CVE-2026-42553
Published: 27 May 2026
Summary
CVE-2026-42553 is a high-severity Improper Input Validation (CWE-20) vulnerability. Its CVSS base score is 7.1 (High).
Operationally, exploitation aligns with the MITRE ATT&CK technique Steal Application Access Token (T1528); ranked at the 21.7th percentile by exploit likelihood (below the median); it is not currently listed in the CISA KEV catalog.
EU & UK References
- 🇪🇺 ENISA EUVD: EUVD-2026-32612
Vulnerability details
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…
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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.
- CWE(s)
Related Threats
MITRE ATT&CK Enterprise TechniquesAI
Why these techniques?
Directly enables theft of Matrix application access token via malicious emote pack URL and unconditional service-worker token attachment.
Affected Assets
Mitigating Controls
Likely Mitigating Controls AI
Per-CVE control mapping for this CVE has not run yet; the list below is derived from the weakness types (CWEs) cited in the NVD entry.
Security testing and developer training directly verify and enforce proper input validation, reducing exploitability of injection and malformed-data weaknesses.
Security testing and evaluation at multiple SDLC stages directly detects missing or flawed input validation, with the required remediation process ensuring fixes are applied.
Directly implements checks on information inputs to reject invalid data before processing.
Spam protection mechanisms perform filtering and detection on inbound/outbound messages, directly compensating for missing or weak input validation of unsolicited content.