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

What is affected, what to do

A flaw was found in msgpack. A remote attacker could trigger a memory leak by sending specially crafted MessagePack data containing rejected map keys during deserialization.

Update Msgpack-python to version 1.2.3.

Affected

• Msgpack-python lt *..1.2.3

Fixed in

• Msgpack-python 1.2.3

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Metrics

Detection rules

No NEOSEC detection rule is available for this vulnerability yet.

Classification under NIS2 and CRA

Rule-based classification from exploitation status and severity. It does not replace a case-by-case assessment.

NIS2

NIS2 is EU Directive (EU) 2022/2555 to raise the level of cybersecurity across the Union. It substantially broadens obligations compared with the first NIS directive: more sectors, mandatory risk management, staged reporting duties and personal accountability of senior management.

Read in glossary

High technical severity: assess and remediate the vulnerability in vulnerability handling under Art. 21(2)(e) NIS2 and document the decision.

CRA

The CRA is EU Regulation (EU) 2024/2847 setting horizontal cybersecurity requirements for „products with digital elements“ — from industrial controllers and software to connected consumer devices. For the first time manufacturers must ensure security across the entire product lifecycle, handle vulnerabilities and provide security updates.

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From 11.12.2027, Annex I Part II CRA requires manufacturers of affected products with digital elements to handle vulnerabilities without delay and provide security updates.

ISO/IEC 27001

ISO/IEC 27001 is the internationally leading standard for an information security management system (ISMS). It describes a risk-based, process-oriented framework to systematically plan, implement, review and improve information security.

Read in glossary

Control A.8.8, management of technical vulnerabilities: assess relevance to your systems, plan remediation and keep evidence.

Description

A flaw was found in msgpack. A remote attacker could trigger a memory leak by sending specially crafted MessagePack data containing rejected map keys during deserialization. Repeated transmission of such malformed inputs can lead to excessive process memory consumption, resulting in a Denial of Service (DoS) for the affected application.

Source: redhat_csaf, Microsoft Security Update Guide, ENISA EUVD, OSV (Open Source Vulnerabilities), GitHub Security Advisories, NVD (NIST), cvelistv5

Vendors and products

• Msgpack

Weakness classes (CWE)

• CWE-401Missing Release of Memory after Effective Lifetime

  Variant

  The product does not sufficiently track and release allocated memory after it has been used, making the memory unavailable for reallocation and reuse.

  Entry at MITRE CWE-401

• CWE-416Use After Free

  Variant

  The product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory "belongs" to the code that operates on the new pointer.

  Entry at MITRE CWE-416

References & sources

• github.commsgpack/msgpack-python/security/advisories/GHSA-j586-36cw-2gc2x_refsource_CONFIRM, web, euvd
• github.commsgpack/msgpack-python/commit/620b0eb4c89cd72d4098564537624d9db…x_refsource_MISC, web, euvd
• github.commsgpack/msgpack-python/releases/tag/v1.2.3x_refsource_MISC, web, euvd
• NVD (NIST)vuln/detail/CVE-2026-114613advisory
• github.commsgpack/msgpack-pythonpackage

Changes to this CVE

Who changed this CVE and when: NVD (analysis, affected products, CVSS), the CVE publisher (CNA) and additional providers such as CISA (ADP).

1. 23.10.2026Record modifiedCISA ADP

   CISA assessment (SSVC): exploitation: none known, automatable: yes

   Show the original change

   Individual changes of this event as published by NVD

   Type	Action	Before	After
   SSVC assessment	added	—	{"id":"CVE-2026-114613","role":"CISA Coordinator","options":[{"exploitation":"none"},{"automatable":"yes"},{"technicalImpact":"partial"}],"version":"2.0.3","timestamp":"2026-10-23T17:32:58.847580Z"}

   Source identifier: 134c704f-9b21-4f2e-91b3-4a467353bcc0

2. 22.10.2026ReceivedGitHub

   CVSS V3.1: AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

   Weakness type: CWE-401

   Description added or changed

   New 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
   Show 7 original changes

   Individual changes of this event as published by NVD

   Type	Action	Before	After
   Description	added	—	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.
   CVSS	added	—	AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
   Weakness type (CWE)	added	—	CWE-401
   Affected versions (publisher)	added	—	New affected value received. <a href="https://github.com/CVEProject/cvelistV5/blob/main/cves/2026/114xxx/CVE-2026-114613.json">CVE-2026-114613</a>
   Reference	added	—	https://github.com/msgpack/msgpack-python/commit/620b0eb4c89cd72d4098564537624d9dbb2ce141
   Reference	added	—	https://github.com/msgpack/msgpack-python/releases/tag/v1.2.3
   Reference	added	—	https://github.com/msgpack/msgpack-python/security/advisories/GHSA-j586-36cw-2gc2

   Source identifier: security-advisories@github.com