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.
- Upgrade
Upgrade
MISP email_otp() endpointto a version that resolves this vulnerability.Patch Missing Rate Limiting in Email OTP Verification Allows Brute-Force Authentication Bypass - Configuration
Apply the patch that updates the email_otp() endpoint to count failed OTP attempts against the user, reject further attempts once the configured threshold is reached, invalidate the active OTP when the attempt budget is exhausted, integrate MISP’s existing brute-force protection into the email OTP flow, and compare OTPs using hash_equals() after validating the submitted value is a string.
MISP email OTP verification flow OTP verification attempt handling = Failed attempts counted; further attempts rejected after configured threshold; active OTP invalidated when attempt budget exhausted; brute-force protection integrated; OTP comparison uses hash_equals() and validates submitted value is a string - Configuration
Ensure the change that prevents blocklisted users from requesting generation of a fresh OTP is applied in the email OTP flow.
MISP email OTP flow Blocklisted users OTP generation = Prevent blocklisted users from requesting fresh OTPs
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.