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MessagepackApplication

CVE-2026-48509

CRITICAL · 9.1 CVSS v3.1 Published 2026-06-22
Fix available
A fix is available. Upgrade to 2.5.301 / 3.1.7 or later.
See remediation →
100/100
Remediation priority · Urgent
Remotely reachable No privileges Zero-click

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
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.

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis · high confidence

MessagePack for C# serializer's parameterless constructor defaults to MessagePackSecurity.TrustedData, which disables protections against hash-collision denial-of-service attacks. Since this formatter is used for ASP.NET Core MVC request bodies crossing HTTP trust boundaries, attackers can exploit dictionary-like model properties via malicious payloads causing severe CPU exhaustion.

MitigationUpgrade MessagePack for C# to version 2.5.301 or 3.1.7 or later, which changes the default to MessagePackSecurity.UntrustedData. Applications can also explicitly configure MessagePackSecurity.UntrustedData in their serializer options.

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.

NVD · CPE data
MessagepackApplication
Affected:< 2.5.301>= 3.0.3, < 3.1.7

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Network
Complexity
Low
Privileges
None
User interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
High
Availability
High

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:H

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

dbcve checks

Work through these to decide whether this CVE applies to you.

  1. Identify installed MessagePack version
    Run 'dotnet list package' in your project directory, or inspect the .csproj file for the MessagePack package reference version number
    Affected if The version listed is less than 2.5.301, or is between 3.0.3 and 3.1.6 inclusive
  2. Check if MessagePack deserializes HTTP request bodies
    Search your codebase for MessagePackSerializer.Deserialize calls that process input from controller action parameters, HttpRequest.Body, or API endpoint request payloads
    Affected if MessagePack is used to deserialize data from HTTP requests into model objects
  3. Inspect serializer instantiation method
    Search for 'new MessagePackSerializer()' or 'MessagePackSerializer.Deserialize' calls without passing a MessagePackSerializerOptions argument
    Affected if The deserialization uses a parameterless constructor or default options rather than explicitly configured options
  4. Verify security configuration
    Search for 'MessagePackSecurity.UntrustedData' in your codebase - look for serializer options configured with '.WithSecurity(MessagePackSecurity.UntrustedData)' or equivalent
    Affected if No explicit MessagePackSecurity.UntrustedData configuration is found, meaning the default TrustedData (vulnerable) setting is in use

You are affected if you use an affected MessagePack version AND deserialize HTTP request bodies using default serializer options without explicitly configuring MessagePackSecurity.UntrustedData.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

dbcve · scoped
Upgrade available Upgrade to 2.5.301 / 3.1.7 or later
Fixed in 2.5.3013.1.7
Interim mitigation

Upgrade MessagePack for C# to version 2.5.301 or 3.1.7 or later, which changes the default to MessagePackSecurity.UntrustedData. Applications can also explicitly configure MessagePackSecurity.UntrustedData in their serializer options.

Recommended fix High confidence

MessagePack for C# 2.5.301 (for 2.x line) or 3.1.7 (for 3.x line)

  1. Identify the current version of MessagePack for C# being used in the project (check .csproj, packages.config, or NuGet package references)
  2. If using version 2.x (< 2.5.301): Upgrade MessagePack to version 2.5.301 or later in the project file or package manager
  3. If using version 3.0.3 - 3.1.6: Upgrade MessagePack to version 3.1.7 or later in the project file or package manager
  4. Run build and tests to verify the upgrade does not introduce compilation errors or runtime failures
  5. Ensure the application properly handles MessagePack deserialization with the updated library

Generated from the published advisory — verify against the referenced sources before acting.

Fix this in Messagepack Scoped from the published advisory
  • Consultation2.0 h
  • Implementation2.0 h
  • Testing4.0 h
  • Review / QA2.0 h
10.0 hours of engineering $1,720
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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2026-48509 in production — separate from our analysis above.

No notes yet

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What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here
  • Verified mitigations, workarounds, and config changes
  • Version or environment caveats, and links to real fixes
  • No weaponised exploit code, or anything meant to cause harm
  • No spam, self-promotion, credentials, or personal data