CVE-2026-43939: YAFNET YetAnotherForum.NET stored XSS in thread posts and replies

CVE-2026-43939 · Severity: high · CVSS 7.3 · Published 2026-05-12

YetAnotherForum.NET (YAF.NET) is an open-source forum software used to host community discussions. A security flaw in the thread posting and reply system allows users to inject malicious scripts into their posts. When other users, including administrators, view these posts, the scripts execute automatically in their browsers, potentially leading to account takeover, theft of sensitive session information, or website defacement.

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

Quick answers

Is CVE-2026-43939 exploited in the wild?
No. Junglewise Threat Intelligence has no report of CVE-2026-43939 being exploited in the wild.
How severe is CVE-2026-43939?
High severity, with a CVSS score of 7.3, 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-2026-43939?
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-2026-43939 attacked?
It is attacked over the network, in a network appliance.
Am I affected by CVE-2026-43939?
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 Cross-Site Scripting (XSS) vulnerability exists in YetAnotherForum.NET (YAF.NET) due to improper neutralization of user-supplied input in forum posts and replies. The application fails to perform adequate HTML sanitization or contextual output encoding before rendering stored content to other users. An authenticated attacker with standard posting privileges can submit a malicious payload (e.g., using broken image tags with onerror handlers) that executes arbitrary JavaScript in the context of any user viewing the thread. This can be used to steal session cookies, perform unauthorized actions on behalf of administrators, or redirect users to malicious sites. The issue is resolved in versions 4.0.5 and 3.2.12.

Affected products

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

Identifiers

  • CVE-2026-43939
  • GHSA-8rq5-wwpp-fmj2

Timeline

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

References

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