CVE-2026-72854

What it is

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.

At a glance

Exploitability

Can be exploited locally on the machine without logging in, but needs a user to act. High risk of data tampering.

Exploit odds

0.2% chance of exploitation in the next 30 days (EPSS).

Fix

No patch or advisory is linked yet. Look at mitigations or restrict exposure.

Timeline

1. 2026-08-20 19:17 UTC PublishedDisclosed publicly on CVE.org and NVD
2. 2026-10-18 20:10 UTC Last updatedMost recent change to the record

Why it ranks here

• Exploited in the wild: no, CISA does not list it.
• Exploit odds: 0.2% in the next 30 days (EPSS).
• Severity: 5.8 MEDIUM (CVSS).
• Fix: none linked yet, which raises it.

Details

Published

2026-08-20 19:17 UTC

Last modified

2026-10-18 20:10 UTC

Exploit probability

0.2% · EPSS, next 30 days

Weakness

CWE-190 · Integer overflow

Record status

Deferred

Attack profile CVSS 4.0 · 5.8

Attack vector

Local

Complexity

High

Attack requirements

Present

Privileges needed

None

User interaction

Passive

Confidentiality

None

Integrity

High

Availability

Low

Is it in your code?

Hand this to your coding agent. It searches your project for the affected products and checks versions. Nothing leaves your machine.

Read https://cve-list.com/agent/cve/CVE-2026-72854.md and follow it. Check whether this project is affected by CVE-2026-72854. Read only. Don't change anything and don't upload my code. Tell me yes, no or not sure, with the file as proof.

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