CVE-2026-43938: YAFNET YetAnotherForum.NET stored XSS in Admin Event Log

CVE-2026-43938 · Severity: high · CVSS 8.1 · Published 2026-05-12

YetAnotherForum.NET (YAF.NET) is an open-source forum software used to host community discussions. A security flaw allows unauthenticated attackers to inject malicious scripts into the forum's administrative logs by sending a specially crafted web request. If an administrator views these logs, the script executes in their browser, potentially allowing the attacker to take full control of the forum, steal user data, or create new administrative accounts.

Technologies: YAFNET.Core (NuGet), YAF.NET Core, YAFNET YetAnotherForum.NET. Vendors: NuGet, YAF.NET, YAFNET.

Quick answers

Is CVE-2026-43938 exploited in the wild?
No. Junglewise Threat Intelligence has no report of CVE-2026-43938 being exploited in the wild.
How severe is CVE-2026-43938?
High severity, with a CVSS score of 8.1, published 12 May 2026.
Which products and versions are affected?
YAFNET.Core (NuGet) ECOSYSTEM:introduced=4.0.0-beta01,fixed=4.0.5; YAFNET YetAnotherForum.NET < 4.0.5, < 3.2.12.
Is there a fix for CVE-2026-43938?
Yes. YAFNET.Core (NuGet): upgrade to 4.0.5 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
How is CVE-2026-43938 attacked?
It is attacked over the network, in a network appliance.
Am I affected by CVE-2026-43938?
Check whether you run YAFNET.Core (NuGet), YAF.NET Core or YAFNET YetAnotherForum.NET at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

How to fix

  • YAFNET.Core (NuGet): upgrade to 4.0.5 or later
    dotnet add package YAFNET.Core --version 4.0.5

Technical details

A stored (second-order) Cross-Site Scripting (XSS) vulnerability exists in YAF.NET's database logger (DbLogger.cs). The application captures the 'User-Agent' HTTP header from incoming requests and stores it as a JSON object in the EventLog table when an exception occurs. The administrative event log page (EventLog.cshtml.cs) later deserializes this JSON and uses @Html.Raw to render the User-Agent value without proper HTML encoding. An unauthenticated attacker can trigger a loggable event (e.g., by requesting a non-existent attachment ID) with a malicious User-Agent header. When an administrator views the Event Log, the payload executes in their security context, leading to potential full site takeover. This issue is fixed in versions 4.0.5 and 3.2.12.

Affected products

  • YAFNET YetAnotherForum.NET (YAF.NET) < 4.0.5, < 3.2.12

Identifiers

  • CVE-2026-43938
  • GHSA-33gv-fc78-qgf5

Timeline

  • 2026-04-26: advisory: GitHub Security Advisory published
  • 2026-05-12: disclosed: CVE published to NVD

References

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