The Logto core library reflected SAML RelayState, SAMLResponse, and actionUrl directly into an auto‑submit HTML form without escaping, creating a reflected cross‑site scripting flaw. Attacker‑supplied data is injected into the user’s browser, allowing script execution in the context of the Logto tenant origin after the user completes SAML authentication. The weakness is a classic instance of CWE‑79.

The vulnerability affects Logto installations (logto‑io:logto) with any version released before 1.41.0. All legacy deployments that have not applied the 1.41.0 migration are susceptible.

The CVSS base score is 6.1, indicating medium severity. The EPSS score is < 1%, suggesting a very low exploitation probability, and the flaw is not listed in CISA KEV. Exploitation requires the attacker to craft a SAML authentication request containing a malicious RelayState, so the attack vector is the SAML login flow exposed by /api/saml/:id/authn. Once the victim completes authentication, the injected script will execute with the user’s privileges on the Logto tenant origin.