GHSA-F27W-VCWJ-C954
Vulnerability from github – Published: 2026-03-19 17:54 – Updated: 2026-03-25 21:36Impact
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 31.
{
"affected": [
{
"database_specific": {
"last_known_affected_version_range": "\u003c= 3.1.21"
},
"package": {
"ecosystem": "RubyGems",
"name": "bcrypt"
},
"ranges": [
{
"events": [
{
"introduced": "0"
},
{
"fixed": "3.1.22"
}
],
"type": "ECOSYSTEM"
}
]
}
],
"aliases": [
"CVE-2026-33306"
],
"database_specific": {
"cwe_ids": [
"CWE-190"
],
"github_reviewed": true,
"github_reviewed_at": "2026-03-19T17:54:17Z",
"nvd_published_at": "2026-03-24T01:17:02Z",
"severity": "MODERATE"
},
"details": "### Impact\n\nAn 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.\n\nThe 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 \u2014 only the initial EksBlowfish key setup and final 64x encryption phase remain.\n\nThe 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.\n\n### Patches\n\nThis problem has been fixed in version 3.1.22\n\n### Workarounds\n\nSet the cost to something less than 31.",
"id": "GHSA-f27w-vcwj-c954",
"modified": "2026-03-25T21:36:33Z",
"published": "2026-03-19T17:54:17Z",
"references": [
{
"type": "WEB",
"url": "https://github.com/bcrypt-ruby/bcrypt-ruby/security/advisories/GHSA-f27w-vcwj-c954"
},
{
"type": "ADVISORY",
"url": "https://nvd.nist.gov/vuln/detail/CVE-2026-33306"
},
{
"type": "WEB",
"url": "https://github.com/bcrypt-ruby/bcrypt-ruby/commit/831ce64cb0a9502130fa93a28bfd9527a5fa45c4"
},
{
"type": "PACKAGE",
"url": "https://github.com/bcrypt-ruby/bcrypt-ruby"
},
{
"type": "WEB",
"url": "https://github.com/bcrypt-ruby/bcrypt-ruby/releases/tag/v3.1.22"
},
{
"type": "WEB",
"url": "https://github.com/rubysec/ruby-advisory-db/blob/master/gems/bcrypt/CVE-2026-33306.yml"
}
],
"schema_version": "1.4.0",
"severity": [
{
"score": "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",
"type": "CVSS_V4"
}
],
"summary": "bcrypt-ruby has an Integer Overflow that Causes Zero Key-Strengthening Iterations at Cost=31 on JRuby"
}
Sightings
| Author | Source | Type | Date |
|---|
Nomenclature
- Seen: The vulnerability was mentioned, discussed, or observed by the user.
- Confirmed: The vulnerability has been validated from an analyst's perspective.
- Published Proof of Concept: A public proof of concept is available for this vulnerability.
- Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
- Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
- Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
- Not confirmed: The user expressed doubt about the validity of the vulnerability.
- Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.