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

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS 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 · score source

EPSS

• CVE-2026-48510: 0.40% estimated exploitation activity in the next 30 days; percentile 34.80 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary 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. Impact Applications are affected when they deserialize attacker-controlled MessagePack payloads with MessagePackCompression.Lz4Block or MessagePackCompression.Lz4BlockArray enabled. In the Lz4Block case, an attacker-controlled integer is used to request the destination span. In the Lz4BlockArray case, per-block uncompressed lengths and their aggregate can be attacker-controlled. Without a cap, the declared output size can be disproportionate to the input size, producing out-of-memory exceptions, process termination on constrained hosts, or severe memory pressure. This advisory is about…

Published by the source

25 Jun 2026, 18:53 UTC

Last modified by the source

25 Jun 2026, 18:53 UTC

Source last checked

26 Oct 2026, 17:04 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-v72x-2h86-7f8m

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# Patch Next advisory brief: CVE-2026-48510

Advisory: MessagePack-CSharp: LZ4 decompression allocates from unbounded declared output lengths
IDs: CVE-2026-48510, GHSA-v72x-2h86-7f8m
Record: https://patchnext.com/advisories/cve-2026-48510 (revision 1, source checked 26 Oct 2026, 17:04 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 6.3)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: messagepack 3.1.7, 2.5.301. A vendor fix does not mean a deployment is fixed.

## Affected components
- NuGet messagepack: affected < 2.5.301; >= 3.0, < 3.1.7; fixed in 2.5.301, 3.1.7

## Suggested inspection (read-only)
- NuGet messagepack: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on messagepack.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-v72x-2h86-7f8m (GitHub-reviewed advisory GHSA-v72x-2h86-7f8m lists affected ranges: NuGet messagepack >=0 =3.0 <3.1.7)

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