CVE-2026-48511
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 · uneditedMessagePack for C# is a MessagePack serializer for C#. Prior to 2.5.301 and 3.1.7, ExpandoObjectFormatter.Deserialize populates System.Dynamic.ExpandoObject by calling IDictionary<string, object>.Add for each map entry. ExpandoObject internally maintains member names in array-like structures, so inserting many distinct keys can require repeated linear scans and array copies. For large attacker-controlled maps, this produces quadratic CPU and allocation behavior. The issue is especially surprising because ExpandoObjectResolver.Options is configured with MessagePackSecurity.UntrustedData, but collision-resistant dictionary comparers cannot protect ExpandoObject insertion internals. 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 confidenceMessagePack for C#'s ExpandoObjectFormatter.Deserialize uses IDictionary<string, object>.Add to populate ExpandoObject instances. Since ExpandoObject stores member names in array-like internal structures, inserting many distinct keys triggers repeated linear scans and array copies, resulting in quadratic CPU and allocation behavior for large attacker-controlled maps. This occurs despite MessagePackSecurity.UntrustedData being enabled, as the protection cannot extend to ExpandoObject's internal insertion mechanism.
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< 2.5.301>= 3.0.3, < 3.1.7CVSS 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
- None
- Availability
- High
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/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 checksWork through these to decide whether this CVE applies to you.
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Identify MessagePack for C# library versionCheck the installed package version via NuGet package manager, project .csproj file, or run: dotnet list package | grep MessagePackAffected if The installed version is less than 2.5.301 or is 3.0.3 through 3.1.6 (inclusive)
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Locate MessagePack library referenceSearch project files (.csproj, .vbproj) or the packages folder for 'msgpack' or 'MessagePack' package referencesAffected if The project contains a vulnerable version of the MessagePack library
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Identify ExpandoObjectFormatter usage in codeSearch source code for 'ExpandoObjectFormatter' class usage or deserialization of types that map to ExpandoObject (dynamic/ExpandoObject in MessagePackObject attributes)Affected if Code deserializes to ExpandoObject or dynamic types using MessagePack
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Check for untrusted data deserialization pathsReview application entry points that receive serialized MessagePack data from external sources (HTTP requests, message queues, file uploads, network sockets)Affected if The application deserializes MessagePack data from untrusted sources
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Verify MessagePackSecurity settingsSearch code for 'MessagePackSecurity.UntrustedData' initialization - note this setting does NOT protect against this specific vulnerabilityAffected if The application uses MessagePackSecurity but processes large maps into ExpandoObjects from untrusted sources
A user is affected if their application uses a vulnerable MessagePack for C# version (below 2.5.301 or 3.0.3-3.1.6) AND deserializes untrusted MessagePack data into ExpandoObject or dynamic types with many distinct keys.
Generated from the published advisory. Verify against your own configuration.
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 · scoped2.5.3013.1.7
Upgrade MessagePack for C# to version 2.5.301 or 3.1.7 or later to obtain the fix for this quadratic deserialization DoS vulnerability.
MessagePack for C# 2.5.301 or 3.1.7 (depending on major version branch in use)
- 1. Identify the current MessagePack for C# version in use by checking the project dependencies (e.g., via .csproj file, packages.config, or dotnet list package)
- 2. For projects using MessagePack version 2.x: upgrade to version 2.5.301 or later
- 3. For projects using MessagePack version 3.x: upgrade to version 3.1.7 or later
- 4. Update the dependency in the project file (e.g., <PackageReference Include="MessagePack" Version="2.5.301" /> or <PackageReference Include="MessagePack" Version="3.1.7" />)
- 5. Run dotnet restore to fetch the updated package
- 6. Rebuild and test the application to verify the upgrade does not break existing functionality
Generated from the published advisory — verify against the referenced sources before acting.
- Consultation2.0 h
- Implementation1.0 h
- Testing3.0 h
- Review / QA2.0 h
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Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2026-48511 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.
References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.
Primary sourcesPractitioner notes
ContributedPeer-ranked notes from engineers who’ve handled CVE-2026-48511 in production — separate from our analysis above.
The advisory tells you what broke. It rarely tells you what actually worked. If you’ve dealt with this one, that detail is what the next engineer is searching for.
- The version that genuinely resolved it — not the one the vendor claimed
- A config change or rule that shut the vector down
- A gotcha in the upgrade path that cost you an afternoon
No notes yet
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- Version or environment caveats, and links to real fixes
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