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BREW-asciidoctor-CVE-2026-54905 — Concurrent Ruby: `ReentrantReadWriteLock` read-count overflow grants a write lock without exclusivity

Affected: Homebrew: asciidoctor

### Summary `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.

### Version Software: concurrent-ruby Version: 1.3.6 Commit: 7a1b78941c081106c20a9ca0144ac73a48d254ab

### Details

The implementation uses a shared counter to track global readers/writers and a per-thread local counter to support reentrancy:

```ruby READER_BITS = 15 WRITER_BITS = 14

WAITING_WRITER = 1 << READER_BITS RUNNING_WRITER = 1 << (READER_BITS + WRITER_BITS) MAX_READERS = WAITING_WRITER - 1 MAX_WRITERS = RUNNING_WRITER - MAX_READERS - 1

WRITE_LOCK_HELD = 1 << READER_BITS READ_LOCK_MASK = WRITE_LOCK_HELD - 1 WRITE_LOCK_MASK = MAX_WRITERS ```

When a thread already holds a lock, `acquire_read_lock` increments `@HeldCount`:

```ruby if (held = @HeldCount.value) > 0 if held & READ_LOCK_MASK == 0 @Counter.update { |c| c + 1 } end @HeldCount.value = held…

Withdrawn: CLEANSTART-2026-DE16221 — Security fixes for CVE-2026-54171, CVE-2026-54297, CVE-2026-54522, CVE-2026-54904, CVE-2026-54905, CVE-2026-54906, ghsa…

Withdrawn by the publisher.

Affected: CleanStart: ruby-fluentd-1.18

Multiple security vulnerabilities affect the ruby-fluentd-1.18 package. These issues are resolved in later releases. See references for individual vulnerability details.

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

Affected: ruby-concurrency concurrent-ruby

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.