How severe is CVE-2026-48516?

No CVSS score has been assigned to CVE-2026-48516 yet. Review the advisory details and affected package list to assess your exposure.

Which packages are affected by CVE-2026-48516?

CVE-2026-48516 affects the following packages: MessagePack (NuGet), MessagePack (NuGet). Ecosystems affected: NuGet.

How do I fix CVE-2026-48516?

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.

How do I detect CVE-2026-48516 in my NuGet dependencies?

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.

How do I mitigate CVE-2026-48516 if there is no patch (or I can't update yet)?

Do not deserialise data from untrusted sources: where the format allows it, restrict deserialisation to an explicit allowlist of expected types, and prefer a data-only format (JSON, Protobuf) over one that can reconstruct arbitrary objects until you can upgrade.

Is CVE-2026-48516 actively exploited in the wild?

No public exploit code has been indexed for CVE-2026-48516 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What is the EPSS score for CVE-2026-48516?

CVE-2026-48516 has an EPSS (Exploit Prediction Scoring System) score of 0.4%, placing it in the 35th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2026-48516?

CVE-2026-48516 is classified as CWE-407 (CWE-407). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2026-48516 published, and has it been updated?

CVE-2026-48516 was published on June 22, 2026 and was last updated on October 28, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

.NET NuGet

Not in CISA KEV

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

Oct 28, 2026

Affected

2 pkgs

Patched

2 / 2

Exploits

None indexed

Exploitation data as of Nov 6, 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

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

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:

Upgrade MessagePack to the patched version for your release line.
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

Ecosystem	Package	Vulnerable range	Fix
.NETNuGet	`MessagePack`	all versions	2.5.301`dotnet add package MessagePack --version 2.5.301`
.NETNuGet	`MessagePack`	≥ 3.0&&< 3.1.7	3.1.7`dotnet add package MessagePack --version 3.1.7`

Affected Products

1 product · 2 configurations

Application

messagepackmessagepack

≥ 3.0.3 && < 3.1.7

range

Detection & mitigation playbook

Open-source dependency

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.
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.
Workarounds
Do not deserialise data from untrusted sources: where the format allows it, restrict deserialisation to an explicit allowlist of expected types, and prefer a data-only format (JSON, Protobuf) over one that can reconstruct arbitrary objects until you can upgrade.

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

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