MessagePack-CSharp: InterfaceLookupFormatter bypasses collision-resistant comparer settings

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-2026-48516: 0.40% estimated exploitation activity in the next 30 days; percentile 34.80 (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 InterfaceLookupFormatter<TKey,TElement constructs an internal Dictionary<TKey, IGrouping<TKey,TElement with the default equality comparer instead of the security-aware comparer supplied by options.Security.GetEqualityComparer<TKey(). Other hash-based collection formatters use the security-aware comparer when MessagePackSecurity.UntrustedData is configured. This formatter omission allows hash-collision CPU denial of service against ILookup<TKey,TElement even when the application has opted into the untrusted-data security posture. Impact Applications are affected when they deserialize untrusted payloads into schemas containing ILookup<TKey,TElement with a key type for which attacker-controlled hash collisions are feasible. Under the default comparer, many colliding keys can degrade dictionary insertion from amortized constant time to quadratic behavior. A payload of colliding keys can consume CPU for a disproportionate amount of time. This bypasses the mitigation that…

Published by the source

25 Jun 2026, 21:29 UTC

Last modified by the source

25 Jun 2026, 21:29 UTC

Source last checked

26 Oct 2026, 17:04 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-q2h6-ghwm-5qm8

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# Patch Next advisory brief: CVE-2026-48516

Advisory: MessagePack-CSharp: InterfaceLookupFormatter bypasses collision-resistant comparer settings
IDs: CVE-2026-48516, GHSA-q2h6-ghwm-5qm8
Record: https://patchnext.com/advisories/cve-2026-48516 (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-q2h6-ghwm-5qm8 (GitHub-reviewed advisory GHSA-q2h6-ghwm-5qm8 lists affected ranges: NuGet messagepack >=0 =3.0 <3.1.7)

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