CVE-2026-85237: Missing Rate Limiting in Email OTP Verification Allows Brute-Force Authentication Bypass
A vulnerability in MISP's email-based one-time password (OTP) authentication flow allowed an attacker to perform an unrestricted number of OTP verification attempts.
The emailotp() endpoint did not apply brute-force protection when validating submitted OTP values. An attacker who had reached the OTP verification stage, for example after successfully providing a user's primary authentication credentials, could repeatedly submit candidate OTP values while the same OTP remained valid. This significantly increased the feasibility of guessing the OTP and bypassing the additional authentication factor, potentially resulting in unauthorized access to the affected user's account.
The issue was exacerbated by the fact that the OTP is associated with the user rather than with an individual pending login session, allowing multiple concurrent sessions to attempt guesses against the same valid OTP.
The patch integrates the existing MISP brute-force protection mechanism into the email OTP flow. Failed OTP attempts are now counted against the user, further attempts are rejected once the configured threshold is reached, and the active OTP is invalidated when the attempt budget is exhausted. Blocklisted users are also prevented from requesting the generation of a fresh OTP. In addition, OTP comparison now uses hashequals() and validates that the submitted value is a string.
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Configuration
Update MISP email OTP flow so that failed OTP attempts are counted against the user and additional OTP verification attempts are rejected once the configured threshold is reached; when the attempt budget is exhausted, invalidate the currently active OTP.
MISP email OTP authentication (email_otp() endpoint) failed OTP attempt counting and rejection threshold = Count failed OTP attempts; reject further attempts once configured threshold is reached; invalidate active OTP when attempt budget is exhausted - Configuration
Modify OTP comparison to use PHP hash_equals() and validate that the submitted OTP value is a string before comparing.
MISP email OTP authentication (OTP verification) OTP verification function = Use hash_equals() and require submitted value to be a string - Configuration
Enable brute-force protection in the email OTP verification endpoint by integrating MISP's existing brute-force protection mechanism into email_otp() validation.
MISP email OTP authentication (email_otp() endpoint) brute-force protection in OTP verification = Integrate existing MISP brute-force protection mechanism into email OTP flow
Event History
Frequently Asked Questions
What must an attacker have before they can exploit this issue?
The attacker must first reach the email OTP verification stage, such as by successfully providing the target user's primary authentication credentials. They can then repeatedly submit candidate OTP values while the active OTP remains valid.
Does the issue affect only one login attempt at a time?
No. The OTP is associated with the user rather than an individual pending login session, so multiple concurrent sessions can submit guesses against the same valid OTP.
What changes after applying the patch?
Failed email OTP attempts are counted against the user through MISP's existing brute-force protection mechanism. Once the configured threshold is reached, further attempts are rejected and the active OTP is invalidated.