MessagePack-CSharp: LZ4 decompression allocates from unbounded declared output lengths

published 2026-06-22 4 mo ago modified 2026-06-25 4 mo ago also GHSA-v72x-2h86-7f8m , EUVD-2026-38386

cvss: 7.5; version 3.1, NVD
cisa kev: Not listed; not in CISA's catalog
epss: 0.4%; EPSS 0.4% chance of exploitation in 30 days, percentile 35%
public exploits: 0; none known

MessagePack for C# is a MessagePack serializer for C#. Prior to 2.5.301 and 3.1.7, when MessagePack-CSharp decompresses Lz4Block or Lz4BlockArray payloads, it reads declared uncompressed lengths from the wire and allocates output buffers based on those lengths before validating that the compressed data is valid or that the declared expansion is reasonable. A small payload can claim a very large uncompressed length and force a large allocation before LZ4 decoding begins. This vulnerability is fixed in 2.5.301 and 3.1.7.

Patch / fix

fixed in 2

nuget MessagePack 2.5.301, 3.1.7
messagepack messagepack 2.5.301, 3.1.7

CISA SSVC

Exploitation: none
Automatable: no
Technical impact: partial
Assessed: 4 mo ago (2026-06-23)

Exploit prediction

0.40% chance of exploitation in the next 30 days, higher than 35% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by	version	score	vector
CNA	4.0	6.3	CVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N
CNA	4.0	6.3	CVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:N/VC:N/VI:N/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
NVD	3.1	7.5	CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
GitHub	3.1	7.5	CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Affected packages

ecosystem	package	affected	fixed in
nuget	MessagePack	<2.5.301 >=3.0, <3.1.7	2.5.301 3.1.7

Affected products

MessagePack-CSharp MessagePack-CSharp

Classification

Weakness

CWE-409 Improper Handling of Highly Compressed Data (Data Amplification)
CWE-770 Allocation of Resources Without Limits or Throttling

References

github.com /MessagePack-CSharp/MessagePack-CSharp/security/advisories/GHSA-v72x-2h86-7f8m
web, mitigation, vendor-advisory
nvd.nist.gov /vuln/detail/CVE-2026-48510
advisory
github.com /MessagePack-CSharp/MessagePack-CSharp
package

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.

Summary