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
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
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
Real-World Exposure
MessagePack.NETMessagePackReal-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, duplicateMESSAGEPACKCSHARP-095
Patches
Fixes are prepared and will be released in coordinated patch versions.
Upgrade guidance:
- Upgrade
MessagePack.AspNetCoreMvcFormatterto 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 postureMESSAGEPACKCSHARP-095: duplicate finding for the same root cause- CWE-1188: Initialization of a Resource with an Insecure Default
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| .NETNuGet | MessagePack | all versions | 2.5.301dotnet add package MessagePack --version 2.5.301 |
| .NETNuGet | MessagePack | ≥ 3.0&&< 3.1.7 | 3.1.7dotnet add package MessagePack --version 3.1.7 |
Detection & mitigation playbook
Open-source dependencyDetect
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
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2026-48509 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2026-48509. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is CVE-2026-48509 in your dependencies?
O3 Security finds CVE-2026-48509 across NuGet dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.