Vulnerability · Published May 12, 2026 · Assigned by GitHub_M

CVE-2026-43939YAF.NET: Stored XSS in Forum Thread Posts/Replies Allowing Arbitrary JavaScript Execution for All Thread Viewers

High 7.3

In plain English

A high-severity cross-site scripting in YAFNET from YAFNET. Proof-of-concept exploit code is public. The chance it is exploited in the next 30 days is estimated at 0.2% (higher than 10% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.

Severity (CVSS 3.1)7.3High

Exploit odds (30 days)0.2%top 90.2% of all CVEs

Exploited in the wildNo reportsCISA SSVC: poc

Attack toolingNoneno public tooling known

Official description

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.

Affected packages & fixed versions

Ecosystem	Package	Fixed in	Advisory
NuGet	yafnet.core	4.0.5	GHSA-8rq5-wwpp-fmj2
NuGet	yafnet.core	3.2.12	GHSA-8rq5-wwpp-fmj2

Affected products (as reported)

Vendor	Product	Versions
YAFNET	YAFNET	>= 4.0.0-beta.1, < 4.0.5< 3.2.12

Weakness: CWE-79, CWE-80, CWE-116 · Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:U/C:H/I:H/A:N

References

https://github.com/YAFNET/YAFNET/security/advisories/GHSA-8rq5-wwpp-fmj2 x_refsource_CONFIRM