CVE-2026-33306: 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 imp...

Description

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.

Metrics

CVSS 3.1

7.5/10

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N

CVSS 4.0

4.5/10

CVSS:4.0/AV:L/AC:H/AT:P/PR:N/UI:N/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N/E:U/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X

EPSS Probability

0.23%

13.5th percentile

Probability of exploitation in the next 30 days. Learn more

Weakness Enumeration

CWE-190

Affected Software

Vendor	Product	Versions
Bcrypt-Ruby Project	Bcrypt-Ruby	< 3.1.22

References

https://github.com/bcrypt-ruby/bcrypt-ruby/commit/831ce64cb0a9502130fa93a28bfd9527a5fa45c4Patch
https://github.com/bcrypt-ruby/bcrypt-ruby/releases/tag/v3.1.22Product, Release Notes
https://github.com/bcrypt-ruby/bcrypt-ruby/security/advisories/GHSA-f27w-vcwj-c954Mitigation, Vendor Advisory

Timeline

Published: Mar 24, 2026
Last Modified: Jun 17, 2026
Status: Analyzed

Frequently Asked Questions

What is CVE-2026-33306?

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.

How severe is CVE-2026-33306?

CVE-2026-33306 has a CVSS score of 4.5/10 (MEDIUM severity). The EPSS model estimates a 0.23% probability of exploitation in the next 30 days.

How do I fix CVE-2026-33306?

Check the vendor references and advisories linked above for patched versions and mitigation guidance. You can also run a Strix scan to test if your systems are affected.