Vulnerability 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.
CVSS Score
CRITICAL
Affected Products
| Vendor | Product | Versions |
|---|---|---|
| Messagepack | Messagepack | < 2.5.301 |
Related Weaknesses (CWE)
References
- https://github.com/MessagePack-CSharp/MessagePack-CSharp/security/advisories/GHSMitigationVendor Advisory
FAQ
What is CVE-2026-48509?
CVE-2026-48509 is a vulnerability with a CVSS score of 9.1 (CRITICAL). 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 MessagePa...
How severe is CVE-2026-48509?
CVE-2026-48509 has been rated CRITICAL with a CVSS base score of 9.1/10. This is considered a critical vulnerability requiring immediate attention.
Is there a patch for CVE-2026-48509?
Check the references section above for vendor advisories and patch information. Affected products include: Messagepack Messagepack.