The flaw in UseMemos version 0.27.1 resides in the SignIn handler for SSO authentication. The code matches the identifier supplied by an external IdP against a local database record but does not verify that the IdP’s stable subject claim is bound to the identifier. An attacker who can control the identifier value can therefore complete the SSO flow as any existing user, gaining full access to that user’s account and all associated data. The vulnerability is a classic authentication bypass (CWE‑287).

UseMemos 0.27.1 is the only version listed as affected. No other versions or products are mentioned in the report. The vulnerability is specific to the SSO authentication branch of the SignIn endpoint.

The flaw is reachable over the network through the public SignIn API, allowing a remote attacker to exercise the vulnerability without local system access. The CVSS score of 9.8 marks it as critical, while the EPSS score of less than 1 percent indicates the current probability of exploitation is low. It is not included in the CISA Known Exploited Vulnerabilities catalog. Nonetheless, any successful exploitation would result in full account takeover and potential data compromise across the entire system.