CVE-2026-33306 bcrypt-ruby has an Integer Overflow that Causes Zero Key-Strengthening Iterations at Cost=31 on JRuby
Integer overflow in JRuby bcrypt implementation causes zero key-strengthening iterations at cost 31, weakening password security.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Local or same network only
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-33306?
CVE-2026-33306 is a medium-severity vulnerability in bcrypt-ruby, published on 24 Mar 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Official 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.
Weakness: CWE-190
Which versions are affected by CVE-2026-33306?
2 open source packages ship an affected version, including bcrypt. It affects 1 product.
| Package | Affected | Fixed in |
|---|---|---|
bcrypt RubyGems | All versions | 3.1.22 |
bcrypt RubyGems | All versions | 3.1.22 |
Products include Bcrypt-Ruby Project Bcrypt-Ruby.
How do I fix CVE-2026-33306?
Upgrade bcrypt to 3.1.22 or later.
Is CVE-2026-33306 being exploited?
There is no known exploitation of CVE-2026-33306 in the wild and no public exploit code.
Sources
Also known as DEBIAN-CVE-2026-33306, GHSA-f27w-vcwj-c954, OESA-2026-1723, UBUNTU-CVE-2026-33306.