MessagePack for C# is a MessagePack serializer for C#. Prior to 2.5.301 and 3.1.7, when MessagePack-CSharp decompresses Lz4Block or Lz4BlockArray payloads, it reads declared uncompressed lengths from the wire and allocates output buffers based on those lengths before validating that the compressed data is valid or that the declared expansion is reasonable. A small payload can claim a very large uncompressed length and force a large allocation before LZ4 decoding begins. This vulnerability is fixed in 2.5.301 and 3.1.7.
Affected: NuGet: MessagePack
## Summary
When MessagePack-CSharp decompresses `Lz4Block` or `Lz4BlockArray` payloads, it reads declared uncompressed lengths from the wire and allocates output buffers based on those lengths before validating that the compressed data is valid or that the declared expansion is reasonable.
A small payload can claim a very large uncompressed length and force a large allocation before LZ4 decoding begins.
## Impact
Applications are affected when they deserialize attacker-controlled MessagePack payloads with `MessagePackCompression.Lz4Block` or `MessagePackCompression.Lz4BlockArray` enabled.
In the `Lz4Block` case, an attacker-controlled integer is used to request the destination span. In the `Lz4BlockArray` case, per-block uncompressed lengths and their aggregate can be attacker-controlled. Without a cap, the declared output size can be disproportionate to the input size, producing out-of-memory exceptions, process termination on constrained hosts, or severe memory pressure.
This advisory is about unbounded allocation from declared decompressed sizes. It is separate from the LZ4 source-buffer over-read issue, which concerns unsafe decoder reads beyond the compressed input buffer.
## Affected components
- Package: `MessagePack` - Feature: LZ4 compressed MessagePack payloads - APIs: `MessagePackSerializer` with `WithCompression(MessagePackCompression.Lz4Block)` or `WithCompression(MessagePackCompression.Lz4BlockArray)` - Internal routine: `MessagePackSerializer.TryDecomp…