ruby-advisory-db/gems/bcrypt/CVE-2026-33306.yml at master · rubysec/ruby-advisory-db · GitHub

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---
gem: bcrypt
cve: 2026-33306
ghsa: f27w-vcwj-c954
url: https://github.com/advisories/GHSA-f27w-vcwj-c954
title: bcrypt-ruby has an Integer Overflow that Causes Zero
  Key-Strengthening Iterations at Cost=31 on JRuby
date: 2026-03-19
description: |
  ### 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.
patched_versions:
  - ">= 3.1.22"
related:
  url:
    - https://github.com/bcrypt-ruby/bcrypt-ruby/security/advisories/GHSA-f27w-vcwj-c954
    - https://github.com/bcrypt-ruby/bcrypt-ruby/commit/5faa2748331d3edc661c127ef2fbb3afcb6b02a4
    - https://github.com/advisories/GHSA-f27w-vcwj-c954