XSS in Yafnet

CVE-2026-43939

YetAnotherForum.NET (YAF.NET) is a C# ASP.NET forum. Prior to 4.0.5 and 3.2.12, the thread posting and reply feature accepts user-supplied content via a a post or reply that is stored server-side and later rendered back into the thread page without adequate HTML sanitization or contextual output encoding. This vulnerability is fixed in 4.0.5 and 3.2.12.

Vulnerability class: XSS (Cross-Site Scripting)

Published · last modified .

CVSS v3 metric

CVSS v3 base score 7.3 (High). Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:U/C:H/I:H/A:N.

EPSS exploit prediction

EPSS: 0.004 (26.4th percentile), scored .

Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 26th percentile — 26.4% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS trend (30 days)EPSS over the last 30 days for CVE-2026-43939: held from 0.002 to 0.004.

EPSS over last 30 days · oldest: 0.002 · newest: 0.004 · change: +0.002

Affected products

  • Yafnet — versions >= 4.0.0-beta.1, < 4.0.5, < 3.2.12

Weakness classification (CWE)

References

Frequently asked questions

What is CVE-2026-43939?
CVE-2026-43939 is a high-severity vulnerability in Yafnet, classified under Cross-site Scripting. CVSS score: 7.3/10. Published 2026-05-12.
How severe is CVE-2026-43939?
High severity. CVSS v3 base score is 7.3 out of 10.