CVE-2026-72854 msgpack-c Integer Overflow in msgpack_unpacker_expand_buffer Causes a False-Success Undersized Reservation
An integer overflow in msgpack-c's buffer expansion allows heap buffer overflows when integrating with length-prefixed transports, potentially leading to code execution.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Local or same network only
- Victim action needed
- A user has to click or open something
- Fix
- Available
What is CVE-2026-72854?
CVE-2026-72854 is a medium-severity vulnerability in msgpack-c, published on 20 Aug 2026. It can only be exploited from the same machine or local network, and only if a user opens a file or follows a link.
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.
Which versions are affected by CVE-2026-72854?
Linux distribution packages (4)
msgpack-c SUSE:Linux Enterprise Server 16.0 | All versions | 7.0.2-160000.1.1 |
msgpack-c SUSE:Linux Enterprise Server for SAP applications 16.0 | All versions | 7.0.2-160000.1.1 |
msgpack-c openSUSE:Leap 16.0 | All versions | 7.0.2-160000.1.1 |
msgpack-c openSUSE:Tumbleweed | All versions | 7.0.2-2.1 |
How do I fix CVE-2026-72854?
Apply the vendor's patch.
Is CVE-2026-72854 being exploited?
It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-72854 exists.
Sources
Also known as AZL-97458, AZL-97512, AZL-97515, DEBIAN-CVE-2026-72854, GHSA-p5jw-mm77-f8jp, MGASA-2026-0533 and 4 more.