---
id: CVE-2026-72854
title: >-
  msgpack_unpacker_expand_buffer in src/unpack.c, reached through the public
  msgpack_unpacker_reserve_buffer API, computes its new buffer size using an
  unchecked size_t addition of the requested size and the amount already used
summary: >-
  msgpack_unpacker_expand_buffer in src/unpack.c, reached through the public
  msgpack_unpacker_reserve_buffer API, computes its new buffer size using an
  unchecked size_t addition of the requested size and the amount already used.
  The doubli…
severity: medium
cvss: 5.3
cvssVector: 'CVSS:3.1/AV:L/AC:H/PR:N/UI:R/S:U/C:N/I:H/A:L'
cwe:
  - CWE-190
  - CWE-787
vendor: msgpack
product: msgpack-c
affected:
  - msgpack-c <= 7.0.1
published: '2026-08-20'
updated: '2026-09-24'
sourceUpdated: '2026-09-24T20:06:30.133'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2026-72854'
references:
  - url: 'https://github.com/msgpack/msgpack-c'
    label: disclosure@vulncheck.com
  - url: >-
      https://github.com/msgpack/msgpack-c/blob/c-7.0.1/example/lib_buffer_unpack.c
    label: disclosure@vulncheck.com
  - url: >-
      https://github.com/msgpack/msgpack-c/blob/c-7.0.1/include/msgpack/unpack.h#L219-L223
    label: disclosure@vulncheck.com
  - url: 'https://github.com/msgpack/msgpack-c/blob/c-7.0.1/src/unpack.c#L429-L502'
    label: disclosure@vulncheck.com
  - url: 'https://github.com/msgpack/msgpack-c/issues/1181'
    label: disclosure@vulncheck.com
  - url: >-
      https://www.vulncheck.com/advisories/msgpack-c-integer-overflow-in-msgpack-unpacker-expand-buffer-causes-a-false-success-undersized-reservation
    label: disclosure@vulncheck.com
  - url: 'https://github.com/msgpack/msgpack-c/issues/1181'
    label: 134c704f-9b21-4f2e-91b3-4a467353bcc0
tags:
  - nvd
  - cve.org
  - exploit-available
epss: 0.00157
epssPercentile: 0.04077
exploitAvailable: true
ssvc:
  exploitation: poc
  automatable: 'no'
  technicalImpact: partial
  timestamp: '2026-08-20T18:52:52.984726Z'
ingestedAt: '2026-09-24T15:45:56.711Z'
---

## Overview

msgpack_unpacker_expand_buffer in src/unpack.c, reached through the public msgpack_unpacker_reserve_buffer API, computes its new buffer size using an unchecked size_t addition of the requested size and the amount already used. The doubling loop guards its own multiplication against overflow, but the addition in the loop condition is unguarded, so a request near SIZE_MAX wraps: the loop condition is already satisfied, the allocation is performed at the small pre-wrap size, and the function returns true. The caller is told the requested capacity was reserved when it was not, so a subsequent write of the requested length overflows the heap buffer. The library's own example/lib_buffer_unpack.c demonstrates the reserve-then-write pattern, and its defensive assert comparing capacity against the request is compiled out under NDEBUG. msgpack-c's own decode entry points do not derive the reservation size from untrusted input, so reaching this requires an integration that passes an attacker-influenced length to the reservation API, such as a length-prefixed streaming transport.

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.