Arnott

**Name of the Vulnerable Software and Affected Versions** MessagePack for C# versions prior to 2.5.301 MessagePack for C# versions prior to 3.1.7 **Description** JSON conversion helpers contain multiple recursion paths that do not consistently enforce a depth limit. Specifically, the `MessagePackSerializer.ConvertFromJson` function recursively processes nested JSON arrays and objects within `FromJsonCore()` without consulting `MessagePackSecurity.MaximumObjectGraphDepth`. Additionally, `TinyJsonReader.ReadNextToken()` recursively consumes comma and colon separator characters, which allows malformed JSON with long separator runs to consume one stack frame per character. Furthermore, while `MessagePackSerializer.ConvertToJson` applies depth checks to arrays and maps, the typeless extension branch for ext-100 recursively calls `ToJsonCore()` without applying `MessagePackSecurity.DepthStep(ref reader)`. These issues allow attacker-controlled input to exhaust the process stack, triggering an uncatchable StackOverflowException instead of a catchable parse or serialization exception. **Recommendations** Update to version 2.5.301. Update to version 3.1.7.

**Name of the Vulnerable Software and Affected Versions** MessagePack for C# versions prior to 2.5.301 MessagePack for C# versions prior to 3.1.7 **Description** The parameterless `MessagePackInputFormatter()` constructor uses default serializer options that resolve to `MessagePackSerializerOptions.Standard` with `MessagePackSecurity.TrustedData`. Because this formatter is designed for ASP.NET Core MVC request bodies that cross HTTP trust boundaries, this default configuration can expose applications to denial-of-service attacks. Specifically, it allows for hash-collision attacks against dictionary-like model properties, which `MessagePackSecurity.UntrustedData` is intended to prevent. **Recommendations** Update to version 2.5.301 or later. Update to version 3.1.7 or later.

**Name of the Vulnerable Software and Affected Versions** MessagePack for C# versions prior to 2.5.301 MessagePack for C# versions prior to 3.1.7 **Description** The `ExpandoObjectFormatter.Deserialize()` function populates `System.Dynamic.ExpandoObject` by calling `IDictionary<string, object>.Add` for each map entry. Because `ExpandoObject` maintains member names in array-like structures, inserting a large number of distinct keys can lead to repeated linear scans and array copies. When processing large attacker-controlled maps, this results in quadratic CPU and memory allocation behavior. This occurs even when `ExpandoObjectResolver.Options` is configured with `MessagePackSecurity.UntrustedData`, as collision-resistant dictionary comparers do not protect the internal insertion mechanisms of `ExpandoObject`. **Recommendations** Update to version 2.5.301. Update to version 3.1.7.

**Name of the Vulnerable Software and Affected Versions** MessagePack for C# versions prior to 2.5.301 MessagePack for C# versions prior to 3.1.7 **Description** 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. Because the allocation of the target multi-dimensional array occurs before the dimensions are checked against the guarded element array header, a small payload can declare large dimensions and provide a tiny or empty inner array, leading to a large heap allocation before element data is validated. **Recommendations** Update to version 2.5.301. Update to version 3.1.7.

**Name of the Vulnerable Software and Affected Versions** MessagePack for C# versions prior to 2.5.301 MessagePack for C# versions prior to 3.1.7 **Description** Runtime-generated union deserializers emitted by `DynamicUnionResolver` fail to call `MessagePackSecurity.DepthStep(ref reader)` and do not decrement `reader.Depth` during recursive deserialization and skip paths. Consequently, union deserialization does not consistently adhere to the maximum object graph depth enforcement used in other recursive formatter paths. When encountering unknown union keys, the emitted deserializer executes `reader.Skip()` on attacker-controlled data without an enclosing depth step. **Recommendations** Update to version 2.5.301. Update to version 3.1.7.

**Name of the Vulnerable Software and Affected Versions** MessagePack for C# versions prior to 2.5.301 MessagePack for C# versions prior to 3.1.7 **Description** The `UnsafeBlitFormatterBase<T>.Deserialize()` function reads an attacker-controlled `byteLength` from an extension payload and allocates an array based on that value before validating it against the extension header length or remaining payload bytes. While the outer extension header is bounded by available input, this bound is not used to constrain the inner `byteLength` before allocation, allowing a small payload to request a very large `T[]` allocation. **Recommendations** Update to version 2.5.301. Update to version 3.1.7.

**Name of the Vulnerable Software and Affected Versions** MessagePack for C# versions prior to 2.5.301 MessagePack for C# versions prior to 3.1.7 **Description** An issue exists in the optional LZ4 decompression path used by compression modes `Lz4Block` and `Lz4BlockArray`. The decoder uses a deprecated fast-decompression algorithm that lacks a source-length bound. A remote attacker can send a crafted MessagePack payload with manipulated LZ4 token/length fields to cause out-of-bounds reads from the compressed input buffer. This can trigger an `AccessViolationException` during decompression, leading to process termination and denial of service. In certain conditions, limited unintended memory disclosure from over-read data may occur before the process fails. This affects applications that deserialize untrusted data with LZ4 compression enabled. **Recommendations** Update to version 2.5.301 or later. Update to version 3.1.7 or later. Disable LZ4 compression for untrusted input paths (`Lz4Block`, `Lz4BlockArray`). Only accept compressed payloads from strongly trusted producers. Isolate deserialization in a separate process or container with restart supervision to limit availability impact.