CVE-2026-33306
bcrypt-ruby has an Integer Overflow that Causes Zero Key-Strengthening Iterations at Cost=31 on JRuby
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.
INFO
Published Date :
March 24, 2026, 1:17 a.m.
Last Modified :
March 24, 2026, 1:17 a.m.
Remotely Exploit :
No
Source :
[email protected]
Affected Products
The following products are affected by CVE-2026-33306
vulnerability.
Even if cvefeed.io is aware of the exact versions of the
products
that
are
affected, the information is not represented in the table below.
No affected product recoded yet
CVSS Scores
| Score | Version | Severity | Vector | Exploitability Score | Impact Score | Source |
|---|---|---|---|---|---|---|
| CVSS 4.0 | MEDIUM | [email protected] |
Solution
- Update bcrypt-ruby gem to version 3.1.22.
- Set bcrypt cost to less than 31.
- Verify hash integrity after changes.
References to Advisories, Solutions, and Tools
Here, you will find a curated list of external links that provide in-depth
information, practical solutions, and valuable tools related to
CVE-2026-33306.
CWE - Common Weakness Enumeration
While CVE identifies
specific instances of vulnerabilities, CWE categorizes the common flaws or
weaknesses that can lead to vulnerabilities. CVE-2026-33306 is
associated with the following CWEs:
Common Attack Pattern Enumeration and Classification (CAPEC)
Common Attack Pattern Enumeration and Classification
(CAPEC)
stores attack patterns, which are descriptions of the common attributes and
approaches employed by adversaries to exploit the CVE-2026-33306
weaknesses.
We scan GitHub repositories to detect new proof-of-concept exploits. Following list is a collection of public exploits and proof-of-concepts, which have been published on GitHub (sorted by the most recently updated).
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The following list is the news that have been mention
CVE-2026-33306 vulnerability anywhere in the article.
The following table lists the changes that have been made to the
CVE-2026-33306 vulnerability over time.
Vulnerability history details can be useful for understanding the evolution of a vulnerability, and for identifying the most recent changes that may impact the vulnerability's severity, exploitability, or other characteristics.
-
New CVE Received by [email protected]
Mar. 24, 2026
Action Type Old Value New Value Added 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. Added CVSS V4.0 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 Added CWE CWE-190 Added Reference https://github.com/bcrypt-ruby/bcrypt-ruby/commit/831ce64cb0a9502130fa93a28bfd9527a5fa45c4 Added Reference https://github.com/bcrypt-ruby/bcrypt-ruby/releases/tag/v3.1.22 Added Reference https://github.com/bcrypt-ruby/bcrypt-ruby/security/advisories/GHSA-f27w-vcwj-c954