Executive brief
Cinny is a web-based chat client for the Matrix communication protocol. A vulnerability in how the app handles custom emojis allows a malicious user to steal another person's login token. If a victim opens the emoji picker in a chat room containing a specially crafted emoji pack, their secret access token is automatically sent to the attacker's server, potentially allowing the attacker to take over their account.
Technical details
Cinny is vulnerable to an information disclosure flaw involving Matrix access tokens. The vulnerability stems from two root causes: first, the EmojiBoard component fails to validate 'pack.meta.avatar' as a proper MXC URL, allowing attackers to inject arbitrary HTTP(S) URLs. Second, the application's service worker automatically attaches the user's Authorization bearer token to any outbound GET request containing specific Matrix media path fragments (download/thumbnail) without verifying if the destination host matches the trusted homeserver. An authenticated attacker with permissions to create room emotes can exploit this by tricking a victim into opening the emoji picker, causing the client to leak the access token to an attacker-controlled server. This is fixed in version 4.10.3.
Affected products
- cinnyapp cinny < 4.10.3
Timeline
- 2026-05-07: disclosed
- 2026-05-07: advisory
- 2026-05-07: patched: Fixed in version 4.10.3