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.
Summary dbcve.org
In concurrent-ruby versions prior to 1.3.7, the ReentrantReadWriteLock uses a single integer where the low 15 bits store the read hold count and bit 15 serves as the WRITE_LOCK_HELD flag. After 32,768 reentrant read acquisitions by a single thread, the read count overflows into the write-lock bit, causing try_write_lock to incorrectly return true without setting the global RUNNING_WRITER bit. This breaks mutual exclusion by allowing false write-lock claims while read locks can still be held or acquired by other threads.
Mitigation
Upgrade concurrent-ruby to version 1.3.7 or later to receive the fix that properly separates read hold counts from the write-lock flag.