CVE-2026-42553
Cinny vulnerable to access token disclosure via invalidated emoji pack avatar URL in service worker
Description
### Impact 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
How to fix CVE-2026-42553
To remediate CVE-2026-42553, upgrade the affected package to a fixed version below.
- —upgrade to 4.10.3 or later
Is CVE-2026-42553 being exploited?
Low — EPSS is 0.2%, meaning exploitation activity has not been observed at scale.
Affected packages (1)
- from 0, < 4.10.3
CVSS scores
| Source | Version | Severity | Vector |
|---|---|---|---|
| osv | CVSS 4.0 | — | CVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:N/VC:H/VI:N/VA:N/SC:N/SI:N/SA:N |