Msgpack Msgpack-C up to 7.0.1 Buffer Expansion src/unpack.c msgpack_unpacker_expand_buffer integer overflow

Summaryinfo

A vulnerability, which was classified as critical, was found in Msgpack Msgpack-C up to 7.0.1. This vulnerability affects the function msgpack_unpacker_expand_buffer of the file src/unpack.c of the component Buffer Expansion. Such manipulation leads to integer overflow. This vulnerability is documented as CVE-2026-72854. The attack can be executed remotely. There is not any exploit available.

Detailsinfo

A vulnerability was found in Msgpack Msgpack-C up to 7.0.1 and classified as critical. Affected by this issue is the function msgpack_unpacker_expand_buffer of the file src/unpack.c of the component Buffer Expansion. The manipulation with an unknown input leads to a integer overflow vulnerability. Using CWE to declare the problem leads to CWE-190. The product performs a calculation that can produce an integer overflow or wraparound, when the logic assumes that the resulting value will always be larger than the original value. This can introduce other weaknesses when the calculation is used for resource management or execution control. Impacted is confidentiality, integrity, and availability. CVE summarizes:

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.

The advisory is shared for download at github.com. This vulnerability is handled as CVE-2026-72854 since 08/10/2026. The exploitation is known to be easy. The attack may be launched remotely. No form of authentication is required for exploitation. There are known technical details, but no exploit is available. The current price for an exploit might be approx. USD $0-$5k (estimation calculated on 08/20/2026).

There is no information about possible countermeasures known. It may be suggested to replace the affected object with an alternative product.

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Productinfo

Vendor

• Msgpack

Name

• Msgpack-C

Version

• 7.0.0
• 7.0.1

Website

• Product: https://github.com/msgpack/msgpack-c/

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