Bug 2450565 (CVE-2026-33306)

Summary: CVE-2026-33306 github.com/bcrypt-ruby/bcrypt-ruby: bcrypt-ruby (JRuby): Weakened password hashing due to integer overflow
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security DevOps Team <prodsec-dev>
Status: NEW --- QA Contact:
Severity: medium Docs Contact:
Priority: medium    
Version: unspecifiedCC: anthomas, ehelms, ggainey, juwatts, mhulan, nmoumoul, osousa, pcreech, rchan, smallamp, tmalecek
Target Milestone: ---Keywords: Security
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Hardware: All   
OS: Linux   
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A flaw was found in bcrypt-ruby, a Ruby binding for the OpenBSD bcrypt() password hashing algorithm, specifically in its JRuby implementation. When the cost parameter is set to 31, an integer overflow occurs, causing the key-strengthening loop to execute zero iterations. This significantly weakens the generated password hashes, effectively reducing the computational effort required to crack them. An attacker could leverage this to more easily compromise user accounts protected by affected applications.
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Description OSIDB Bzimport 2026-03-24 01:01:50 UTC 
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.