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

CVSS 8.1 · High EPSS 0.03% · P10

Possible ATT&CK Techniques 1AI

T1190 · Exploit Public-Facing Application

Affected Version Matrix 2

VendorProductVersion RangeStatus
YAFNETYAFNET>= 4.0.0-beta.1, < 4.0.5affected
< 3.2.12affected
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I. Basic Information for CVE-2026-43938

Vulnerability Information
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Vulnerability Title
YAF.NET: Unauthenticated Stored Second-Order XSS in Admin Event Log via Reflected `User-Agent` Header
Source: NVD (National Vulnerability Database)
Vulnerability 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.
Source: NVD (National Vulnerability Database)
CVSS Information
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:N
Source: NVD (National Vulnerability Database)
Vulnerability Type
在Web页面生成时对输入的转义处理不恰当(跨站脚本)
Source: NVD (National Vulnerability Database)
Vulnerability Title
YAFNET 跨站脚本漏洞
Source: CNNVD (China National Vulnerability Database)
Vulnerability Description
YAFNET是YAFNET个人开发者的ASP.NET 开源论坛解决方案。 YAFNET 4.0.5之前版本和3.2.12之前版本存在跨站脚本漏洞，该漏洞源于数据库日志记录器将用户代理标头序列化为JSON后未编码直接渲染，可能导致跨站脚本攻击。
Source: CNNVD (China National Vulnerability Database)
CVSS Information
N/A
Source: CNNVD (China National Vulnerability Database)
Vulnerability Type
N/A
Source: CNNVD (China National Vulnerability Database)

Affected Products

VendorProductAffected VersionsCPESubscribe
YAFNETYAFNET >= 4.0.0-beta.1, < 4.0.5 -

II. Public POCs for CVE-2026-43938

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III. Intelligence Information for CVE-2026-43938

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Same Patch Batch · YAFNET · 2026-05-12 · 3 CVEs total

CVE-2026-439378.8 HIGHYAF.NET: Pre-Handler Authorization Bypass on Admin Pages Enabling Blind SQL Execution via
CVE-2026-439397.3 HIGHYAF.NET: Stored XSS in Forum Thread Posts/Replies Allowing Arbitrary JavaScript Execution

IV. Related Vulnerabilities

Same product: YAFNET
Same vendor: YAFNET
Same weakness: CWE-79

V. Comments for CVE-2026-43938

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