Vulnerability · Published May 12, 2026 · Assigned by GitHub_M

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

High 8.1

In plain English

A high-severity cross-site scripting in YAFNET from YAFNET. Proof-of-concept exploit code is public. The chance it is exploited in the next 30 days is estimated at 0.3% (higher than 21% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.

Severity (CVSS 3.1)8.1High

Exploit odds (30 days)0.3%top 79.1% of all CVEs

Exploited in the wildNo reportsCISA SSVC: poc

Attack toolingNoneno public tooling known

Official 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 packages & fixed versions

Ecosystem	Package	Fixed in	Advisory
NuGet	yafnet.core	4.0.5	GHSA-33gv-fc78-qgf5
NuGet	yafnet.core	3.2.12	GHSA-33gv-fc78-qgf5

Affected products (as reported)

Vendor	Product	Versions
YAFNET	YAFNET	>= 4.0.0-beta.1, < 4.0.5< 3.2.12

Weakness: CWE-79, CWE-80, CWE-116 · Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:N

References

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