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HIGH severity

GHSA-v72x-2h86-7f8m

HIGH

GHSA-v72x-2h86-7f8m is a high-severity (CVSS 7.5) CWE-409 vulnerability in MessagePack. O3 Security confirms whether GHSA-v72x-2h86-7f8m is actually reachable in your code before you act, and blocks exploitation at runtime until you patch.

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

Also known asCVE-2026-48510
Published
Jun 25, 2026
Updated
Jun 25, 2026
Affected
2 pkgs
Patched
2 / 2
Exploits
None indexed
Exploitation data as of Jun 25, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

Real-World Exposure

2 pkgs affected
.NETMessagePack.NETMessagePack

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects NuGet packages — download data is not available via public APIs for these ecosystems.

Description

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 unbounded allocation from declared decompressed sizes. It is separate from the LZ4 source-buffer over-read issue, which concerns unsafe decoder reads beyond the compressed input buffer.

Affected components

  • Package: MessagePack
  • Feature: LZ4 compressed MessagePack payloads
  • APIs: MessagePackSerializer with WithCompression(MessagePackCompression.Lz4Block) or WithCompression(MessagePackCompression.Lz4BlockArray)
  • Internal routine: MessagePackSerializer.TryDecompress
  • Finding ID: MESSAGEPACKCSHARP-OPEN-004

Patches

Fixes are prepared and will be released in coordinated patch versions.

Upgrade guidance:

  1. Upgrade MessagePack to the patched version for your release line.
  2. Upgrade companion MessagePack packages in the same dependency graph to the coordinated patched versions.

The fix should reject negative and excessive uncompressed lengths before allocation. It should also cap aggregate decompressed size for block arrays and expose or honor an appropriate maximum decompressed length policy.

Workarounds

Patching is recommended.

Until a patched version is available, do not enable MessagePack-CSharp's built-in LZ4 compression modes for untrusted inputs. If compression is required, enforce strict compressed and decompressed size limits outside MessagePack-CSharp before deserialization.

Resources

  • MESSAGEPACKCSHARP-OPEN-004: LZ4 decompression allocation from unbounded uncompressed length
  • MESSAGEPACKCSHARP-011: duplicate decompression-bomb finding
  • CWE-409: Improper Handling of Highly Compressed Data
  • CWE-770: Allocation of Resources Without Limits or Throttling

Affected Packages

2 total 2 fixed
EcosystemPackageVulnerable rangeFix
.NETNuGetMessagePackall versions2.5.301
.NETNuGetMessagePack≥ 3.0&&< 3.1.73.1.7

Detection & mitigation playbook

Open-source dependency

  1. Detect

    Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for MessagePack. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.

  2. Fix

    Update MessagePack to 2.5.301 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-v72x-2h86-7f8m is resolved across your whole dependency graph.

  3. Workarounds

    If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

  4. How O3 protects you

    O3 pinpoints whether GHSA-v72x-2h86-7f8m is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.

Tailored to GHSA-v72x-2h86-7f8m. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.

Frequently Asked Questions

## 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.
O3 Security · Impact-Aware SCA

Is GHSA-v72x-2h86-7f8m in your dependencies?

O3 detects GHSA-v72x-2h86-7f8m across NuGet dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.

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GHSA-v72x-2h86-7f8m: MessagePack (High… | O3 Security