Summary
MessagePack-CSharp: JSON conversion APIs can recurse without consistent depth enforcement
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:
MessagePackSerializer.ConvertFromJsonrecursively processes nested JSON arrays and objects inFromJsonCore()without consultingMessagePackSecurity.MaximumObjectGraphDepth.TinyJsonReader.ReadNextToken()recursively consumes comma and colon separator characters, allowing even malformed JSON with long separator runs to consume one stack frame per character.MessagePackSerializer.ConvertToJsonapplies depth checks to arrays and maps, but the typeless extension branch for ext-100 recursively callsToJsonCore()without applyingMessagePackSecurity.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.
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
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:ConvertFromJsonunbounded structural recursionMESSAGEPACKCSHARP-091:TinyJsonReader.ReadNextTokenseparator self-recursionMESSAGEPACKCSHARP-092:ConvertToJsonext-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.
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 versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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See it in your environmentNew to Kodem? Get a demo →Remediation advice
Fixes are prepared and will be released in coordinated patch versions.
Upgrade guidance:
- Upgrade
MessagePackto the patched version for your release line. - 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.
Frequently Asked Questions
- What is CVE-2026-48512? CVE-2026-48512 is a medium-severity security vulnerability in MessagePack (nuget), affecting versions < 2.5.301. It is fixed in 2.5.301, 3.1.7.
- Which versions of MessagePack are affected by CVE-2026-48512? MessagePack (nuget) versions < 2.5.301 is affected.
- Is there a fix for CVE-2026-48512? Yes. CVE-2026-48512 is fixed in 2.5.301, 3.1.7. Upgrade to this version or later.
- Is CVE-2026-48512 exploitable, and should I be worried? Whether CVE-2026-48512 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2026-48512 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2026-48512?
- Upgrade
MessagePackto 2.5.301 or later - Upgrade
MessagePackto 3.1.7 or later
- Upgrade