9.1

CVE-2026-48509: messagepack-csharp

CVSS: 9.1v3.1CriticalAnalyzed
AliasesGHSA-2f33-pr97-265q
Published 22-06-2026Last modified 25-06-2026
Open JSON

Vulnerability summary

CVE-2026-48509 is a critical (CVSS 9.1) vulnerability in messagepack-csharp. EPSS estimates a 0.42% chance of exploitation in the next 30 days, higher than 34.5% of scored vulnerabilities.

Overall risk
70/100
0100
High · default modelHow is this calculated?
CVSS score
9.1/10
010
Critical · v3.1 (NVD)
EPSS score
0.42%
0%100%
Low · 0.00 pp in 30 days
KEVNot listedIn no KEV catalog
RansomwareNone recordedNo known use recorded
Public exploits0No public exploits recorded

Timeline

  1. 22-06-2026
    PublishedVulnerability first disclosed.
  2. 25-06-2026
    Last modifiedVulnerability information updated.

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.

CVSS metrics

4 scores · 3 sources · 2 versions

EPSS trend

Probability

0.42%

Chance of exploitation in the next 30 days

Percentile

35%

Versus all other CVEs

30-day change

0.00 pp

From 0.42% on 20-09-2026

20-09-202620-10-2026
EPSS score
0.42%

Known exploited

Compare KEV catalogs

Not in any of the 3 tracked catalogs.

Public exploits

0 found

No public exploits indexed.

Weaknesses

CWE · root cause

Affected systems

Messagepack1 product
Messagepack-csharp1 product
NuGet1 product
  • MessagePack< 2.5.301 | ≥ 3.0, < 3.1.7

3 vendors · 3 products

References

3 links