msgpack5: Decoding negative int64 values mutates the input buffer

published 2026-10-08 31 d ago modified 2026-10-14 26 d ago also EUVD-2026-95107 , GHSA-qw35-55vc-rhgj

cvss: 3.7; version 3.1, CNA
cisa kev: Not listed; not in CISA's catalog
epss: 0.3%; EPSS 0.3% chance of exploitation in 30 days, percentile 20%
public exploits: 0; none known

msgpack5 is a msgpack v5 implementation for node.js and the browser. Prior to 6.1.0, decoding a negative signed 64-bit integer modifies the corresponding bytes in the caller-provided input buffer while computing the value. Applications that retain or reuse encoded input for integrity checks, logging, or later processing can observe silently corrupted data, while positive integers and other MessagePack value types are unaffected. This issue is fixed in version 6.1.0.

Patch / fix

fixed in 2 links 2

npm msgpack5 6.1.0
msgpack5_project msgpack5 6.1.0

github.com/mcollina/msgpack5@82b7039 commit
github.com/mcollina/msgpack5/releases/tag/v6.1.0 release

CISA SSVC

Exploitation: none
Automatable: no
Technical impact: partial
Assessed: 26 d ago (2026-10-14)

Exploit prediction

0.28% chance of exploitation in the next 30 days, higher than 20% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by	version	score	vector
CNA	3.1	3.7	CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:N
GitHub	3.1	3.7	CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:N

Affected packages

ecosystem	package	affected	fixed in
npm	msgpack5	<6.1.0	6.1.0

Affected products

mcollina msgpack5

Classification

Weakness

CWE-471 Modification of Assumed-Immutable Data (MAID)

References

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.

CPEs and fixed versions use Anchore's correction of NVD's data: Added CPE configurations because not yet analyzed by NVD.

Summary