CVE-2026-48509
CRITICALDescription
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.
How to fix
TridentStack Control can deploy fixes like this automatically across your Windows, macOS, and Linux fleet. See how it works
Remediation is compiled from vendor and distribution security advisories. Always confirm against the linked source for your exact version and platform.
CVSS v3 Vector
Exploitability
Impact
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:H
Exploit Intelligence
Low risk: more likely to be exploited than 35% of all known CVEs.
References
Related Vulnerabilities
Other CWE-1188 vulnerabilities, ordered by exploit likelihood. View all
| CVE | Severity | CVSS | EPSS | Exploited | Fix |
|---|---|---|---|---|---|
| CVE-2020-13927 | Critical | 9.8 | 100% | KEV | Fix |
| CVE-2023-27524 | High | 8.9 | 97% | KEV | - |
| CVE-2022-24706 | Critical | 9.8 | 92% | KEV | Fix |
| CVE-2020-11532 | Critical | 9.8 | 77% | - | Fix |
| CVE-2018-16752 | High | 8.8 | 43% | - | - |
| CVE-2020-14011 | Critical | 9.8 | 29% | - | - |
Common questions
How do I fix CVE-2026-48509?
Upgrade messagepack to 3.1.7 or later.
Is CVE-2026-48509 being actively exploited?
Not that we know of. CVE-2026-48509 is not in the CISA Known Exploited Vulnerabilities catalog. Its EPSS score of 0.42% is the estimated probability that it will be exploited in the next 30 days. That is higher than 35% of all scored CVEs.
How severe is CVE-2026-48509?
CVE-2026-48509 has a CVSS v3 base score of 9.1, rated critical. CVSS rates the technical impact if the vulnerability is exploited, not how likely that is, so weigh it alongside the exploit-prediction score when you decide what to patch first.
What does CVE-2026-48509 affect?
Published advisories record a fix for messagepack (NVD). Only products with a sourced advisory are listed, so treat this as what we can cite rather than a complete inventory.
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This product uses NVD data but is not endorsed or certified by the NVD. EPSS scores courtesy of FIRST.org (https://www.first.org/epss). Source: CISA KEV Catalog. Data as of 2026-06-25.