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.
Cinny: Access token disclosure via invalidated emoji pack avatar URL in service worker
Problem type
Affected products
cinnyapp
< 4.10.3 - AFFECTED
References
https://github.com/cinnyapp/cinny/security/advisories/GHSA-j944-w549-3453
https://github.com/cinnyapp/cinny/releases/tag/v4.10.3
GitHub Security Advisories
GHSA-j944-w549-3453
Cinny vulnerable to access token disclosure via invalidated emoji pack avatar URL in service worker
https://github.com/advisories/GHSA-j944-w549-3453Impact
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.
The root causes are:
(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
(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).
Impacted 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).
Patches
Version with fixes: https://github.com/cinnyapp/cinny/releases/tag/v4.10.3
JSON source
https://cveawg.mitre.org/api/cve/CVE-2026-42553Click to expand
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