CVE-2026-33306
HighCVSS 7.5Summary
bcrypt-ruby (prior to 3.1.22) has an integer overflow in the Java BCrypt implementation for JRuby that causes zero iterations in the strengthening loop when cost=31. This collapses bcrypt to effectively constant-time computation, and the resulting hash looks valid and verifies correctly. The issue occurs only when cost=31 is used or when verifying a $2a$31$ hash.
Risk Assessment
If an application uses cost 31, passwords are practically unprotected against brute-force attacks, potentially leading to rapid password cracking. The flaw is invisible to the application because the hash appears valid.
Recommendation
Update bcrypt-ruby to version 3.1.22 or later. As a workaround, set the cost to a value less than 31.
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Original NVD description (English source)
bcrypt-ruby is a Ruby binding for the OpenBSD bcrypt() password hashing algorithm. Prior to version 3.1.22, an integer overflow in the Java BCrypt implementation for JRuby can cause zero iterations in the strengthening loop. Impacted applications must be setting the cost to 31 to see this happen. The JRuby implementation of bcrypt-ruby (`BCrypt.java`) computes the key-strengthening round count as a signed 32-bit integer. When `cost=31` (the maximum allowed by the gem), signed integer overflow causes the round count to become negative, and the strengthening loop executes **zero iterations**. This collapses bcrypt from 2^31 rounds of exponential key-strengthening to effectively constant-time computation — only the initial EksBlowfish key setup and final 64x encryption phase remain. The resulting hash looks valid (`$2a$31$...`) and verifies correctly via `checkpw`, making the weakness invisible to the application. This issue is triggered only when cost=31 is used or when verifying a `$2a$31$` hash. This problem has been fixed in version 3.1.22. As a workaround, set the cost to something less than 31.
Vulnerability data from NVD (NIST) · CISA KEV · EPSS