CVE-2026-48509
CVE-2026-48509 is a critical-severity vulnerability in Messagepack with a CVSS 3.x base score of 9.1. It is not currently listed as actively exploited by CISA, and its EPSS exploit-prediction score is low. The underlying weakness is classified as CWE-1188.
Key facts
- Severity: Critical (CVSS 3.x base score 9.1)
- CVSS v4: 6.3
- EPSS exploit prediction: 0% (15th percentile)
- Actively exploited: Not listed in CISA KEV
- EU (EUVD) id: EUVD-2026-38387
- Weakness: CWE-1188
- Affected product: Messagepack
- Published:
- Last modified:
Description
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.
Frequently asked questions
- What is 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.
- How severe is CVE-2026-48509?
- CVE-2026-48509 has a CVSS 3.x base score of 9.1, rated critical severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is none, integrity high, and availability high.
- Is CVE-2026-48509 being actively exploited?
- It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (15th percentile), an estimate of the probability of exploitation in the next 30 days.
- What products are affected by CVE-2026-48509?
- CVE-2026-48509 affects Messagepack. See the affected-products list for the exact vulnerable versions.
- How do I fix CVE-2026-48509?
- Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its critical severity, prioritise patching exposed systems.
- Does CVE-2026-48509 have an EU (EUVD) identifier?
- Yes. CVE-2026-48509 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-38387.
- When was CVE-2026-48509 published?
- CVE-2026-48509 was published on 2026-06-22 and last updated on 2026-06-25.
References
Affected products (1)
- cpe:2.3:a:messagepack:messagepack:*:*:*:*:*:c\#:*:*
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