Summary
MessagePack-CSharp: ASP.NET Core MessagePackInputFormatter defaults to TrustedData for HTTP request bodies
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.
Affected components
- Package:
MessagePack.AspNetCoreMvcFormatter - API:
MessagePackInputFormatter()parameterless constructor - Scenario: ASP.NET Core MVC model binding from HTTP request bodies
- Finding IDs:
MESSAGEPACKCSHARP-OPEN-009, duplicateMESSAGEPACKCSHARP-095
Workarounds
Do not use the parameterless constructor on affected versions. Register the formatter with explicit untrusted-data options, for example:
options.InputFormatters.Add(
new MessagePackInputFormatter(
MessagePackSerializerOptions.Standard.WithSecurity(MessagePackSecurity.UntrustedData)));
Also apply normal HTTP request-size limits and model validation appropriate for your service.
Resources
MESSAGEPACKCSHARP-OPEN-009: MVC input formatter defaults to trusted-data security postureMESSAGEPACKCSHARP-095: duplicate finding for the same root cause- CWE-1188: Initialization of a Resource with an Insecure Default
Impact
Applications are affected when they register new MessagePackInputFormatter() without explicitly passing serializer options configured for untrusted data.
An unauthenticated or otherwise untrusted HTTP client can send MessagePack request bodies that are deserialized using the trusted-data posture. For models containing hash-based collections, this can enable algorithmic complexity attacks using colliding keys. The default constructor makes the unsafe posture easy to use at the exact boundary where request bodies should be treated as untrusted.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
Already deployed Kodem?
See it in your environmentNew to Kodem? Get a demo →Remediation advice
Fixes are prepared and will be released in coordinated patch versions.
Upgrade guidance:
- Upgrade
MessagePack.AspNetCoreMvcFormatterto the patched version for your release line. - Upgrade companion MessagePack packages in the same dependency graph to the coordinated patched versions.
The fix should default the parameterless constructor to MessagePackSerializerOptions.Standard.WithSecurity(MessagePackSecurity.UntrustedData), or require callers to pass explicit options so the trust posture is deliberate.
Frequently Asked Questions
- What is CVE-2026-48509? CVE-2026-48509 is a medium-severity security vulnerability in MessagePack (nuget), affecting versions < 2.5.301. It is fixed in 2.5.301, 3.1.7.
- Which versions of MessagePack are affected by CVE-2026-48509? MessagePack (nuget) versions < 2.5.301 is affected.
- Is there a fix for CVE-2026-48509? Yes. CVE-2026-48509 is fixed in 2.5.301, 3.1.7. Upgrade to this version or later.
- Is CVE-2026-48509 exploitable, and should I be worried? Whether CVE-2026-48509 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2026-48509 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2026-48509?
- Upgrade
MessagePackto 2.5.301 or later - Upgrade
MessagePackto 3.1.7 or later
- Upgrade