Vulnerability · Published Oct 8, 2026 · Assigned by GitHub_M

CVE-2026-107299msgpack5: Reserved byte can cause unbounded stream buffering

Medium 5.9

In plain English

A medium-severity security flaw in msgpack5 from mcollina. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.4% (higher than 33% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.

Severity (CVSS 3.1)5.9Medium

Exploit odds (30 days)0.4%top 66.8% of all CVEs

Exploited in the wildNo reportsCISA SSVC: none

Attack toolingNoneno public tooling known

Official description

msgpack5 is a msgpack v5 implementation for node.js and the browser. Prior to 6.1.0, the streaming decoder treats the reserved MessagePack byte 0xc1 as incomplete input instead of invalid input. When 0xc1 begins a stream, subsequent data remains buffered while the decoder waits for bytes that cannot make the value valid, allowing a remote peer to exhaust memory. This issue is fixed in version 6.1.0.

Affected packages & fixed versions

Ecosystem	Package	Fixed in	Advisory
npm	msgpack5	6.1.0	GHSA-26wq-p25c-j6fv

Affected products (as reported)

Vendor	Product	Versions
mcollina	msgpack5	< 6.1.0

Weakness: CWE-228 · Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H

References

https://github.com/mcollina/msgpack5/security/advisories/GHSA-26wq-p25c-j6fv x_refsource_CONFIRM
https://github.com/mcollina/msgpack5/commit/85da345bf1acc18ca441741e5cf4aa0ed0d69314 x_refsource_MISC
https://github.com/mcollina/msgpack5/releases/tag/v6.1.0 x_refsource_MISC