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CVE-2026-48509

Messagepack vulnerability

9.1 CRITICALEPSS 0.42%
  1. Published22 Jun 2026

Description

MessagePack for C# is a MessagePack serializer for C#. Prior to 2.5.301 and 3.1.7, 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. This vulnerability is fixed in 2.5.301 and 3.1.7.

CVSS 3.1 breakdown

Attack vectorNetwork
Attack complexityLow
Privileges requiredNone
User interactionNone
ScopeUnchanged
ConfidentialityNone
IntegrityHigh
AvailabilityHigh
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:H

Affected products

Messagepack≥ 3.0.3 < 3.1.7 · < 2.5.301

Remediation

Fixed in
Messagepack≥ 3.1.7

References

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Frequently asked

Is CVE-2026-48509 being actively exploited?

CVE-2026-48509 is not in the CISA KEV catalog. Its EPSS exploit probability is 0.42%.

How severe is CVE-2026-48509?

CVE-2026-48509 has a CVSS 3.1 base score of 9.1, rated critical. EPSS estimates a 0.42% probability of exploitation.

Which products are affected by CVE-2026-48509?

Affected products include Messagepack.

How do I fix CVE-2026-48509?

Upgrade affected software to a fixed version, for example Messagepack 3.1.7 or later, then confirm no affected versions remain in your inventory.

How Secuno handles this

Secuno correlates vulnerabilities like CVE-2026-48509 against the real software inventory on every managed device, every day. Rather than a static severity, each match is scored in context: exploit availability, EPSS, the asset's exposure, and business criticality decide where it lands in the queue. If an affected version is present anywhere across your portfolio, it surfaces automatically with a prioritised remediation path.

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Data from the NVD, CISA KEV catalog and FIRST EPSS. CVSS 3.1 base score shown where available.