CVE-2026-54522: messagepack Use-After-Free

Description

MessagePack for Ruby is an implementation of the MessagePack binary serialization format. Prior to 1.8.2, MessagePack::Buffer#clear in ext/msgpack/buffer.c leaves rmem_last, rmem_end, and rmem_owner stale after _msgpack_buffer_shift_chunk returns an rmem page to the shared pool, allowing a subsequent Buffer#write and a second MessagePack::Buffer to alias the page and disclose or corrupt cross-buffer data. This issue is fixed in version 1.8.2.

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CVSS v3.1 Score

5.4

MEDIUM

CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:N

EPSS — Exploit Prediction

0.0016

Probability of exploitation

0.06%

Percentile rank

EPSS estimates the probability that this vulnerability will be exploited in the wild within the next 30 days. A higher score means more likely to be exploited.

Weakness Type (CWE)

CWE-416 Use After Free

Affected Products

Vendor	Product	Versions
msgpack	messagepack	< 1.8.2

References

Advisories & Patches

https://github.com/msgpack/msgpack-ruby/commit/5627d71606b565641d2dd501b82aae862f4abe90 https://github.com/msgpack/msgpack-ruby/security/advisories/GHSA-4mrv-5p47-p938 https://github.com/msgpack/msgpack-ruby/security/advisories/GHSA-4mrv-5p47-p938

Exploits

https://github.com/msgpack/msgpack-ruby/security/advisories/GHSA-4mrv-5p47-p938 https://github.com/msgpack/msgpack-ruby/security/advisories/GHSA-4mrv-5p47-p938

Frequently Asked Questions

What is CVE-2026-54522? +

MessagePack for Ruby is an implementation of the MessagePack binary serialization format. Prior to 1.8.2, MessagePack::Buffer#clear in ext/msgpack/buffer.c leaves rmem_last, rmem_end, and rmem_owner stale after _msgpack_buffer_shift_chunk returns an rmem page to the shared pool, allowing a subsequent Buffer#write and a second MessagePack::Buffer to alias the page and disclose or corrupt cross-buffer data. This issue is fixed in version 1.8.2. It has a CVSS v3.1 base score of 5.4 (MEDIUM).

How severe is CVE-2026-54522? +

CVE-2026-54522 has a CVSS v3.1 score of 5.4 out of 10, rated MEDIUM. This is a medium-severity vulnerability that should be remediated as part of regular maintenance. The EPSS score is 0.0016, placing it in the 0th percentile for exploitation probability.

What products are affected by CVE-2026-54522? +

CVE-2026-54522 affects products from msgpack, specifically: messagepack. Check the affected products table above for specific version ranges.

How do I check if I'm vulnerable to CVE-2026-54522? +

You can use Secably's free Website Scanner to check your website for known vulnerabilities. For infrastructure scanning, use the Port Scanner to identify exposed services that may be affected. Check the vendor advisories linked above for specific patch and version information.

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