Concurrent Ruby: ReentrantReadWriteLock read-count overflow grants a write lock without exclusivity

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS 4.0

2.0 · CVSS:4.0/AV:L/AC:L/AT:P/PR:L/UI:N/VC:L/VI:L/VA:L/SC:N/SI:N/SA:N · score source

EPSS

• CVE-2026-54905: 0.15% estimated exploitation activity in the next 30 days; percentile 4.46 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary Concurrent::ReentrantReadWriteLock can incorrectly grant a write lock after one thread acquires the read lock 32,768 times. The lock stores a thread's local read and write hold counts in one integer. The low 15 bits are used for the read hold count, and bit 15 is used as WRITELOCKHELD. After 32,768 reentrant read acquisitions, the local read count crosses into the write-lock bit. trywritelock then treats the thread as already holding a write lock and returns true without setting the global RUNNINGWRITER bit. This breaks the core mutual-exclusion guarantee: the caller is told it has a write lock, but other threads can still hold or acquire read locks at the same time. Version Software: concurrent-ruby Version: 1.3.6 Commit: 7a1b78941c081106c20a9ca0144ac73a48d254ab Details The implementation uses a shared counter to track global readers/writers and a per-thread local counter to support reentrancy: When a thread already holds a lock, acquirereadlock increments @HeldCount: After…

Published by the source

19 Jun 2026, 20:47 UTC

Last modified by the source

20 Jul 2026, 13:37 UTC

Source last checked

26 Oct 2026, 15:52 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-wv3x-4vxv-whpp

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# Patch Next advisory brief: CVE-2026-54905

Advisory: Concurrent Ruby: `ReentrantReadWriteLock` read-count overflow grants a write lock without exclusivity
IDs: CVE-2026-54905, GHSA-wv3x-4vxv-whpp
Record: https://patchnext.com/advisories/cve-2026-54905 (revision 1, source checked 26 Oct 2026, 15:52 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Low (CVSS 2.0)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: concurrent-ruby 1.3.7. A vendor fix does not mean a deployment is fixed.

## Affected components
- RubyGems concurrent-ruby: affected < 1.3.7; fixed in 1.3.7

## Suggested inspection (read-only)
- RubyGems concurrent-ruby: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on concurrent-ruby.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-wv3x-4vxv-whpp (GitHub-reviewed advisory GHSA-wv3x-4vxv-whpp lists affected ranges: RubyGems concurrent-ruby >=0 <1.3.7)

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