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CVE-2026-33306— bcrypt-ruby has an Integer Overflow that Causes Zero Key-Strengthening Iterations at Cost=31 on JRuby

Quick assessment

Affected
bcrypt-ruby bcrypt-ruby
Exploitation
No confirmed in-the-wild exploitation; assess based on exposure
Recommended action
Check the vendor advisory and references for a fixed version. If immediate upgrade is impossible, restrict exposure and increase monitoring.

bcrypt-ruby是bcrypt-ruby开源的一个安全的密码哈希工具。 bcrypt-ruby 3.1.22之前版本存在输入验证错误漏洞，该漏洞源于JRuby实现中的整数溢出导致强化循环零迭代，可能削弱密码哈希强度。

AI Predicted 7.5 Difficulty: Easy EPSS 0.26% · P17

Possible ATT&CK Techniques 1 AI

T1556 · Modify Authentication Process
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I. Basic Information for CVE-2026-33306

Vulnerability Information
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Vulnerability Title
bcrypt-ruby has an Integer Overflow that Causes Zero Key-Strengthening Iterations at Cost=31 on JRuby
Source: CVE Program / CVE List V5
Vulnerability 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.
Source: CVE Program / CVE List V5
CVSS Information
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
Source: CVE Program / CVE List V5
Vulnerability Type
整数溢出或超界折返
Source: CVE Program / CVE List V5
Vulnerability Title
bcrypt-ruby 输入验证错误漏洞
Source: CNNVD (China National Vulnerability Database)
Vulnerability Description
bcrypt-ruby是bcrypt-ruby开源的一个安全的密码哈希工具。 bcrypt-ruby 3.1.22之前版本存在输入验证错误漏洞，该漏洞源于JRuby实现中的整数溢出导致强化循环零迭代，可能削弱密码哈希强度。
Source: CNNVD (China National Vulnerability Database)
CVSS Information
N/A
Source: CNNVD (China National Vulnerability Database)
Vulnerability Type
N/A
Source: CNNVD (China National Vulnerability Database)

Affected Products

Vendor Product Affected Versions CPE Subscribe
bcrypt-ruby bcrypt-ruby < 3.1.22 -

II. Public POCs for CVE-2026-33306

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III. Intelligence Information for CVE-2026-33306

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Patches & Fixes for CVE-2026-33306 (1)

Vendor Advisories for CVE-2026-33306 (1)

Other References for CVE-2026-33306 (1)

IV. Related Vulnerabilities

Same weakness: CWE-190

V. Comments for CVE-2026-33306

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