CVE-2026-43938: YAFNET has Unauthenticated Stored Second-Order XSS in Admin Event Log via Reflected `User-Agent` Header

Description: Stored (second-order) Cross-Site Scripting (XSS) occurs when attacker-controlled input is persisted through one component of an application and later rendered, without proper sanitization or contextual output encoding, by a completely different component — often one that implicitly trusts the stored data. Because the dangerous sink is typically a privileged administrative interface, the payload executes in the browser of a high-value user (such as an administrator) and inherits their authenticated session. This class of issue is especially severe when the entry point is an HTTP header on an unauthenticated endpoint, since the attack surface extends to any anonymous attacker on the internet with no prerequisites.

Issue Details: The application’s database logger (YAFNET.Core/Logger/DbLogger.cs) captures the incoming request’s User-Agent header into a JObject, serializes it with JsonConvert, and stores the result in the EventLog.Description column whenever an event (e.g., an unhandled exception) is logged. The admin event-log page (YetAnotherForum.NET/Pages/Admin/EventLog.cshtml.cs) later deserializes that JSON in FormatStackTrace() and interpolates the UserAgent value directly into an HTML string with no encoding, and the Razor view EventLog.cshtml emits the result through @Html.Raw. Critically, the attacker does not need to be authenticated: the unauthenticated endpoint GET /api/Attachments/GetAttachment?attachmentId=2147483647 reliably triggers a server-side exception that is written to dbo.EventLog together with the attacker-controlled User-Agent. A single anonymous request such as curl -A "<img src=x onerror=alert('XSS')>" "http://<target>/api/Attachments/GetAttachment?attachmentId=2147483647" is sufficient to persist the payload, and when an administrator later visits /Admin/EventLog the malicious markup is rendered as live HTML in the admin’s authenticated session.

Impact: An entirely unauthenticated attacker, with no account, no CSRF token, and no prior access, can stage JavaScript that will execute in an administrator’s browser the next time the Event Log is viewed. Because the script runs in the admin’s authenticated origin, it can perform any action the admin can: creating new administrative accounts, modifying site-wide settings, exfiltrating user data through admin-only endpoints. This effectively converts a single anonymous HTTP request into a full forum-takeover primitive, and the lack of any authentication requirement makes it exploitable at internet scale, including by automated scanners.

Likelihood: Exploitation requires only the ability to send a single HTTP request to a public, unauthenticated endpoint, which any anonymous attacker on the internet can do. Administrators routinely review the Event Log as part of normal operations, so payload delivery is highly probable with negligible attacker effort, making the overall likelihood very high.

Steps to Reproduce:

• From an unauthenticated session (no cookies, no tokens), send the following request with a malicious User-Agent header:
• curl.exe -sS -A "<img src=x onerror=alert('XSS')>" "http://yetanotherforum.internal:8080/api/Attachments/GetAttachment?attachmentId=2147483647"
• Confirm the server returns an error response and that a row has been written to dbo.EventLog containing the attacker-controlled UserAgent value inside the JSON Description.
• As an administrator (e.g., Superuser), navigate to /Admin/EventLog.
• Observe that the rendered page contains the raw <img src=x onerror="alert('XSS')"> element inside the event’s badge block, and that the onerror handler executes in the admin’s authenticated session, confirming cross-session, cross-privilege script execution from an unauthenticated source.