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TI-2026-31995 Aliases CVE-2026-43938GHSA-33gv-fc78-qgf5

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

Priority HIGH 70CWE-116Published · Updated

Key facts

Attack vector
Network
Privileges required
None
User interaction
Required
Attack complexity
Low
Impact type
Cross-site scripting
Confidentiality · integrity · availability
High · High · None
Fix status
Fixed

Attack profile from CVSS 3.1 · CNA. Fix status from CNA and GitHub. Impact type from CWE-79.

Summary

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.

Why it matters now

Exploitation Proof of concept CISA SSVC · not in CISA KEV
EPSS 0.00282 20th percentileScored 2026-11-07
Fix Fixed YAFNET (YAFNET): 4.0.5, 3.2.12YAFNET.Core (NuGet): 4.0.5, 3.2.12

SSVC decision points

CISA ADP · SSVC 2.0.3

Exploitation
Poc
Automatable
No
Technical impact
Total

The decision (Track, Attend, Act) also depends on your mission and well-being impact; see CISA's SSVC guide.

Affected products

VendorProductAffected versionsFixed inStatusSource
YAFNETYAFNET4.0.0-beta.1 to before 4.0.5; before 3.2.124.0.5, 3.2.12Patch availableCNA
—YAFNET.Core (NuGet)4.0.0-beta01 to before 4.0.54.0.5Patch availableGHSA-33gv-fc78-qgf5
—YAFNET.Core (NuGet)before 3.2.123.2.12Patch availableGHSA-33gv-fc78-qgf5
  • YAFNET VendorYAFNET Affected 4.0.0-beta.1 to before 4.0.5; before 3.2.12 Fixed in 4.0.5, 3.2.12 Patch available CNA
  • YAFNET.Core (NuGet) Vendor— Affected 4.0.0-beta01 to before 4.0.5 Fixed in 4.0.5 Patch available GHSA-33gv-fc78-qgf5
  • YAFNET.Core (NuGet) Vendor— Affected before 3.2.12 Fixed in 3.2.12 Patch available GHSA-33gv-fc78-qgf5

Remediation

  • Fix · CNA YAFNET (YAFNET) Fixed in 4.0.5, 3.2.12
  • Fix · GHSA-33gv-fc78-qgf5 YAFNET.Core (NuGet) Fixed in 4.0.5 Advisory →
  • Fix · GHSA-33gv-fc78-qgf5 YAFNET.Core (NuGet) Fixed in 3.2.12 Advisory →

Timeline

  1. 2026-10-02 Threatist record created from CVE-2026-43938
  2. 2026-10-03 Alias added: GHSA-33gv-fc78-qgf5

References

Advisory 1

Other 4

  • TI-2026-31994 YAF.NET: Pre-Handler Authorization Bypass on Admin Pages Enabling Blind SQL Execution via `/Admin/RunSql` CVSS 3.1HIGH 8.8
  • TI-2026-31996 YAF.NET: Stored XSS in Forum Thread Posts/Replies Allowing Arbitrary JavaScript Execution for All Thread Viewers CVSS 3.1HIGH 7.3

Sources

This product uses the NVD API but is not endorsed or certified by the NVD. Sources and licences