msgpack-c Integer Overflow in msgpack_unpacker_expand_buffer Causes a False-Success Undersized Reservation
Summary
| CVE | CVE-2026-72854 |
|---|---|
| State | PUBLISHED |
| Assigner | VulnCheck |
| Source Priority | CVE Program / NVD first with legacy fallback |
| Published | 2026-08-20 19:17:01 UTC |
| Updated | 2026-08-20 19:17:01 UTC |
| 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. |
Risk And Classification
Primary CVSS: v4.0 5.8 MEDIUM from [email protected]
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/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X
Problem Types: CWE-190 | CWE-787 | CWE-190 Integer Overflow or Wraparound | CWE-787 Out-of-bounds Write
| Version | Source | Type | Score | Severity | Vector |
|---|---|---|---|---|---|
| 4.0 | [email protected] | Secondary | 5.8 | MEDIUM | 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/E:X/C... |
| 4.0 | CNA | CVSS | 5.8 | MEDIUM | 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 |
| 3.1 | [email protected] | Primary | 5.3 | MEDIUM | CVSS:3.1/AV:L/AC:H/PR:N/UI:R/S:U/C:N/I:H/A:L |
| 3.1 | CNA | CVSS | 5.3 | MEDIUM | CVSS:3.1/AV:L/AC:H/PR:N/UI:R/S:U/C:N/I:H/A:L |
CVSS v4.0 Breakdown
Attack Vector
LocalAttack Complexity
HighAttack Requirements
PresentPrivileges Required
NoneUser Interaction
PassiveConfidentiality
NoneIntegrity
HighAvailability
LowSub Conf.
NoneSub Integrity
NoneSub Availability
NoneCVSS:4.0/AV:L/AC:H/AT:P/PR:N/UI:P/VC:N/VI:H/VA:L/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X
CVSS v3.1 Breakdown
Attack Vector
LocalAttack Complexity
HighPrivileges Required
NoneUser Interaction
RequiredScope
UnchangedConfidentiality
NoneIntegrity
HighAvailability
LowCVSS:3.1/AV:L/AC:H/PR:N/UI:R/S:U/C:N/I:H/A:L
Vendor Declared Affected Products
References
| Reference | Source | Link | Tags |
|---|---|---|---|
| github.com/msgpack/msgpack-c/blob/c-7.0.1/example/lib_buffer_unpack.c | [email protected] | github.com | |
| github.com/msgpack/msgpack-c/blob/c-7.0.1/include/msgpack/unpack.h | [email protected] | github.com | |
| github.com/msgpack/msgpack-c/blob/c-7.0.1/src/unpack.c | [email protected] | github.com | |
| github.com/msgpack/msgpack-c | [email protected] | github.com | |
| github.com/msgpack/msgpack-c/issues/1181 | 134c704f-9b21-4f2e-91b3-4a467353bcc0 | github.com | |
| www.vulncheck.com/advisories/msgpack-c-integer-overflow-in-msgpack-unpacker-exp... | [email protected] | www.vulncheck.com | |
| CVE Program record | CVE.ORG | www.cve.org | canonical |
| NVD vulnerability detail | NVD | nvd.nist.gov | canonical, analysis |
Vendor Comments And Credit
Discovery Credit
CNA: mtholmquist (en)
There are currently no legacy QID mappings associated with this CVE.