MISP contains a race condition in the email-based one-time password (OTP) login flow. When two HTTP requests carrying the same valid OTP are submitted concurrently, both can successfully authenticate and establish a session. The root cause is that the OTP value is read from the shared store, validated, and then deleted in separate non-atomic steps, allowing a second in-flight request to read the same value before the first request's deletion takes effect. Preconditions: - The target MISP instance has email OTP login enabled. - The attacker possesses a valid, unexpired OTP (e.g., via email interception or social engineering). - The attacker can issue two HTTP POST requests in close temporal proximity. Impact: - The one-time-use guarantee of the OTP is violated; a single code can yield two authenticated sessions. - This weakens the authentication control and may facilitate unauthorized access if the OTP is shared or intercepted. Affected versions: <2.5.48