RabbitMQ is a messaging and streaming broker. Prior to versions 3.13.15, 4.0.20, 4.1.11, and 4.2.6, The management UI uses EJS 1.0 in which <%= ... %> does NOT HTML-escape. connection.ejs:135 renders <%= connection.ssl details.peer cert subject %> (and peer cert issuer) directly into the page. The same pattern appears in streamConnection.ejs:102,106,110. The values come from rabbit ssl:peer cert subject/1 which formats the DN as a string without HTML escaping. The verifier corrected the original researcher's claim: this is reachable only when the listener is configured with verify peer (so the certificate must be signed by a CA in the broker's trust store, not arbitrary self-signed); however, in deployments using mTLS for client authentication, any user who can request a certificate from the organisational CA controls the Subject CN. An attacker who can obtain a TLS client certificate signed by a CA the broker trusts (with verify peer enabled) can embed JavaScript in the certificate's Subject DN. When any administrator views that connection in the management UI, the script executes in the admin's browser session, allowing full account takeover (create users, export definitions, etc.). The management UI's CSP includes 'unsafe-inline', so inline script execution is not blocked. Preconditions include TLS listener configured with ssl options.verify = verify peer Attacker can obtain a CA-signed client certificate with attacker-chosen Subject (e.g. self-service corporate PKI, or rabbitmq trust store plugin in use) Administrator views the connection detail page. This issue is fixed in versions 3.13.15, 4.0.20, 4.1.11, and 4.2.6.