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

• NuGet MessagePack: < 2.5.301 → fixed in 2.5.301 • NuGet MessagePack: >= 3.0, < 3.1.7 → fixed in 3.1.7

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 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