label on it (note the label id from
   the API response).
2. As bob, create any repo with an issue, and a token with write:issue.
3. curl -u bob:$T -X DELETE https://<gitea>/api/v1/repos/bob/pub/issues/1/labels/<alice_label_id> → 204.
4. curl -u bob:$T -X DELETE https://<gitea>/api/v1/repos/bob/pub/issues/1/labels/99999999 → 422.
5. The differing status across an ID bob cannot otherwise see is the oracle.

Cross-tenant authorization-key bypass producing a label-ID existence/enumeration oracle: an authenticated user can
determine whether arbitrary label IDs (including in private repositories and organizations they cannot access) exist,
and enumerate the instance-wide label population.

Scope the loader like every sibling handler, returning 404 for a label that is not in the URL repository (or its
owning organization), so the status no longer distinguishes existence:

 1// routers/api/v1/repo/issue_label.go — in DeleteIssueLabel, after loading the label
 2if label.RepoID != ctx.Repo.Repository.ID && label.OrgID != ctx.Repo.Repository.OwnerID {
 3    ctx.APIErrorNotFound()
 4    return
 5}

(Equivalently, resolve via GetLabelInRepoByID and, for org repositories, also accept the repo owner's org labels —
mirroring the scoping in GetLabel/EditLabel/DeleteLabel.)