CVE-2026-54906: concurrent-ruby: ReadWriteLock allows wrong-thread write release and stray read-release counter corruption

Severity: critical (CVSS 9.8 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.2% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-06-24 · updated 2026-10-25
Weakness: CWE-414

Description

concurrent-ruby is a modern concurrency tools for Ruby. Prior to 1.3.7, Concurrent::ReadWriteLock#release_write_lock does not verify that the calling thread acquired the write lock. Any thread with access to the lock object can release an active write lock held by another thread. A second writer can then enter its critical section while the first writer is still running. Concurrent::ReadWriteLock#release_read_lock also decrements the shared counter even when no read lock is held. Calling it on a fresh lock changes the counter from 0 to -1, after which normal read acquisition raises Concurrent::ResourceLimitError. This is a synchronization correctness issue in the public Concurrent::ReadWriteLock API. This vulnerability is fixed in 1.3.7.

Affected products

ruby-concurrency concurrent-ruby: before 1.3.7 — fixed in 1.3.7
Rubygems Concurrent-Ruby: before 1.3.7; 0.0.1, 0.1.0, 0.1.1, 0.1.1.pre.1, 0.1.1.pre.2, 0.1.1.pre.3 (+71) — fixed in 1.3.7

How to fix

Upgrade to a fixed version: ruby-concurrency concurrent-ruby 1.3.7; Rubygems Concurrent-Ruby 1.3.7. Check the vendor advisory for mitigations.

References

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