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GHSA-2f33-pr97-265q

GHSA-2f33-pr97-265q is a CWE-1188 vulnerability in MessagePack. O3 Security confirms whether GHSA-2f33-pr97-265q is actually reachable in your code before you act, and blocks exploitation at runtime until you patch.

MessagePack-CSharp: ASP.NET Core MessagePackInputFormatter defaults to TrustedData for HTTP request bodies

Also known asCVE-2026-48509
Published
Jun 25, 2026
Updated
Jun 25, 2026
Affected
2 pkgs
Patched
2 / 2
Exploits
None indexed
Exploitation data as of Jun 25, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

Real-World Exposure

2 pkgs affected
.NETMessagePack.NETMessagePack

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects NuGet packages — download data is not available via public APIs for these ecosystems.

Description

Summary

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.

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 components

  • Package: MessagePack.AspNetCoreMvcFormatter
  • API: MessagePackInputFormatter() parameterless constructor
  • Scenario: ASP.NET Core MVC model binding from HTTP request bodies
  • Finding IDs: MESSAGEPACKCSHARP-OPEN-009, duplicate MESSAGEPACKCSHARP-095

Patches

Fixes are prepared and will be released in coordinated patch versions.

Upgrade guidance:

  1. Upgrade MessagePack.AspNetCoreMvcFormatter to the patched version for your release line.
  2. 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.

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 posture
  • MESSAGEPACKCSHARP-095: duplicate finding for the same root cause
  • CWE-1188: Initialization of a Resource with an Insecure Default

Affected Packages

2 total 2 fixed
EcosystemPackageVulnerable rangeFix
.NETNuGetMessagePackall versions2.5.301
.NETNuGetMessagePack≥ 3.0&&< 3.1.73.1.7

Detection & mitigation playbook

Open-source dependency

  1. Detect

    Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for MessagePack. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.

  2. Fix

    Update MessagePack to 2.5.301 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-2f33-pr97-265q is resolved across your whole dependency graph.

  3. Workarounds

    If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

  4. How O3 protects you

    O3 pinpoints whether GHSA-2f33-pr97-265q is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.

Tailored to GHSA-2f33-pr97-265q. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.

Frequently Asked Questions

## Summary 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. ## Impact Applications are affected when they register `new MessagePackIn
O3 Security · Impact-Aware SCA

Is GHSA-2f33-pr97-265q in your dependencies?

O3 detects GHSA-2f33-pr97-265q across NuGet dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.

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GHSA-2f33-pr97-265q: MessagePack | O3 Security