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CVE-2026-48516 — MessagePack

CVE-2026-48516 is a CWE-407 vulnerability in MessagePack. A fix is available for MessagePack — see the affected versions and patch details below.

MessagePack-CSharp: InterfaceLookupFormatter bypasses collision-resistant comparer settings

Also known asGHSA-q2h6-ghwm-5qm8
Published
Jun 22, 2026
Updated
Aug 27, 2026
Affected
2 pkgs
Patched
2 / 2
Exploits
None indexed
Exploitation data as of Sep 22, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

Exploitation Status

No confirmed exploitation observed yet

  • CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.

Exploitation and automatability from CISA’s SSVC triage for CVE-2026-48516.

EPSS Exploitation Probability

via FIRST.org ↗
0.4%probability of exploitation in next 30 days
Lower Risk0.00%
Lower risk than most CVEs35th percentile — riskier than 35% of all scored CVEsHighest risk

EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.

Real-World Exposure

2 pkgs affected
.NETMessagePack.NETMessagePack

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects NuGet packages — download data is not available via public APIs for these ecosystems.

Description

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 developers intentionally enabled by using MessagePackSecurity.UntrustedData.

Affected components

  • Package: MessagePack
  • API: InterfaceLookupFormatter<TKey,TElement>.Create
  • Data type: ILookup<TKey,TElement>
  • Finding ID: MESSAGEPACKCSHARP-041

Patches

Fixes are prepared and will be released in coordinated patch versions.

Upgrade guidance:

  1. Upgrade MessagePack to the patched version for your release line.
  2. Upgrade companion MessagePack packages in the same dependency graph to the coordinated patched versions.

The fix should create the internal dictionary with options.Security.GetEqualityComparer<TKey>(), matching the sibling dictionary and lookup formatter behavior.

Workarounds

Patching is recommended.

Until a patched version is available, avoid exposing ILookup<TKey,TElement> in DTOs that deserialize untrusted data. Use collection shapes that are already protected by the security-aware comparer path, or validate and cap collection sizes at the transport boundary.

Resources

  • MESSAGEPACKCSHARP-041: InterfaceLookupFormatter missing security comparer
  • CWE-407: Inefficient Algorithmic Complexity

Affected Packages

2 total 2 fixed
EcosystemPackageVulnerable rangeFix
.NETNuGetMessagePackall versions2.5.301dotnet add package MessagePack --version 2.5.301
.NETNuGetMessagePack≥ 3.0&&< 3.1.73.1.7dotnet add package MessagePack --version 3.1.7

Detection & mitigation playbook

Open-source dependency

  1. Detect

    Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for MessagePack, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

  2. Fix

    Update MessagePack to 2.5.301 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-48516 is resolved across your whole dependency graph.

  3. Workarounds

    If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

  4. How O3 protects you

    O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2026-48516 can be triaged on real exposure rather than presence alone.

Tailored to CVE-2026-48516. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.

Frequently Asked Questions

## 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
O3 Security · Impact-Aware SCA

Is CVE-2026-48516 in your dependencies?

O3 Security finds CVE-2026-48516 across NuGet dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.

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CVE-2026-48516: MessagePack DoS | O3 Security