Updated advisory posts against rubysec/ruby-advisory-db@2a45afd

Author: jasnow

advisories/_posts/2026-03-19-CVE-2026-33306.md

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+---
+layout: advisory
+title: 'CVE-2026-33306 (bcrypt): bcrypt-ruby has an Integer Overflow that Causes Zero
+  Key-Strengthening Iterations at Cost=31 on JRuby'
+comments: false
+categories:
+- bcrypt
+advisory:
+  gem: bcrypt
+  cve: 2026-33306
+  ghsa: f27w-vcwj-c954
+  url: https://github.com/advisories/GHSA-f27w-vcwj-c954
+  title: bcrypt-ruby has an Integer Overflow that Causes Zero Key-Strengthening Iterations
+    at Cost=31 on JRuby
+  date: 2026-03-19
+  description: |
+    ### Impact
+
+    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.
+  patched_versions:
+  - ">= 3.1.22"
+  related:
+    url:
+    - https://github.com/bcrypt-ruby/bcrypt-ruby/security/advisories/GHSA-f27w-vcwj-c954
+    - https://github.com/bcrypt-ruby/bcrypt-ruby/commit/5faa2748331d3edc661c127ef2fbb3afcb6b02a4
+    - https://github.com/advisories/GHSA-f27w-vcwj-c954
+---