What’s Vulnerable › Vulnerabilities

CVE-2026-114613: MessagePack: Unbounded memory leak in msgpack C extension on strict_map_key validation failure

• Severity: high (CVSS 7.5 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: 2026-10-22 · updated 2026-10-23
• Weakness: CWE-401

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.

Affected products

• Pypi Msgpack: before 1.2.3; 0.5.0, 0.5.1, 0.5.2, 0.5.3, 0.5.4, 0.5.5 (+28) — fixed in 1.2.3
• msgpack-python: before 1.2.3

How to fix

Upgrade to a fixed version: Pypi Msgpack 1.2.3. Check the vendor advisory for mitigations.

References

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

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