{"id":"CVE-2026-42553","aliases":["GHSA-j944-w549-3453"],"url":"https://o3.security/vulnerability/CVE-2026-42553","summary":"Cinny: Access token disclosure via invalidated emoji pack avatar URL in service worker","details":"### Impact\nA remote authenticated attacker who shares a room with a victim and has permissions to create room emotes (for example in a DM) can cause the victim's client to send their Matrix access token to an attacker-controlled server. This occurs when the victim opens the emoji or sticker picker for the room containing a malicious emote pack. \n\nThe root causes are: \n\n(1) an incorrect fallback in EmojiBoard that uses untrusted `pack.meta.avatar` (user-controlled) without converting/validating it as an MXC URL, allowing arbitrary HTTP(S) URLs to be used; and \n\n(2) the service worker attaching the user's Authorization bearer token to all outbound GET requests whose URL contains `/_matrix/client/v1/media/download` or `/_matrix/client/v1/media/thumbnail` without verifying the request host matches the configured homeserver origin. An attacker-controlled URL containing those path fragments and permissive CORS will receive the victim's Authorization header (access token). \n\nImpacted users: anybody using affected Cinny web app versions who opens the emoji/sticker picker in a room containing a malicious emote pack and who is logged in (authenticated).\n\n### Patches\nVersion with fixes: https://github.com/cinnyapp/cinny/releases/tag/v4.10.3","published":"2026-05-27T17:27:26.740Z","modified":"2026-08-12T03:51:17.390385133Z","cvss":null,"epss":{"score":0.00302,"percentile":0.22682,"asOf":"2026-08-14"},"cisaKev":null,"exploitsKnown":0,"affectedPackages":[{"ecosystem":"npm","name":"cinny","fixedVersion":"4.10.3"}],"fix":null,"references":[{"type":"WEB","url":"https://github.com/cinnyapp/cinny/releases/tag/v4.10.3"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/42xxx/CVE-2026-42553.json"},{"type":"ADVISORY","url":"https://github.com/cinnyapp/cinny/security/advisories/GHSA-j944-w549-3453"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-42553"},{"type":"PACKAGE","url":"https://github.com/cinnyapp/cinny"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-08-12T03:51:17.390385133Z"}}