MessagePack-CSharp's multi-dimensional array formatters (TwoDimensionalArrayFormatter<T>.Deserialize, ThreeDimensionalArrayFormatter<T>.Deserialize, FourDimensionalArrayFormatter<T>.Deserialize) allocate arrays based on dimension lengths read directly from untrusted payloads before validating that the product of these dimensions matches the encoded element count. This unchecked allocation can cause large heap allocations, out-of-memory exceptions, and CPU overhead from zero-initializing oversized arrays. The vulnerability affects deserialization of multi-dimensional arrays (T[,], T[,,], T[,,,]) in MessagePack versions <2.5.301 and >=3.0 <3.1.7. Coordinated patches are prepared but not yet released. The fix will validate dimension values and their product against the element count and payload size before allocation. Workarounds include avoiding deserialization of untrusted payloads into multi-dimensional arrays and using bounded or jagged arrays instead.

Exploitation of this vulnerability can cause applications to allocate excessive memory during deserialization of untrusted MessagePack payloads containing multi-dimensional arrays. This can lead to out-of-memory exceptions, termination of containerized applications on memory-constrained hosts, increased large object heap pressure, and high CPU usage due to zero-initialization of large arrays. There is no general allocation cap in the current MessagePackSecurity.UntrustedData path to prevent this. The impact is limited to scenarios where untrusted data is deserialized into multi-dimensional arrays.

A patch is prepared and will be released in coordinated versions of MessagePack. Users should upgrade to the patched versions once available. Until then, avoid deserializing untrusted MessagePack payloads into models containing multi-dimensional arrays (T[,], T[,,], T[,,,]). Prefer using schema shapes that allow validation before allocation, such as bounded lists, dictionaries with application-level count limits, or jagged arrays with limits. Message-size limits can reduce but not eliminate the risk. Monitor vendor advisories for the release of patched versions and upgrade accordingly.