What’s Vulnerable › Vulnerabilities

CVE-2026-43938: YAF.NET: Unauthenticated Stored Second-Order XSS in Admin Event Log via Reflected `User-Agent` Header

• Severity: high (CVSS 8.1 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.3% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-05-12 · updated 2026-06-17
• Weakness: CWE-79

Description

YetAnotherForum.NET (YAF.NET) is a C# ASP.NET forum. Prior to 4.0.5 and 3.2.12, the application's database logger (YAFNET.Core/Logger/DbLogger.cs) captures the incoming request's User-Agent header into a JObject, serializes it with JsonConvert, and stores the result in the EventLog.Description column whenever an event (e.g., an unhandled exception) is logged. The admin event-log page (YetAnotherForum.NET/Pages/Admin/EventLog.cshtml.cs) later deserializes that JSON in FormatStackTrace() and interpolates the UserAgent value directly into an HTML string with no encoding, and the Razor view EventLog.cshtml emits the result through @Html.Raw. This vulnerability is fixed in 4.0.5 and 3.2.12.

Affected products

• YAFNET: from 4.0.0-beta.1, < 4.0.5; before 3.2.12

References

• https://github.com/YAFNET/YAFNET/security/advisories/GHSA-33gv-fc78-qgf5
• https://github.com/YAFNET/YAFNET/security/advisories/GHSA-33gv-fc78-qgf5

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