Bug 2520682 (CVE-2026-72854)

Summary: CVE-2026-72854 msgpack-c: msgpack-c: Heap buffer overflow via integer overflow in buffer expansion.
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security <prodsec-ir-bot>
Status: NEW --- QA Contact:
Severity: medium Docs Contact:
Priority: medium    
Version: unspecifiedCC: rhel-process-autobot, watson-tool-maintainers
Target Milestone: ---Keywords: Security
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Hardware: All   
OS: Linux   
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A flaw was found in msgpack-c. An integer overflow vulnerability exists in the `msgpack_unpacker_expand_buffer` function. This flaw can lead to an undersized memory buffer being allocated, even though the system reports a successful reservation. If an attacker can influence the length provided to the reservation function, a subsequent write operation could overflow the heap buffer, potentially leading to integrity impact and limited availability.
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oVirt Team: --- RHEL 7.3 requirements from Atomic Host:
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Bug Depends On: 2521811    
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Description OSIDB Bzimport 2026-08-20 18:44:06 UTC 
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.