Concurrent RubyApplication · Rubyconcurrency
CVE-2026-54905
Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.
NVD · uneditedconcurrent-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.
Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.
dbcve analysis · high confidenceIn 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.
Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.
Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.
NVD · CPE data
Concurrent RubyApplication
< 1.3.7CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.
From the vector- Attack vector
- Local
- Complexity
- Low
- Privileges
- Low
- User interaction
- None
- Scope
- Unchanged
- Confidentiality
- High
- Integrity
- None
- Availability
- None
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:N
Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.
dbcve checksWork through these to decide whether this CVE applies to you.
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Identify if concurrent-ruby is usedCheck your Gemfile, gemspec, or run `gem list concurrent-ruby` to see if the concurrent-ruby gem is installedAffected if The gem is not installed or the version cannot be determined
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Determine installed version of concurrent-rubyRun `gem list concurrent-ruby` or check your Gemfile.lock for the exact version numberAffected if The installed version is lower than 1.3.7 (e.g., 1.3.6, 1.3.0, etc.)
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Check for ReentrantReadWriteLock usageSearch your codebase for references to ReentrantReadWriteLock, typically in files requiring concurrent-ruby synchronization featuresAffected if Your code uses ReentrantReadWriteLock from the concurrent-ruby library
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Identify high-volume read lock scenariosReview code paths where read_lock or try_read_lock is called repeatedly in loops or recursively by a single thread, as this could trigger the overflowAffected if The application has code paths that could result in more than 32,768 reentrant read acquisitions by a single thread
You are affected if you use concurrent-ruby version below 1.3.7 AND your application uses ReentrantReadWriteLock in scenarios where a single thread could acquire read locks more than 32,768 times, causing write-lock detection to fail and break mutual exclusion.
Generated from the published advisory. Verify against your own configuration.
Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.
dbcve · scoped1.3.7
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.
concurrent-ruby version 1.3.7
- Check the current version of the concurrent-ruby gem in your project by running `gem list concurrent-ruby` or checking your Gemfile
- Update the concurrent-ruby gem to version 1.3.7 or later by running `gem install concurrent-ruby -v '~> 1.3.7'` for system-wide install, or update your Gemfile with `gem 'concurrent-ruby', '~> 1.3.7'` and run `bundle update concurrent-ruby`
- Verify the upgrade was successful by running `gem list concurrent-ruby` and confirming the installed version is 1.3.7 or higher
- Run your test suite to ensure the upgrade does not break existing functionality
Generated from the published advisory — verify against the referenced sources before acting.
Fix this in Concurrent Ruby
Scoped from the published advisory
- Consultation1.0 h
- Implementation2.0 h
- Testing3.0 h
- Review / QA1.0 h
An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $1,904.
Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2026-54905 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.
References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.
Primary sourcesPractitioner notes
ContributedPeer-ranked notes from engineers who’ve handled CVE-2026-54905 in production — separate from our analysis above.
The advisory tells you what broke. It rarely tells you what actually worked. If you’ve dealt with this one, that detail is what the next engineer is searching for.
- The version that genuinely resolved it — not the one the vendor claimed
- A config change or rule that shut the vector down
- A gotcha in the upgrade path that cost you an afternoon
No notes yet
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