YetAnotherForum.NET XSS via User-Agent Logging (CVE-2026-43938)
A critical Cross-Site Scripting (XSS) vulnerability, tracked as CVE-2026-43938, has been identified in YetAnotherForum.NET (YAF.NET), a C# ASP.NET forum. The National Vulnerability Database (NVD) reports that versions prior to 4.0.5 and 3.2.12 are affected. The flaw stems from how the application’s database logger handles the incoming User-Agent header, which is captured, serialized into a JObject, and stored in the EventLog.Description column upon various events, such as unhandled exceptions.
The core issue, as detailed by NVD, arises when the admin event-log page later deserializes this JSON and interpolates the UserAgent value directly into an HTML string without proper encoding. This unencoded output is then emitted through @Html.Raw in the Razor view, creating a clear XSS vector. An attacker could craft a malicious User-Agent string to execute arbitrary JavaScript in an administrator’s browser, potentially leading to session hijacking, data exfiltration, or further compromise of the forum’s administrative interface. The vulnerability carries a high CVSS score of 8.1.
This is a classic case of improper input sanitization and output encoding, compounded by the dangerous practice of deserializing untrusted data. Defenders should recognize that the User-Agent header is user-controlled input and must be treated as such. The fix, according to NVD, is available in YAF.NET versions 4.0.5 and 3.2.12, which address this flaw by presumably implementing proper encoding before rendering the User-Agent string.
What This Means For You
- If your organization uses YetAnotherForum.NET, you are exposed to CVE-2026-43938. Immediately check your YAF.NET version and prioritize upgrading to 4.0.5 or 3.2.12 to mitigate this high-severity XSS vulnerability. An attacker only needs to get an admin to view the event logs to trigger code execution.
Related ATT&CK Techniques
🛡️ Detection Rules
3 rules · 6 SIEM formats3 detection rules auto-generated for this incident, mapped to MITRE ATT&CK. Sigma YAML is free — export to any SIEM format via the Intel Bot.
CVE-2026-43938 - YetAnotherForum.NET XSS via User-Agent Logging
title: CVE-2026-43938 - YetAnotherForum.NET XSS via User-Agent Logging
id: scw-2026-05-12-ai-1
status: experimental
level: critical
description: |
Detects attempts to exploit CVE-2026-43938 in YetAnotherForum.NET by observing requests to the EventLog page containing a malicious User-Agent string embedded within the referer header. This indicates an attempt to inject JavaScript via the User-Agent logging mechanism, which is then rendered unsanitized by the application.
author: SCW Feed Engine (AI-generated)
date: 2026-05-12
references:
- https://shimiscyberworld.com/posts/nvd-CVE-2026-43938/
tags:
- attack.initial_access
- attack.t1190
logsource:
category: webserver
detection:
selection:
cs-uri|contains:
- '/YetAnotherForum.NET/Pages/Admin/EventLog.cshtml'
uri|contains:
- 'YetAnotherForum.NET/Pages/Admin/EventLog.cshtml'
sc-status|exact:
- 200
selection_user_agent:
referer|contains:
- '"UserAgent":"<script>alert(1)</script>"'
condition: selection AND selection_user_agent
falsepositives:
- Legitimate administrative activity
Source: Shimi's Cyber World · License & reuse
Indicators of Compromise
| ID | Type | Indicator |
|---|---|---|
| CVE-2026-43938 | XSS | YetAnotherForum.NET (YAF.NET) versions prior to 4.0.5 and 3.2.12 |
| CVE-2026-43938 | XSS | Vulnerable component: YAFNET.Core/Logger/DbLogger.cs (captures User-Agent) |
| CVE-2026-43938 | XSS | Vulnerable component: YetAnotherForum.NET/Pages/Admin/EventLog.cshtml.cs (deserializes and interpolates User-Agent in FormatStackTrace()) |
| CVE-2026-43938 | XSS | Attack vector: Malicious User-Agent header leading to stored XSS in EventLog.Description column |
Written by SCW Vulnerability Desk
Source & Attribution
| Source Platform | NVD |
| Channel | National Vulnerability Database |
| Published | May 12, 2026 at 18:16 UTC |
This content was AI-rewritten and enriched by Shimi's Cyber World based on the original source. All intellectual property rights remain with the original author.
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