CVE-2026-48509 MessagePack-CSharp: ASP.NET Core MessagePackInputFormatter defaults to TrustedData for HTTP request bodies
Insecure default security settings in MessagePack-CSharp allow attackers to cause denial of service via hash-collision attacks on ASP.NET Core request bodies.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-48509?
CVE-2026-48509 is a medium-severity denial of service 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, the parameterless MessagePackInputFormatter() constructor uses default serializer options, which resolve to MessagePackSerializerOptions.Standard with MessagePackSecurity.TrustedData. The formatter is designed for ASP.NET Core MVC request bodies, which commonly cross an HTTP trust boundary. This insecure default can expose applications to denial-of-service attacks that MessagePackSecurity.UntrustedData is intended to mitigate, such as hash-collision attacks against dictionary-like model properties. This vulnerability is fixed in 2.5.301 and 3.1.7.
Weakness: CWE-1188
Which versions are affected by CVE-2026-48509?
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 | 3.1.7 |
MessagePack NuGet | From 3.0 | 3.1.7 |
Products include Messagepack Messagepack.
How do I fix CVE-2026-48509?
Upgrade MessagePack to a fixed version for your release line: 2.5.301 or 3.1.7.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2026-48509 being exploited?
There is no known exploitation of CVE-2026-48509 in the wild and no public exploit code.
Other vulnerabilities in MessagePack-CSharp
- CVE-2026-48109 MessagePack-CSharp: LZ4 decompression may fail with AccessViolationException after dereferencing memory from bad input
- 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-2026-48510 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-e60c-e8f0-b786, GHSA-2f33-pr97-265q.