# 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.

- **Severity:** Medium 4.5 (`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`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 24 Mar 2026
- **Updated:** 24 Mar 2026
- **Also known as:** DEBIAN-CVE-2026-33306, GHSA-f27w-vcwj-c954, OESA-2026-1723, UBUNTU-CVE-2026-33306

## 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 (Integer Overflow or Wraparound)

## Which versions are affected by CVE-2026-33306?

2 open source packages ship an affected version, including bcrypt. It affects 1 product.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `bcrypt` | RubyGems | All versions | 3.1.22 |
| `bcrypt` | RubyGems | All versions | 3.1.22 |

Products: 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.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-33306 and fix it if it is affected.

What it is: 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.
Exploitation: There is no known exploitation of CVE-2026-33306 in the wild and no public exploit code.

Affected packages and fixed versions:
- bcrypt (RubyGems): affected all versions; fixed in 3.1.22
- bcrypt (RubyGems): affected all versions; fixed in 3.1.22

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-33306/ (Markdown: https://insights.sbomify.com/CVE-2026-33306/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-33306
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-33306
- OSV: https://osv.dev/vulnerability/CVE-2026-33306

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-33306/
Summaries are generated and may contain mistakes; the official description above is the record.