CVE-2026-48512: MessagePack-CSharp: JSON conversion APIs can recurse without consistent depth enforcement

Summary

MessagePack-CSharp's JSON conversion helpers contain multiple recursion paths that do not consistently enforce a depth limit. These paths are in the JSON conversion component rather than normal typed MessagePack deserialization.

Three related issues are covered by this advisory:

1. MessagePackSerializer.ConvertFromJson recursively processes nested JSON arrays and objects in FromJsonCore() without consulting MessagePackSecurity.MaximumObjectGraphDepth. 2. TinyJsonReader.ReadNextToken() recursively consumes comma and colon separator characters, allowing even malformed JSON with long separator runs to consume one stack frame per character. 3. MessagePackSerializer.ConvertToJson applies depth checks to arrays and maps, but the typeless extension branch for ext-100 recursively calls ToJsonCore() without applying MessagePackSecurity.DepthStep(ref reader).

Each path can allow attacker-controlled input to exhaust the process stack and trigger an uncatchable StackOverflowException instead of failing with a catchable parse or serialization exception.

Impact

Applications are affected when they call MessagePack-CSharp JSON conversion APIs on attacker-controlled data. This includes gateways, diagnostics endpoints, migration tools, logging paths, and services that convert between external JSON and MessagePack payloads.

For JSON-to-MessagePack conversion, deeply nested JSON arrays or objects can recurse through FromJsonCore() without applying the configured object graph depth limit. Separately, long runs of comma or colon separator characters can recurse through TinyJsonReader.ReadNextToken() before normal structural validation rejects the input.

For MessagePack-to-JSON conversion, nested typeless extension wrappers can recurse through ToJsonCore() without the depth guard that the same function applies to arrays and maps.

MessagePackSecurity.UntrustedData does not fully mitigate these conversion paths because the missing checks occur inside JSON conversion and tokenization branches that do not consistently use the configured depth policy.

Affected components

- Package: MessagePack - APIs: MessagePackSerializer.ConvertFromJson, MessagePackSerializer.ConvertToJson - Internal routines: FromJsonCore, ToJsonCore, TinyJsonReader.ReadNextToken - Data shapes: deeply nested JSON arrays/objects, long JSON separator runs, and nested typeless MessagePack extension values converted to JSON - Finding IDs: MESSAGEPACKCSHARP-090, MESSAGEPACKCSHARP-091, MESSAGEPACKCSHARP-092

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.

The JSON-to-MessagePack fix should add explicit JSON nesting-depth accounting to FromJsonCore, using the configured maximum object graph depth or an equivalent limit, or rewrite the conversion to use an iterative bounded stack.

The tokenizer fix should replace separator self-recursion in TinyJsonReader.ReadNextToken() with an iterative loop so consecutive commas, colons, and whitespace do not consume stack frames.

The MessagePack-to-JSON fix should apply DepthStep and matching reader.Depth-- cleanup around recursive ToJsonCore() calls made from the typeless extension branch, consistent with the existing array and map conversion branches.

Workarounds

Patching is recommended.

Until a patched version is available, do not pass untrusted JSON directly to ConvertFromJson, and do not call ConvertToJson on untrusted MessagePack payloads that may contain typeless extension values. Validate JSON nesting depth with a parser that enforces depth limits before calling MessagePack-CSharp, reject malformed JSON before conversion, and apply strict input-size limits.

Input-size limits reduce exposure but do not remove the recursive behavior in affected versions.

References

- MESSAGEPACKCSHARP-090: ConvertFromJson unbounded structural recursion - MESSAGEPACKCSHARP-091: TinyJsonReader.ReadNextToken separator self-recursion - MESSAGEPACKCSHARP-092: ConvertToJson ext-100 branch missing depth enforcement - CWE-674: Uncontrolled Recursion

CVE split rationale

These issues are grouped because they affect the same JSON conversion feature area and share the same failure mode: recursive conversion/tokenization paths do not consistently enforce depth or iteration bounds for attacker-controlled input. They are distinct from normal binary MessagePack skip recursion, dynamic union formatter depth accounting, DateTime stack allocation, and allocation-oriented denial-of-service issues.