Medium, CVSS 4.5RubyGemsPublished
bcrypt-ruby has an Integer Overflow that Causes Zero Key-Strengthening Iterations at Cost=31 on JRuby
CVE-2026-33306 · GHSA-f27w-vcwj-c954
- Affected
- RubyGems bcrypt <= 3.1.21
- Fixed in
- 3.1.22· Vendor fix available
- Issue
- 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…
Recommended action: Upgrade to 3.1.22 or later, then redeploy and verify.A vendor fix does not resolve your finding until you verify it in your own systems.