CVE-2026-54522· NVD / CVE Program· CNA GitHub_M

MessagePack::Buffer#clear Use-After-Free that Enables Cross-Buffer Disclosure

Noroxi assessment

Priority: Monitor (action score 8/100)

The sources we track (CISA KEV, Exploit-DB, Metasploit, Nuclei, GitHub) show no public exploit code or exploitation in the wild.
Probability of exploitation in the next 30 days: 0.22% (FIRST EPSS).
Someone with local access to the system can trigger it. No account or password is required. No user action is required. No special conditions are required; it is repeatable.
Fixed version: msgpack msgpack-ruby 1.8.2; azl3 rubygem-msgpack 1.7.2-1 on Azure Linux 3.0 1.8.4-1.

Low priority for now. Upgrade to msgpack msgpack-ruby 1.8.2; azl3 rubygem-msgpack 1.7.2-1 on Azure Linux 3.0 1.8.4-1 or later.

Generated automatically from the record's data: CVSS (NVD and CNA), CISA KEV and SSVC, FIRST EPSS, exploit sources, fixed versions. How we calculate it

Primary sources: NVD · CVE.org · FIRST EPSS

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.

LowCVSS 2.1 · v4.0—No exploit Fix available

Memory safety

GHSA-4mrv-5p47-p938

Published: Jul 30, 2026
Updated: Aug 5, 2026
EPSS: 0.2% · 12th percentile
CWE: CWE-416

Report tools

JSON

Action score

Monitor

Low priority for now.

CVSS: 8 / 40 · 2.1 / 10
CISA KEV: 0 / 30 · Not listed
EPSS: 0 / 30 · 0.2%

CISA SSVC decision

Exploitation: proof of concept
Automatable: no
Technical impact: partial

Vulnrichment: CISA's decision-tree inputs.

CNA vs NVD score

NVD: —
CNA · GitHub_M: 2.1; NVD has not scored this yet; the score shown is the CNA’s.

CVSS:4.0/AV:L/AC:L/AT:P/PR:N/UI:N/VC:L/VI:N/VA:N/SC:N/SI:N/SA:N

The score given by the assigning authority versus NVD’s independent score. A gap means the severity is contested.

Get told when this record changes

Not in KEV: no record of exploitation in the wild
No public exploit code on record
A fix has been released

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Noroxi analysis

What does it mean in your environment?

The assessment above is generated automatically from the record's data; it doesn't know your environment. Our team verifies on your systems whether this vulnerability is actually exploitable and which mitigation is enough.

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Affected systems

Is your version affected?

Pick the product and type the version you run. It's compared with this record's version ranges in your browser; the version isn't sent anywhere.

Vendor	Product	CPE
msgpack	messagepack	cpe:2.3:a:msgpack:messagepack

Affected versions

NVD version ranges (for catalog products). Add the product to your stack with a version and matching uses these.

msgpack messagepackbefore 1.8.2

Versions reported by the vendor

Affected version ranges reported by the assigning authority (GitHub_M). Independent of NVD's CPE analysis and usually ahead of it.

msgpack msgpack-ruby
- < 1.8.2affected

Package-level exposure

OSV and GitHub Advisory data: ecosystem, package and range. SBOM matching uses this table.

Ecosystem	Package	Affected range	Fix
Debian:12	ruby-msgpack	all versions	—
Debian:14	ruby-msgpack	before 1.8.3-1	1.8.3-1
RubyGems	msgpack	before 1.8.2	1.8.2

Same product

msgpack: all records

Other highest-scoring records for the same primary product.

CVE-2026-57585MessagePack: Out-of-bounds read/crash on Unpacker reuse after caught error
30Monitor
CVE-2026-21452MessagePack-Java Vulnerable to Remote Denial of Service via Malicious .msgpack Model File Triggering Unbounded EXT Payload Allocation
30Monitor

Remediation

Which version to upgrade to

Fix versions compiled from the vendor, package registries and Microsoft. Verify the vendor's note before upgrading.

Product / package	Fixed version	Source
msgpack msgpack-ruby	1.8.2 · description	Vendor (CNA)
azl3 rubygem-msgpack 1.7.2-1 on Azure Linux 3.0	1.8.4-1 · CBL-Mariner Releases	Microsoft (MSRC)
debian:ruby-msgpack	1.8.3-1 · Debian:14	Package registry (OSV)
RubyGems:msgpack	1.8.2	Package registry (OSV)

Exploit status

No known public exploit

No public exploit has been observed yet. That doesn't mean you're safe, only that the bar is a little higher.

Research context

For pentesters and researchers: attack profile, score disagreement, timeline, patch commits, credits, variant and chain candidates, bug bounty scope. All derived from existing data; no exploit code.

Timeline

From publication to today: proof of concept, Metasploit module, CISA KEV and fix record. Dates are as reported by the sources.

No dated events beyond publication.

EPSS, last 120 days

FIRST EPSS daily score; only changes of 0.01 or more are recorded (step chart).

Patch and commit links

Commit, PR and diff links among the references. A starting point for patch-diffing and variant hunting; fixes, not exploits.

msgpack/msgpack-rubycommit 5627d71

Credits

All researchers

Finders, reporters and analysts named in the CNA record. Click a name for that researcher’s other records.

No credits in the CNA record.

Variant candidates

Same product, same weakness class, within 18 months. If the patch missed the root cause, the sibling bug is here.

CVE-2026-5758530 days apartMessagePack: Out-of-bounds read/crash on Unpacker reuse after caught error
30Monitor

Chain candidates

An authentication bypass and a privilege-requiring bug in the same product, published close together: combined they may become an unauthenticated path.

—

Bug bounty scope

No known public program.

Source: bounty-targets-data (public HackerOne, Bugcrowd, Intigriti, YesWeHack listings).

Technical details

Attack conditions

Someone with local access to the system can trigger it.
No account or password is required.
No user action is required.
No special conditions are required; it is repeatable.

If successful

Confidentiality
Integrity
Availability

Attack vector: Local
Attack complexity: Low
Privileges required: None
User interaction: None
Scope: X

Weakness class (CWE)

CWE-416 · Use After Free

CVSS vector

CVSS:4.0/AV:L/AC:L/AT:P/PR:N/UI:N/VC:L/VI:N/VA:N/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X

nvd-secondary

Root cause

After a memory region is freed, another reference pointing to it is still used. By then, the same region may have been allocated for different data.

The bug in code

The whole CWE-416 class

A representative example of this vulnerability class. Not the vendor's source code; it shows the faulty pattern and its fix.

Vulnerable

1free(dev->buf);2/* ... another thread is still running ... */3process(dev->buf);

Fixed

1lock(&dev->lock);2free(dev->buf);3dev->buf = NULL;4unlock(&dev->lock);

Attack context

MITRE CAPEC attack patterns and ATT&CK techniques for this weakness class (CWE). A starting point for detection rules and threat hunting.

MITRE has no CAPEC/ATT&CK mapping for this CWE.

Change log

Fix✗ → ✓Sep 28, 2026, 10:45 PM UTC

For records you follow, these changes also arrive as notifications. →

References

github.com/msgpack/msgpack-ruby/commit/5627d71606b565641d2dd501b82aae862f4abe90

Vendor advisories and official records. Exploit/PoC links are deliberately left out.

All records