CVE-2026-72854msgpack-c Integer Overflow in msgpack_unpacker_expand_buffer Causes a False-Success Undersized Reservation

Official 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.

Affected packages & fixed versions

Affected products (as reported)

References

• GitHub Issue #1181 issue-tracking
• msgpack_unpacker_expand_buffer at c-7.0.1 technical-description
• msgpack_unpacker_reserve_buffer capacity contract at c-7.0.1 technical-description
• Shipped example demonstrating the reserve-then-write pattern technical-description
• https://github.com/msgpack/msgpack-c product
• VulnCheck Advisory: msgpack-c Integer Overflow in msgpack_unpacker_expand_buffer Causes a False-Success Undersized Reservation third-party-advisory