{"id":"CVE-2026-72854","summary":"msgpack-c Integer Overflow in msgpack_unpacker_expand_buffer Causes a False-Success Undersized Reservation","details":"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.","modified":"2026-08-22T03:58:11.445568315Z","published":"2026-08-20T18:19:24.266Z","database_specific":{"osv_generated_from":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/72xxx/CVE-2026-72854.json","cna_assigner":"VulnCheck","cwe_ids":["CWE-190","CWE-787"]},"references":[{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/72xxx/CVE-2026-72854.json"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-72854"},{"type":"ADVISORY","url":"https://www.vulncheck.com/advisories/msgpack-c-integer-overflow-in-msgpack-unpacker-expand-buffer-causes-a-false-success-undersized-reservation"},{"type":"REPORT","url":"https://github.com/msgpack/msgpack-c/issues/1181"},{"type":"PACKAGE","url":"https://github.com/msgpack/msgpack-c"},{"type":"ARTICLE","url":"https://github.com/msgpack/msgpack-c/blob/c-7.0.1/example/lib_buffer_unpack.c"},{"type":"ARTICLE","url":"https://github.com/msgpack/msgpack-c/blob/c-7.0.1/include/msgpack/unpack.h#L219-L223"},{"type":"ARTICLE","url":"https://github.com/msgpack/msgpack-c/blob/c-7.0.1/src/unpack.c#L429-L502"}],"affected":[{"ranges":[{"type":"GIT","repo":"https://github.com/msgpack/msgpack-c","events":[{"introduced":"0"},{"last_affected":"1be50ee5dadc4b85f79945c51df362582ccb2631"}],"database_specific":{"extracted_events":[{"introduced":"0"},{"last_affected":"7.0.1"}],"source":"AFFECTED_FIELD"}}],"versions":["c-7.0.1","c-7.0.0","c-6.1.0","c-6.0.2","c-6.0.1","c-6.0.0","c-5.0.0","c-4.0.0","cpp-3.3.0","cpp-3.2.1","cpp-3","cpp-3.2.0","cpp-3.1.1","cpp-3.1.0","cpp-3.0.1","cpp-3.0.0","cpp-2.1.5","cpp-2.1.4","2.1.4","cpp-2.1.3","cpp-2.1.2","cpp-2.1.1","cpp-2.1.0","cpp-2.0.0","cpp-1.4.0","cpp-1.3.0","cpp-1.2.0","cpp-1.1.0","cpp-1.0.1","cpp-1.0.0","cpp-0.5.9","cpp-0.5.8","cpp-0.5.5","cpp-0.5.0","cpp-0.4.3","cpp-0.4.2","cpp-0.4.1","cpp-0.4.0","cpp-0.3.8","cpp-0.3.7","cpp-0.3.6","cpp-0.3.5","cpp-0.3.4","0.3.2","0.3.1","0.3.0"],"database_specific":{"source":"https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-72854.json"}}],"schema_version":"1.9.0","severity":[{"type":"CVSS_V4","score":"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"}]}