CVE-2026-72854

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.

References

• https://github.com/msgpack/msgpack-c
• https://github.com/msgpack/msgpack-c/blob/c-7.0.1/example/lib_buffer_unpack.c
• 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/src/unpack.c#L429-L502
• https://github.com/msgpack/msgpack-c/issues/1181