How severe is CVE-2026-48509?

No CVSS score has been assigned to CVE-2026-48509 yet. Review the advisory details and affected package list to assess your exposure.

Which packages are affected by CVE-2026-48509?

CVE-2026-48509 affects the following packages: MessagePack (NuGet), MessagePack (NuGet). Ecosystems affected: NuGet.

How do I fix CVE-2026-48509?

Update MessagePack to 2.5.301 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-48509 is resolved across your whole dependency graph.

How do I detect CVE-2026-48509 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-2026-48509 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-2026-48509 actively exploited in the wild?

No public exploit code has been indexed for CVE-2026-48509 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-2026-48509?

CVE-2026-48509 has an EPSS (Exploit Prediction Scoring System) score of 0.4%, placing it in the 36th 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-2026-48509?

CVE-2026-48509 is classified as CWE-1188 (CWE-1188). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2026-48509 published, and has it been updated?

CVE-2026-48509 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-2026-48509 — MessagePack

CVE-2026-48509 is a CWE-1188 vulnerability in MessagePack. A fix is available for MessagePack — see the affected versions and patch details below.

MessagePack-CSharp: ASP.NET Core MessagePackInputFormatter defaults to TrustedData for HTTP request bodies

Also known asGHSA-2f33-pr97-265q

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-2026-48509.

EPSS Exploitation Probability

via FIRST.org ↗

0.4%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs36th percentile — riskier than 36% 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

The parameterless MessagePackInputFormatter() constructor uses default serializer options, which resolve to MessagePackSerializerOptions.Standard with MessagePackSecurity.TrustedData. The formatter is designed for ASP.NET Core MVC request bodies, which commonly cross an HTTP trust boundary.

This insecure default can expose applications to denial-of-service attacks that MessagePackSecurity.UntrustedData is intended to mitigate, such as hash-collision attacks against dictionary-like model properties.

Impact

Applications are affected when they register new MessagePackInputFormatter() without explicitly passing serializer options configured for untrusted data.

An unauthenticated or otherwise untrusted HTTP client can send MessagePack request bodies that are deserialized using the trusted-data posture. For models containing hash-based collections, this can enable algorithmic complexity attacks using colliding keys. The default constructor makes the unsafe posture easy to use at the exact boundary where request bodies should be treated as untrusted.

Affected components

Package: MessagePack.AspNetCoreMvcFormatter
API: MessagePackInputFormatter() parameterless constructor
Scenario: ASP.NET Core MVC model binding from HTTP request bodies
Finding IDs: MESSAGEPACKCSHARP-OPEN-009, duplicate MESSAGEPACKCSHARP-095

Patches

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

Upgrade guidance:

Upgrade MessagePack.AspNetCoreMvcFormatter 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 default the parameterless constructor to MessagePackSerializerOptions.Standard.WithSecurity(MessagePackSecurity.UntrustedData), or require callers to pass explicit options so the trust posture is deliberate.

Workarounds

Do not use the parameterless constructor on affected versions. Register the formatter with explicit untrusted-data options, for example:

options.InputFormatters.Add(
    new MessagePackInputFormatter(
        MessagePackSerializerOptions.Standard.WithSecurity(MessagePackSecurity.UntrustedData)));

Also apply normal HTTP request-size limits and model validation appropriate for your service.

Resources

MESSAGEPACKCSHARP-OPEN-009: MVC input formatter defaults to trusted-data security posture
MESSAGEPACKCSHARP-095: duplicate finding for the same root cause
CWE-1188: Initialization of a Resource with an Insecure Default

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-2026-48509 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 The parameterless `MessagePackInputFormatter()` constructor uses default serializer options, which resolve to `MessagePackSerializerOptions.Standard` with `MessagePackSecurity.TrustedData`. The formatter is designed for ASP.NET Core MVC request bodies, which commonly cross an HTTP trust boundary. This insecure default can expose applications to denial-of-service attacks that `MessagePackSecurity.UntrustedData` is intended to mitigate, such as hash-collision attacks against dictionary-like model properties. ## Impact Applications are affected when they register `new MessagePackIn

O3 Security · Impact-Aware SCA

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