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Fencer Vulns
high 8.1

CVE-2026-43938 json

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

published 2026-05-05 6 mo ago modified 2026-06-17 4 mo ago also EUVD-2026-29480 , GHSA-33gv-fc78-qgf5

cvss
8.1
version 3.1, CNA
cisa kev
Not listed
not in CISA's catalog
epss
0.3%
EPSS 0.3% chance of exploitation in 30 days, percentile 20%
public exploits
0
none known

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.

Patch / fix

  • nuget YAFNET.Core 3.2.12, 4.0.5
  • yetanotherforum yaf.net 3.2.12, 4.0.5

CISA SSVC

Exploitation
Proof of concept is public
Automatable
no
Technical impact
total
Assessed
5 mo ago

Exploit prediction

0.28% chance of exploitation in the next 30 days, higher than 20% of all scored CVEs (FIRST EPSS, 2026-11-08).

Exploits and checks

  • github.com https://github.com/YAFNET/YAFNET/security/advisories/GHSA-33gv-fc78-qgf5 referenced PoC, tagged "exploit" by a source, unverified

CVSS scores

assigned by version score vector
CNA 3.1 8.1 CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:N
GitHub 3.1 8.1 CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:N

Affected packages

ecosystem package affected fixed in
nuget YAFNET.Core
>=4.0.0-beta01, <4.0.5
<3.2.12
4.0.5
3.2.12

Affected products

  • YAFNET YAFNET

Classification

Weakness
  • CWE-79 Improper Neutralization of Input During Web Page Generation
  • CWE-80 Improper Neutralization of Script-Related HTML Tags in a Web Page
  • CWE-116 Improper Encoding or Escaping of Output

References

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.

CPEs and fixed versions use Anchore's correction of NVD's data: Added CPE configurations because not yet analyzed by NVD.