CVE-2026-54905: concurrent-ruby Race Condition Vulnerability

CVE-2026-54905 Overview

CVE-2026-54905 affects the concurrent-ruby gem, a widely used concurrency library for Ruby applications. Versions prior to 1.3.7 contain an integer field overflow in Concurrent::ReentrantReadWriteLock [CWE-128]. The lock packs per-thread read and write hold counts into a single integer, reserving the low 15 bits for the read count and bit 15 as WRITE_LOCK_HELD. After 32,768 reentrant read acquisitions on the same thread, the read counter overflows into the write-lock bit. The try_write_lock method then incorrectly reports success without setting the global RUNNING_WRITER flag, breaking mutual exclusion.

Critical Impact
A local thread holding excessive reentrant read locks can falsely acquire a write lock while other threads still hold read locks, corrupting protected shared state.

Affected Products

concurrent-ruby gem versions prior to 1.3.7
Ruby applications using Concurrent::ReentrantReadWriteLock
Downstream libraries and frameworks that depend on concurrent-ruby for synchronization

Discovery Timeline

2026-06-24 - CVE-2026-54905 published to NVD
2026-06-24 - Last updated in NVD database

Technical Details for CVE-2026-54905

Vulnerability Analysis

The flaw is a classic integer field overflow rooted in bit-packing two counters into a single integer. Concurrent::ReentrantReadWriteLock stores each thread's local read hold count in the low 15 bits of a per-thread counter. Bit 15 of the same integer represents the WRITE_LOCK_HELD flag. The implementation does not bound the read hold counter before it crosses the boundary into the write-lock bit.

Once a thread performs 32,768 reentrant read acquisitions, the read count value sets bit 15. From the perspective of try_write_lock, the thread now appears to already hold a write lock. The method short-circuits and returns true without performing the global write-lock acquisition that sets the RUNNING_WRITER bit.

The caller proceeds under the assumption that mutual exclusion is guaranteed. However, other threads can still hold or acquire read locks concurrently, leading to data races on the resource the lock is meant to protect.

Root Cause

The root cause is unsafe field packing without overflow validation. The read counter and the write-held flag share adjacent bits in one integer, and reentrant read acquisitions increment the counter without checking that it remains below the bit-15 boundary.

Attack Vector

Exploitation requires local code execution within the Ruby process and the ability to drive a thread through 32,768 reentrant read acquisitions on the same lock. This is a low-privilege, local condition that is unlikely to be reached in typical workloads but can occur in long-running services with recursive read patterns or in code paths an attacker can influence.

No verified public exploit code is available. See the GitHub Security Advisory GHSA-wv3x-4vxv-whpp for the upstream technical description.

Detection Methods for CVE-2026-54905

Indicators of Compromise

Unexplained data corruption or inconsistent reads in structures guarded by Concurrent::ReentrantReadWriteLock
Application logs showing intermittent assertion failures or invariant violations near read/write lock usage
Crashes or exceptions originating from code paths protected by reentrant read-write locks

Detection Strategies

Inventory installed Ruby gems and flag any concurrent-ruby version earlier than 1.3.7
Static analysis of application code for use of Concurrent::ReentrantReadWriteLock combined with recursive or loop-based read acquisition patterns
Dependency scanners and Software Composition Analysis (SCA) tools configured with the GHSA-wv3x-4vxv-whpp advisory identifier

Monitoring Recommendations

Track Gemfile.lock changes in CI/CD pipelines and fail builds on vulnerable concurrent-ruby versions
Monitor process telemetry for Ruby workers exhibiting unexpected thread contention or data inconsistency
Aggregate application error logs in a centralized SIEM to correlate lock-related exceptions across hosts

How to Mitigate CVE-2026-54905

Immediate Actions Required

Upgrade concurrent-ruby to version 1.3.7 or later across all Ruby applications and services
Audit transitive dependencies, since many Ruby frameworks pull in concurrent-ruby indirectly
Rebuild and redeploy affected containers and application images after upgrading

Patch Information

The maintainers fixed the issue in concurrent-ruby 1.3.7. Update the gem entry in Gemfile and run bundle update concurrent-ruby, then verify the resolved version in Gemfile.lock. Full details are available in the GitHub Security Advisory GHSA-wv3x-4vxv-whpp.

Workarounds

Avoid using Concurrent::ReentrantReadWriteLock in code paths that may perform deeply recursive read acquisitions until the patched version is deployed
Refactor recursive read-lock patterns to acquire the read lock once at the outer scope
Replace with alternative synchronization primitives such as Mutex or Concurrent::ReadWriteLock where reentrancy is not required

bash

# Configuration example: upgrade concurrent-ruby in a Bundler project
bundle update concurrent-ruby
bundle list | grep concurrent-ruby   # verify version >= 1.3.7