MessagePack-CSharp: DynamicUnionResolver-generated deserializers miss depth enforcement

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:N/VA:L/SC:N/SI:N/SA:N · score source

EPSS

• CVE-2026-48513: 0.40% estimated exploitation activity in the next 30 days; percentile 34.81 (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 Runtime-generated union deserializers emitted by DynamicUnionResolver do not call MessagePackSecurity.DepthStep(ref reader) and do not decrement reader.Depth around recursive deserialization and skip paths. This means union deserialization does not consistently participate in the maximum object graph depth enforcement that protects other recursive formatter paths. For unknown union keys, the emitted deserializer calls reader.Skip() on attacker-controlled data without an enclosing depth step. Impact Applications are affected when they deserialize untrusted payloads into object graphs containing [Union]-decorated interfaces or abstract classes handled by DynamicUnionResolver. An attacker can provide a union payload with an unknown key and a deeply nested value. Because the generated union formatter does not enter the depth accounting scope before skipping or recursively processing the value, configured depth limits can be bypassed. In combination with recursive skip behavior,…

Published by the source

25 Jun 2026, 21:22 UTC

Last modified by the source

25 Jun 2026, 21:22 UTC

Source last checked

26 Oct 2026, 17:04 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-wfr3-xj75-pfwh

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# Patch Next advisory brief: CVE-2026-48513

Advisory: MessagePack-CSharp: DynamicUnionResolver-generated deserializers miss depth enforcement
IDs: CVE-2026-48513, GHSA-wfr3-xj75-pfwh
Record: https://patchnext.com/advisories/cve-2026-48513 (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-wfr3-xj75-pfwh (GitHub-reviewed advisory GHSA-wfr3-xj75-pfwh lists affected ranges: NuGet messagepack >=0 =3.0 <3.1.7)

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