CVE-2026-42553Cinny: Access token disclosure via invalidated emoji pack avatar URL in service worker
- 7.1HighCVSS base score
- 0.30%Exploitation in 30 daysEPSS · top 77% of all CVEs
- NoKnown exploitednot in CISA's KEV catalog
- NonePublic exploitsnone known yet
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.
Timeline
ID reserved
By GitHub_M
- 29 days later
· 29 days later
Published
Changes after publication (a new score, a public exploit, EPSS crossing 1, 10 or 50 %, leaving KEV) are kept since November 2026; earlier ones are not known.
Scores
| Scored by | CVSS | Score | Vector |
|---|---|---|---|
| GitHub_MCNA (assigner) | 4.0 | High7.1 | 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 |
| NVDNVD · Deferred | 4.0 | High7.1 | 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/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X |
- EPSS (exploitation in 30 days)
- 0.30% · top 77% of all CVEs (14 Nov 2026)
- CISA SSVC
- Exploitation: none · Automatable: no · Technical impact: partial
The list uses the assigner's score when there is one, then CISA's, then NVD's: High 7.1.
Affected products
Affected packages
From reviewed advisories of the GitHub Advisory Database.
References
References2
Sources
The same CVE in the primary databases and aggregators, to check against.
- CVE.org record (opens in a new tab)
- CVE List JSON (GitHub) (opens in a new tab)
- NVD (opens in a new tab)
- FIRST EPSS (opens in a new tab)
- EUVD EUVD-2026-32612 (opens in a new tab)
- OSV.dev (opens in a new tab)
- GitHub Advisory GHSA-j944-w549-3453 (opens in a new tab)
- CIRCL Vulnerability-Lookup (opens in a new tab)
Badge for a README or a ticket
[](https://cveledger.app/cve/CVE-2026-42553)Record last updated .