CVE-2026-42553 Cinny: Access token disclosure via invalidated emoji pack avatar URL in service worker

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...

CVE-2026-42553

Cinny (Matrix client) before version 4.10.3 is affected by a token-disclosure vulnerability in two parts: (1) EmojiBoard fallback uses an untrusted pack.meta.avatar as a MXC URL, enabling an attacker-controlled HTTP(S) URL in a malicious emote pack; (2) the service worker attaches the user’s Auth...

CVE-2026-42553 Cinny: Access token disclosure via invalidated emoji pack avatar URL in service worker

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...