Summary 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. Severity - 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 / patched versions - Affected: through 1.26.3 (latest at time of report). - Patched: none yet. Details "DeleteIssueLabel" resolves the label with a global loader and never checks its scope: // routers/api/v1/repo/issue_label.go (DeleteIssueLabel) label, 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. Proof of Concept 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. bob DELETE /api/v1/repos/bob/pub/issues/1/labels/1 -> HTTP 204 (alice's PRIVATE label id exists) bob 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. Impact 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. Remediation 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: // routers/api/v1/repo/issue_label.go — in DeleteIssueLabel, after loading the label if label.RepoID != ctx.Repo.Repository.ID && label.OrgID != ctx.Repo.Repository.OwnerID { ctx.APIErrorNotFound() return } (Equivalently, resolve via "GetLabelInRepoByID" and, for org repositories, also accept the repo owner's org labels — mirroring the scoping in "GetLabel"/"EditLabel"/"DeleteLabel".)