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

CVE-2026-54905 is fixed in ruby-concurrent on 1 platform by DEBIAN-CVE-2026-54905 (version 1.3.7-1), released July 3, 2026. It is not reported as exploited. Patch Radar priority: Routine.

- Web page: https://patchblox.com/security/cve/CVE-2026-54905
- Priority: Routine
- CVSS: 5.5 (Low)
- EPSS: 0.0015
- CISA KEV: not listed
- Exploited (vendor report): no
- Publicly disclosed: no
- Published: Jun 24, 2026
- Weakness: Wrap-around Error
- Vendor advisory: https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-54905

## 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.

## Where it's fixed

| Platform | Product | Fixed in | Fix | Released |
|---|---|---|---|---|
| Linux | ruby-concurrent Debian 14 | 1.3.7-1 | [DEBIAN-CVE-2026-54905](https://patchblox.com/security/data/fix/linux/debian-cve-2026-54905.md) | Jul 3, 2026 |

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