TOCTOU Race Condition in fastschema Allows OTP Bypass

Executive Summary

This is a time-of-check/time-of-use (TOCTOU) race condition in fastschema v0.15.1. It allows unauthenticated remote attackers to bypass the one-time password (OTP) attempt limit during account recovery. The vulnerability exists because the verifyOTPSession function in pkg/auth/local.go reads and increments the attempt counter in separate non-atomic operations, enabling concurrent requests to exploit the same attempt count before it is updated.

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Detection Guidance

Detecting this TOCTOU vulnerability requires monitoring for concurrent OTP verification attempts that bypass attempt limits. Check application logs for multiple rapid OTP verification requests from the same IP or user account. Look for patterns where the attempt counter does not increment between requests despite reaching the limit.

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Impact Analysis

An attacker could exploit this to bypass OTP limits and perform brute-force attacks on 6-digit OTP codes, potentially gaining unauthorized access to user accounts. This undermines account security and could lead to data breaches or unauthorized actions on behalf of legitimate users.

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Compliance Impact

This vulnerability could lead to unauthorized access to personal or sensitive data, violating confidentiality requirements in GDPR and HIPAA. Organizations using affected versions of fastschema may face compliance violations, potential fines, and reputational damage due to insufficient protection against brute-force attacks.

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Mitigation Strategies

Upgrade fastschema to a patched version beyond v0.15.1. Implement atomic operations for OTP attempt counters. Add rate limiting at the network level for OTP endpoints. Disable account recovery via OTP if not essential. Monitor for unusual OTP verification spikes.

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