MessagePack for C# is a MessagePack serializer for C#. Prior to 2.5.301 and 3.1.7, MessagePack-CSharp's multi-dimensional array formatters read dimension lengths directly from the payload and allocate T[,], T[,,], or T[,,,] before validating that the dimension product matches the encoded element count. The formatter reads a guarded element array header, but allocation of the target multi-dimensional array happens before the dimensions are checked against that element count. A small payload can therefore declare large dimensions, provide an empty or tiny inner array, and cause a large heap allocation before element data is validated. This vulnerability is fixed in 2.5.301 and 3.1.7.

Affected: NuGet: MessagePack

## Summary

MessagePack-CSharp's multi-dimensional array formatters read dimension lengths directly from the payload and allocate `T[,]`, `T[,,]`, or `T[,,,]` before validating that the dimension product matches the encoded element count.

The formatter reads a guarded element array header, but allocation of the target multi-dimensional array happens before the dimensions are checked against that element count. A small payload can therefore declare large dimensions, provide an empty or tiny inner array, and cause a large heap allocation before element data is validated.

## Impact

Applications are affected when they deserialize untrusted MessagePack payloads into models containing multi-dimensional arrays such as `T[,]`, `T[,,]`, or `T[,,,]`.

An attacker can encode large dimension integers and a small guarded element array. The formatter allocates the target array from the dimensions before confirming that the product of dimensions is consistent with the element count.

The result can be out-of-memory exceptions, container termination on memory-constrained hosts, large object heap pressure, or severe CPU cost from zero-initializing oversized arrays. `MessagePackSecurity.UntrustedData` does not provide a general allocation cap for this path.

## Affected components

- Package: `MessagePack` - APIs: `TwoDimensionalArrayFormatter<T>.Deserialize`, `ThreeDimensionalArrayFormatter<T>.Deserialize`, `FourDimensionalArrayFormatter<T>.Deserialize` - Data shapes: `T[,]`, `T[,,]`, and…