# CVE-2026-48109: MessagePack for C# out-of-bounds read in LZ4 decompression

Source: https://junglewise.ai/threats/cve-2026-48109-messagepack-for-c-out-of-bounds-read-in-lz4-decompression  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-06-22. Updated: 2026-06-22  
License: CC BY 4.0 with a link back

CVE-2026-48109 · Severity: high · CVSS 8.2 · Published 2026-06-22

MessagePack for C# is a high-performance library used to convert data into a compact binary format for storage or transmission. A security flaw in its data compression feature allows an attacker to send a specially crafted message that crashes the application. In some cases, this could also lead to the accidental exposure of small amounts of sensitive information from the computer's memory.

Technologies: [MessagePack-CSharp MessagePack for C#](https://junglewise.ai/threats/technologies/messagepack-for-c), [MessagePack (NuGet)](https://junglewise.ai/threats/technologies/messagepack). Vendors: [MessagePack-CSharp](https://junglewise.ai/threats/vendors/messagepack-csharp), [NuGet](https://junglewise.ai/threats/vendors/nuget).

## Quick answers

**Is CVE-2026-48109 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-48109 being exploited in the wild.

**How severe is CVE-2026-48109?** High severity, with a CVSS score of 8.2, published 22 June 2026.

**Which products and versions are affected?** MessagePack-CSharp MessagePack for C# < 2.5.301, >= 3.0.0 < 3.1.7, < 2.5.301, >= 3.0.214-rc.1, < 3.1.7; MessagePack (NuGet) ECOSYSTEM:introduced=0,fixed=2.5.301.

**Is there a fix for CVE-2026-48109?** Yes. MessagePack (NuGet): upgrade to 2.5.301 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-48109 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-48109?** Check whether you run MessagePack-CSharp MessagePack for C# or MessagePack (NuGet) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## How to fix

- MessagePack (NuGet): upgrade to 2.5.301 or later

  ```sh
  dotnet add package MessagePack --version 2.5.301
  ```


## Technical details

An out-of-bounds read vulnerability exists in the LZ4 decompression implementation of MessagePack for C#. The root cause is the use of a deprecated fast-decompression algorithm that lacks source-length bound checks. A remote, unauthenticated attacker can exploit this by sending a crafted MessagePack payload with manipulated LZ4 token or length fields. This can result in an AccessViolationException, leading to process termination (Denial of Service), and potentially limited memory disclosure of over-read data. The issue is fixed in versions 2.5.301 and 3.1.7.

## Affected products

- MessagePack-CSharp MessagePack for C# < 2.5.301, >= 3.0.0 < 3.1.7

## Identifiers

- CVE-2026-48109
- GHSA-hv8m-jj95-wg3x

## Timeline

- 2026-06-09: advisory: GitHub advisory published
- 2026-06-22: disclosed: NVD publication date

## References

- [https://github.com/MessagePack-CSharp/MessagePack-CSharp/security/advisories/GHSA-hv8m-jj95-wg3x](https://github.com/MessagePack-CSharp/MessagePack-CSharp/security/advisories/GHSA-hv8m-jj95-wg3x)

## Related threats

- [MessagePack-CSharp memory exhaustion in nested array deserialization](https://junglewise.ai/threats/cve-2026-92707-messagepack-nested-arrays-can-blow-past-buffer-length-limits)
- [MessagePack for C# security bypass in typeless deserialization](https://junglewise.ai/threats/cve-2026-48517-messagepack-for-c-security-bypass-in-typeless-deserialization)
- [MessagePack for C# hash-collision DoS in InterfaceLookupFormatter](https://junglewise.ai/threats/cve-2026-48516-messagepack-for-c-hash-collision-dos-in-interfacelookupformatter)
- [MessagePack for C# resource exhaustion in multi-dimensional array formatters](https://junglewise.ai/threats/cve-2026-48515-messagepack-for-c-resource-exhaustion-in-multi-dimensional-array)
- [MessagePack for C# denial of service in UnsafeBlitFormatterBase](https://junglewise.ai/threats/cve-2026-48514-messagepack-for-c-denial-of-service-in-unsafeblitformatterbase)