---
cve: "CVE-2026-72854"
severity: "MEDIUM"
cvss: 5.8
epss: "11%"
vendor: "msgpack"
kev: false
exploited: false
published: "2026-08-20 19:17:01"
tags: [cve, security, medium]
source: tsecurity.de CVE-Dossier
exported: "2026-09-18T01:59:11+02:00"
---

# CVE-2026-72854

> 5.8 MEDIUM · 🧪 PoC

## Beschreibung

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.

## CVSS-Vektor

```
CVSS:4.0/AV:L/AC:H/AT:P/PR:N/UI:P/VC:N/VI:H/VA:L/SC:N/SI:N/SA:N
```

| Metrik | Wert | Bewertung |
|---|---|---|
| AV Angriffsvektor | Lokal | good |
| AC Komplexität | Hoch | good |
| PR Privilegien | Keine | bad |

## Referenzen

- <https://github.com/msgpack/msgpack-c/issues/1181>
- <https://github.com/msgpack/msgpack-c/blob/c-7.0.1/src/unpack.c#L429-L502>
- <https://github.com/msgpack/msgpack-c/blob/c-7.0.1/include/msgpack/unpack.h#L219-L223>
- <https://github.com/msgpack/msgpack-c/blob/c-7.0.1/example/lib_buffer_unpack.c>
- <https://github.com/msgpack/msgpack-c>
- <https://www.vulncheck.com/advisories/msgpack-c-integer-overflow-in-msgpack-unpacker-expand-buffer-causes-a-false-success-undersized-reservation>

---
_Exportiert aus dem [tsecurity.de CVE-Dossier](https://tsecurity.de/cve?cve=CVE-2026-72854) · Datenquellen: EUVD (ENISA), NVD, OSV, CISA KEV, FIRST EPSS, Exploit-DB, BSI BITS_