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

Severity: medium (CVSS 5.9 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.4% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-10-08 · updated 2026-10-09
Weakness: CWE-228

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 products

mcollina msgpack5: before 6.1.0

References

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

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