RCE in Messagepack
CVE-2026-48509
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.
Published · last modified .
CVSS v3 metric
CVSS v3 base score 9.1 (Critical). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:H.
EPSS exploit prediction
EPSS: 0.004 (33.4th percentile), scored .
Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 33rd percentile — 33.4% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.
EPSS over last 30 days · oldest: 0.004 · newest: 0.004 · change: 0.000
Affected products
- Messagepack
- Messagepack-Csharp — versions >= 3.1.7, < 3.1.7, >= 3.0, < 3.1.7, < 2.5.301
Weakness classification (CWE)
References
- github.com/MessagePack-CSharp/MessagePack-CSharp/security/advisories/GHSA-2f33-… (Mitigation, Vendor Advisory)
Frequently asked questions
- What is CVE-2026-48509?
- CVE-2026-48509 is a critical-severity vulnerability in Messagepack, classified under Initialization of a Resource with an Insecure Default. CVSS score: 9.1/10. Published 2026-06-22.
- How severe is CVE-2026-48509?
- Critical severity. CVSS v3 base score is 9.1 out of 10.