CVE-2026-33306
Impact
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.
Patches
This problem has been fixed in version 3.1.22
Workarounds
Set the cost to something less than 31.
Package Versions Affected
Automatically patch vulnerabilities without upgrading
CVSS Version
Related Resources
References
https://github.com/bcrypt-ruby/bcrypt-ruby/security/advisories/GHSA-f27w-vcwj-c954, https://nvd.nist.gov/vuln/detail/CVE-2026-33306, https://github.com/bcrypt-ruby/bcrypt-ruby/commit/831ce64cb0a9502130fa93a28bfd9527a5fa45c4, https://github.com/bcrypt-ruby/bcrypt-ruby, https://github.com/bcrypt-ruby/bcrypt-ruby/releases/tag/v3.1.22, https://github.com/rubysec/ruby-advisory-db/blob/master/gems/bcrypt/CVE-2026-33306.yml
Basic Information
