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

> An integer overflow in concurrent-ruby allows a thread to bypass write lock exclusivity, potentially leading to data corruption or race conditions.

- **Severity:** Low 2.0 (`CVSS:4.0/AV:L/AC:L/AT:P/PR:L/UI:N/VC:L/VI:L/VA:L/SC:N/SI:N/SA:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 24 Jun 2026
- **Updated:** 24 Jun 2026
- **Also known as:** AZL-91290, CLEANSTART-2026-CH23308, CLEANSTART-2026-VX25818, DEBIAN-CVE-2026-54905, GHSA-wv3x-4vxv-whpp, MINI-485g-xhv3-g79h, MINI-59pm-452g-j8r3, MINI-6c59-gr25-78f6, MINI-6v8p-9g89-8x8x, MINI-7mmf-3r64-v9wr, MINI-8r3f-chcf-3m68, MINI-cpg5-2q8x-vm25, MINI-g4rx-2xwj-7p47, MINI-g9qq-753f-q3cx, MINI-gh2j-vm93-h5g4, MINI-h8x8-rgmx-9x66, MINI-ppp2-v9gf-6x5x, MINI-prh8-ghfv-7h83, MINI-qwwm-wwrq-8j83, MINI-v324-75xr-r22h, MINI-vp6f-p4mw-w5f9, MINI-vrpf-45q5-x8g9, MINI-wq42-fq77-87pq, UBUNTU-CVE-2026-54905

## What is CVE-2026-54905?

CVE-2026-54905 is a low-severity vulnerability in ruby-concurrency concurrent-ruby, published on 24 Jun 2026. It can only be exploited from the same machine or local network, with no action from the victim.

### Official 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.

Weakness: CWE-128 (Wrap-around Error)

## Which versions are affected by CVE-2026-54905?

2 open source packages ship an affected version, including concurrent-ruby. It affects 1 product.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `concurrent-ruby` | RubyGems | All versions | 1.3.7 |
| `concurrent-ruby` | RubyGems | All versions | 1.3.7 |

Products: Rubyconcurrency Concurrent Ruby.

## How do I fix CVE-2026-54905?

Upgrade concurrent-ruby to 1.3.7 or later.

## Is CVE-2026-54905 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-54905 exists.

## Other vulnerabilities in ruby-concurrency concurrent-ruby

- [CVE-2026-54904](https://insights.sbomify.com/CVE-2026-54904/): concurrent-ruby: `AtomicReference#update` livelocks when the stored value is `Float::NAN` (high 8.2)
- [CVE-2026-54906](https://insights.sbomify.com/CVE-2026-54906/): concurrent-ruby: ReadWriteLock allows wrong-thread write release and stray read-release counter corruption (low 2.1)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-54905 and fix it if it is affected.

What it is: CVE-2026-54905 is a low-severity vulnerability in ruby-concurrency concurrent-ruby, published on 24 Jun 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-54905 exists.

Affected packages and fixed versions:
- concurrent-ruby (RubyGems): affected all versions; fixed in 1.3.7
- concurrent-ruby (RubyGems): affected all versions; fixed in 1.3.7

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-54905/ (Markdown: https://insights.sbomify.com/CVE-2026-54905/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-54905
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-54905
- OSV: https://osv.dev/vulnerability/CVE-2026-54905

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-54905/
Summaries are generated and may contain mistakes; the official description above is the record.