CVE-2026-48109 MessagePack-CSharp: LZ4 decompression may fail with AccessViolationException after dereferencing memory from bad input
A remote attacker can cause denial of service and potential memory disclosure in MessagePack-CSharp by sending crafted payloads to the optional LZ4 decompression path.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-48109?
CVE-2026-48109 is a high-severity memory corruption vulnerability in MessagePack-CSharp, published on 22 Jun 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official 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.
Weakness: CWE-20
Which versions are affected by CVE-2026-48109?
4 open source packages ship an affected version, including MessagePack. It affects 1 product.
| Package | Affected | Fixed in |
|---|---|---|
MessagePack NuGet | All versions | 2.5.301 |
MessagePack NuGet | All versions | 2.5.301 |
MessagePack NuGet | From 3.0.214-rc.1 | 3.1.7 |
MessagePack NuGet | From 3.0.214-rc.1 | 3.1.7 |
Products include Messagepack Messagepack.
How do I fix CVE-2026-48109?
Upgrade MessagePack to a fixed version for your release line: 2.5.301 or 3.1.7.
Upgrade to the fixed version. Memory-safe languages, compiler hardening and fuzzing prevent most new cases. More about Memory corruption.
Is CVE-2026-48109 being exploited?
There is no known exploitation of CVE-2026-48109 in the wild and no public exploit code.
Other vulnerabilities in MessagePack-CSharp
- CVE-2026-48502 MessagePack-CSharp: Denial of service vulnerabilities can swamp the CPU or crash the process with stack and heap overflows
- CVE-2026-48506 MessagePack-CSharp: MessagePackReader.Skip can recurse without enforcing maximum object graph depth
- CVE-2026-48509 MessagePack-CSharp: ASP.NET Core MessagePackInputFormatter defaults to TrustedData for HTTP request bodies
- CVE-2026-48510 MessagePack-CSharp: LZ4 decompression allocates from unbounded declared output lengths
- CVE-2026-48511 MessagePack-CSharp: ExpandoObject formatter can perform quadratic insertion work on untrusted maps
Sources
Also known as ECHO-664b-126f-78fd, GHSA-hv8m-jj95-wg3x, MINI-2m69-gr77-5447.