GHSA-2x83-8g95-xh59

## 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