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EUVD-2026-330292

2026-10-06T16:43:13.131099+00:00

EUVD-2026-330292

fkie_cve-2026-48511

2026-10-06T16:43:13.131144+00:00

MessagePack for C# is a MessagePack serializer for C#. Prior to 2.5.301 and 3.1.7, ExpandoObjectFormatter.Deserialize populates System.Dynamic.ExpandoObject by calling IDictionary.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. This vulnerability is fixed in 2.5.301 and 3.1.7.

GHSA-2x83-8g95-xh59 — MessagePack-CSharp: ExpandoObject formatter can perform quadratic insertion work on untrusted maps

2026-10-06T16:43:13.131196+00:00

Affected: NuGet: MessagePack

## Summary

`ExpandoObjectFormatter.Deserialize` populates `System.Dynamic.ExpandoObject` by calling `IDictionary.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 coor…