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.NET
.NET NuGet
Not in CISA KEV
HIGH severity

GHSA-2x83-8g95-xh59

HIGH

GHSA-2x83-8g95-xh59 is a high-severity (CVSS 7.5) CWE-407 vulnerability in MessagePack. O3 Security confirms whether GHSA-2x83-8g95-xh59 is actually reachable in your code before you act, and blocks exploitation at runtime until you patch.

MessagePack-CSharp: ExpandoObject formatter can perform quadratic insertion work on untrusted maps

Also known asCVE-2026-48511
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

ExpandoObjectFormatter.Deserialize populates System.Dynamic.ExpandoObject by calling IDictionary<string, object>.Add for each map entry. ExpandoObject internally maintains member names in array-like structures, so inserting many distinct keys can require repeated linear scans and array copies.

For large attacker-controlled maps, this produces quadratic CPU and allocation behavior. The issue is especially surprising because ExpandoObjectResolver.Options is configured with MessagePackSecurity.UntrustedData, but collision-resistant dictionary comparers cannot protect ExpandoObject insertion internals.

Impact

Applications are affected when they deserialize untrusted MessagePack maps into ExpandoObject using ExpandoObjectResolver or related resolver options.

A hostile payload containing many distinct keys can cause CPU exhaustion and allocation churn disproportionate to the input size. This can make a server unresponsive or exhaust memory under concurrent request load.

This is not a hash-collision attack against a configurable dictionary comparer. The super-linear behavior comes from ExpandoObject's insertion model, so MessagePackSecurity.UntrustedData does not eliminate the cost.

Affected components

  • Package: MessagePack
  • APIs: ExpandoObjectFormatter.Deserialize, ExpandoObjectResolver
  • Data type: System.Dynamic.ExpandoObject
  • Finding ID: MESSAGEPACKCSHARP-102

Patches

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

Upgrade guidance:

  1. Upgrade MessagePack to the patched version for your release line.
  2. Upgrade companion MessagePack packages in the same dependency graph to the coordinated patched versions.

Potential fixes include applying a map-entry count limit for ExpandoObject under untrusted-data settings, buffering into a security-aware dictionary before materializing a bounded ExpandoObject, or otherwise rejecting maps large enough to trigger quadratic behavior.

Workarounds

Patching is recommended.

Until a patched version is available, avoid deserializing untrusted payloads into ExpandoObject. Prefer strongly typed DTOs or dictionaries with security-aware comparers and explicit count limits. Enforce request-size and map-entry limits at the transport or application layer.

Resources

  • MESSAGEPACKCSHARP-102: ExpandoObjectFormatter quadratic insertion behavior
  • CWE-407: Inefficient Algorithmic Complexity

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-2x83-8g95-xh59 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-2x83-8g95-xh59 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-2x83-8g95-xh59. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.

Frequently Asked Questions

## Summary `ExpandoObjectFormatter.Deserialize` populates `System.Dynamic.ExpandoObject` by calling `IDictionary<string, object>.Add` for each map entry. `ExpandoObject` internally maintains member names in array-like structures, so inserting many distinct keys can require repeated linear scans and array copies. For large attacker-controlled maps, this produces quadratic CPU and allocation behavior. The issue is especially surprising because `ExpandoObjectResolver.Options` is configured with `MessagePackSecurity.UntrustedData`, but collision-resistant dictionary comparers cannot protect `Exp
O3 Security · Impact-Aware SCA

Is GHSA-2x83-8g95-xh59 in your dependencies?

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GHSA-2x83-8g95-xh59: MessagePack (High… | O3 Security