Cinny, a Matrix client, contains a flaw that enables a remote authenticated attacker to cause a victim’s client to transmit the victim’s Matrix access token to the attacker’s server. The flaw stems from an unsafe fallback in the emoji picker that accepts an untrusted avatar URL without validating it as an MXC URI, and a service worker that attaches the bearer token to any GET request that includes a media‑download or thumbnail path without ensuring the request host matches the client’s homeserver. If an attacker embeds a malicious emoji pack in a room shared with the victim, opening the picker triggers the victim’s browser to send the token to the attacker. This results in the exposure of the victim’s credentials, allowing the attacker to impersonate the victim on the Matrix network.

Cinny (Matrix client) versions prior to 4.10.3 are impacted. The vulnerability exists in the user‑facing application bundled with those releases and is resolved in v4.10.3 and later.

The CVSS score of 7.1 indicates a moderate to high risk, but the EPSS score is currently unavailable and the vulnerability is not listed in CISA’s KEV catalog. Exploitation requires an attacker to be authenticated into the same Matrix room as the victim and to be permitted to create or upload emoji, which is a realistic scenario in many direct‑message or public rooms. Once the attacker supplies a crafted URL, the victim’s browser will automatically send the access token to the attacker’s host, granting the attacker full client‑level access to the victim’s account without additional effort.