CVE-2026-42553 Cinny: Access token disclosure via invalidated emoji pack avatar URL in service worker
An attacker can steal a victim's Matrix access token by tricking them into opening a malicious emoji pack, leading to account takeover.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-42553?
CVE-2026-42553 is a high-severity vulnerability in cinnyapp cinny, published on 27 May 2026. Any signed-in user can exploit it over the network, with no action from the victim.
Official description
Cinny is a Matrix client. Prior to 4.10.3, A 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. This is caused by 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. Also, 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). This vulnerability is fixed in 4.10.3.
Weakness: CWE-20
Which versions are affected by CVE-2026-42553?
2 open source packages ship an affected version, including cinny.
| Package | Affected | Fixed in |
|---|---|---|
cinny npm | All versions | 4.10.3 |
cinny npm | All versions | 4.10.3 |
How do I fix CVE-2026-42553?
Upgrade cinny to 4.10.3 or later.
Is CVE-2026-42553 being exploited?
There is no known exploitation of CVE-2026-42553 in the wild and no public exploit code.
Sources
Also known as GHSA-j944-w549-3453.