MessagePack-CSharp: Typeless deserialization type restrictions do not recurse into arrays or generic arguments

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS 4.0

6.3 · CVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:N/VC:N/VI:L/VA:N/SC:N/SI:N/SA:N · score source

EPSS

• CVE-2026-48517: 0.35% estimated exploitation activity in the next 30 days; percentile 28.41 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary 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. 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…

Published by the source

25 Jun 2026, 21:31 UTC

Last modified by the source

25 Jun 2026, 21:31 UTC

Source last checked

26 Oct 2026, 17:04 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-qhmf-xw27-6rqr

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# Patch Next advisory brief: CVE-2026-48517

Advisory: MessagePack-CSharp: Typeless deserialization type restrictions do not recurse into arrays or generic arguments
IDs: CVE-2026-48517, GHSA-qhmf-xw27-6rqr
Record: https://patchnext.com/advisories/cve-2026-48517 (revision 1, source checked 26 Oct 2026, 17:04 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 6.3)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: messagepack 3.1.7, 2.5.301. A vendor fix does not mean a deployment is fixed.

## Affected components
- NuGet messagepack: affected < 2.5.301; >= 3.0, < 3.1.7; fixed in 2.5.301, 3.1.7

## Suggested inspection (read-only)
- NuGet messagepack: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on messagepack.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-qhmf-xw27-6rqr (GitHub-reviewed advisory GHSA-qhmf-xw27-6rqr lists affected ranges: NuGet messagepack >=0 =3.0 <3.1.7)

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