CVE-2026-48509: medium-severity vulnerability in MessagePack-CSharp
MessagePack-CSharp: ASP.NET Core MessagePackInputFormatter defaults to TrustedData for HTTP request bodies
Published · Updated
13Vexday Risk Score
No sign of exploitation. No public exploitation artifact known so far.
ssvc Trackcvss 6.3epss 0.4%
exploitation probability
0.4%top 66% of all CVEs
observed exploitation
nono source reports it
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:4.0/AV:N/AC:H/AT:P/PR:N/UI:N/VC:N/VI:L/VA:L/SC:N/SI:N/SA:N
Affected products
MessagePack-CSharp · MessagePack-CSharpRelated CVEs — MessagePack-CSharp
In the same product, most dangerous first.
CVE-2026-48109HIGHMessagePack-CSharp: LZ4 decompression may fail with AccessViolationException after dereferencing memory from bad inputEPSS 0.5%CVE-2026-48506HIGHMessagePack-CSharp: MessagePackReader.Skip can recurse without enforcing maximum object graph depthEPSS 0.5%CVE-2026-48502HIGHMessagePack-CSharp: Denial of service vulnerabilities can swamp the CPU or crash the process with stack and heap overflowsEPSS 0.4%CVE-2026-48510MEDIUMMessagePack-CSharp: LZ4 decompression allocates from unbounded declared output lengthsEPSS 0.4%CVE-2026-48511MEDIUMMessagePack-CSharp: ExpandoObject formatter can perform quadratic insertion work on untrusted mapsEPSS 0.4%CVE-2026-48512MEDIUMMessagePack-CSharp: JSON conversion APIs can recurse without consistent depth enforcementEPSS 0.4%