This flaw allows a malicious remote server to compel a local Matrix homeserver to sign arbitrary events when a user interacts with rooms. The server blindly trusts data from the remote server, missing validation of critical fields such as origin, initiation timestamps, and membership content. As a result, an attacker can forge signed events that appear to originate from legitimate users, potentially enabling unauthorized actions such as membership manipulation or replay attacks. The weakness is a classic Confused‑Deputy scenario (CWE‑441).

Vulnerable servers include all Conduit‑derived homeservers written in Rust, such as continuwuity, Conduit, Grapevine, and Tuwunel. Affected versions are prior to continuwuity 0.5.1, Conduit 0.10.11, Grapevine 0aae932b, and Tuwunel 1.4.9.

The severity score of 9.3 indicates critical impact for affected deployments. Exploitability probability, as measured by EPSS, is less than 1‑percent, suggesting low likelihood of widespread attacks but not zero; attackers would need to run a malicious server and convince victims to join, leave, or knock on rooms. The vulnerability is not listed in CISA’s Known Exploited Vulnerabilities catalog, but the high CVSS and potential loss of integrity make proactive mitigation essential.