Summary
MessagePack-CSharp: Typeless deserialization type restrictions do not recurse into arrays or generic arguments
MessagePack-CSharp's typeless deserialization includes MessagePackSerializerOptions.ThrowIfDeserializingTypeIsDisallowed(Type) as a safety check for dangerous types. The default implementation checks the outer type name, but it does not recursively inspect array element types or generic type arguments.
As a result, a type that would be blocked directly can be wrapped inside an array or constructed generic type and pass the outer type check. The formatter machinery can then materialize formatters for the inner blocked type.
Affected components
- Package:
MessagePack - Feature: typeless deserialization
- APIs:
MessagePackSerializerOptions.ThrowIfDeserializingTypeIsDisallowed,TypelessFormatter - Finding IDs:
MESSAGEPACKCSHARP-030, duplicate/open variantMESSAGEPACKCSHARP-OPEN-007
Workarounds
Patching is recommended.
Avoid typeless deserialization for untrusted data. If typeless support is unavoidable, configure an explicit allowlist that rejects any type not approved by the application and ensure the allowlist recursively validates array elements and generic arguments. Do not rely on exact outer-type blocklists as a complete security boundary.
Resources
MESSAGEPACKCSHARP-030: typeless disallowed-type check is not recursiveMESSAGEPACKCSHARP-OPEN-007: duplicate/open finding for typeless blocklist gaps- CWE-502: Deserialization of Untrusted Data
- CWE-470: Use of Externally-Controlled Input to Select Classes or Code
CVE split rationale
This vulnerability is independently fixable in typeless type-policy enforcement. It is separate from MVC default options, collection allocation, LZ4 decoding, and recursion-depth issues.
Impact
Applications are affected when they deserialize untrusted payloads using typeless serialization features such as MessagePackSerializer.Typeless, TypelessObjectResolver, or related typeless resolver options.
Typeless deserialization is already a high-risk feature for untrusted data, but the presence of a disallowed-type hook creates an expectation that blocked types remain blocked. This issue weakens that mitigation because the check is not applied structurally to nested type components. An attacker who can supply typeless ext-100 payloads may bypass exact outer-type blocklist checks by naming wrapper types such as arrays or generic containers.
The consequence depends on which type is reached and what the application allows typeless deserialization to instantiate. The original findings describe bypasses involving blocked or user-blocklisted gadget types.
Untrusted serialized data is processed by a deserializer that can instantiate arbitrary objects or execute code as a side effect. Typical impact: arbitrary code execution or logic abuse.
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.
Already deployed Kodem?
See it in your environmentNew to Kodem? Get a demo →Remediation advice
Fixes are available via versions 2.5.301 and 3.1.7.
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 fix should apply type-disallow checks recursively to array element types, pointer/byref element types where applicable, nullable underlying types, and constructed generic type arguments. Formatter paths that materialize types supplied by the wire should not instantiate inner types that fail the configured policy.
Frequently Asked Questions
- What is CVE-2026-48517? CVE-2026-48517 is a medium-severity insecure deserialization vulnerability in MessagePack (nuget), affecting versions < 2.5.301. It is fixed in 2.5.301, 3.1.7. Untrusted serialized data is processed by a deserializer that can instantiate arbitrary objects or execute code as a side effect.
- Which versions of MessagePack are affected by CVE-2026-48517? MessagePack (nuget) versions < 2.5.301 is affected.
- Is there a fix for CVE-2026-48517? Yes. CVE-2026-48517 is fixed in 2.5.301, 3.1.7. Upgrade to this version or later.
- Is CVE-2026-48517 exploitable, and should I be worried? Whether CVE-2026-48517 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-2026-48517 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-2026-48517?
- Upgrade
MessagePackto 2.5.301 or later - Upgrade
MessagePackto 3.1.7 or later
- Upgrade