CVE-2026-43937: YAF.NET SQL Injection Vulnerability

CVE-2026-43937 Overview

CVE-2026-43937 is a SQL injection vulnerability in YetAnotherForum.NET (YAF.NET), a C# ASP.NET forum application. Versions prior to 4.0.5 execute admin handler side effects before a ResultFilterAttribute rewrites the response to a 302 redirect to /Info/4. The most impactful abuse path is the /Admin/RunSql endpoint, where the OnPostRunQuery handler binds the Editor field from the POST body and passes it directly to IDbAccess.RunSql without any caller authorization check. Any authenticated low-privileged user can execute arbitrary SQL queries against the forum database. The issue is tracked as [CWE-89] and is fixed in YAF.NET 4.0.5.

Critical Impact
Low-privileged authenticated users can execute arbitrary SQL against the YAF.NET backend database, compromising confidentiality, integrity, and availability of all forum data.

Affected Products

YetAnotherForum.NET (YAF.NET) versions prior to 4.0.5
C# ASP.NET deployments using YAF.NET admin handlers
Forums exposing the /Admin/RunSql endpoint to authenticated users

Discovery Timeline

2026-05-12 - CVE-2026-43937 published to NVD
2026-05-13 - Last updated in NVD database

Technical Details for CVE-2026-43937

Vulnerability Analysis

The vulnerability stems from incorrect ordering between authorization enforcement and handler execution in YAF.NET's admin controllers. ASP.NET Core executes the OnPost… handler logic and its side effects before the ResultFilterAttribute rewrites the HTTP response to a 302 redirect pointing to /Info/4. The redirect was intended to block unauthorized access, but it runs only after the handler has already completed its work.

The most severe abuse target is the /Admin/RunSql administrative diagnostic endpoint. The OnPostRunQuery handler model-binds the Editor field from the POST body and passes the value straight to IDbAccess.RunSql. No role or privilege check guards the call. An authenticated user with any forum role can submit arbitrary SQL statements and have them executed against the backing database.

Root Cause

The root cause is reliance on a result filter for access control instead of authorization middleware or explicit checks inside the handler. Because the filter runs after the handler, the authorization decision is made too late to prevent execution. This is a classic broken access control pattern combined with unsanitized input flowing into a SQL execution API [CWE-89].

Attack Vector

The attack vector is network-accessible and requires only low-privileged authentication. An attacker authenticates with a standard forum account and issues an HTTP POST request to /Admin/RunSql containing an Editor parameter holding the SQL payload. The handler executes the query before the 302 redirect is written to the response. Refer to the GitHub Security Advisory GHSA-xhw7-j96h-c3g5 for handler-level details.

Detection Methods for CVE-2026-43937

Indicators of Compromise

HTTP POST requests to /Admin/RunSql originating from non-administrator user sessions
Application logs showing IDbAccess.RunSql invocations followed immediately by 302 responses to /Info/4
Unexpected SQL statements in database audit logs that reference forum tables such as yaf_User, yaf_UserGroup, or schema metadata queries
New or modified administrative accounts created outside the normal admin UI workflow

Detection Strategies

Inspect web server and application logs for POST traffic to any /Admin/* route paired with a 302 redirect to /Info/4 and a non-admin authentication context
Enable SQL Server or equivalent database auditing for statements executed under the YAF.NET application account and alert on DDL or privilege-related queries
Correlate forum session identity with database query patterns to detect low-privileged users issuing administrative SQL

Monitoring Recommendations

Forward IIS or Kestrel access logs and YAF.NET application logs to a centralized SIEM for retention and correlation
Alert on bursts of POST requests to /Admin/RunSql, /Admin/ReIndex, or other admin handlers from any single session
Baseline normal admin activity and flag deviations such as admin endpoint hits outside business hours or from new IP ranges

How to Mitigate CVE-2026-43937

Immediate Actions Required

Upgrade YAF.NET to version 4.0.5 or later, which adds explicit authorization checks inside the affected handlers
Restrict network access to /Admin/* routes at the reverse proxy or WAF layer until the upgrade is deployed
Rotate database credentials used by the YAF.NET application if exploitation is suspected
Audit forum user accounts and database contents for unauthorized changes

Patch Information

The maintainers fixed the issue in YAF.NET 4.0.5 by enforcing authorization before handler side effects execute. Review the YAF.NET GitHub Security Advisory for full patch notes and upgrade guidance.

Workarounds

Block access to /Admin/RunSql and other admin POST handlers via WAF rules that filter on authenticated user role
Run the YAF.NET database account with the least privilege required, removing DDL and cross-database rights
Temporarily disable the admin diagnostic SQL feature in deployment configuration where supported

bash

# Example NGINX rule to block non-admin access to YAF admin endpoints until patching
location ~* ^/Admin/ {
    if ($http_x_forum_role != "Administrator") { return 403; }
    proxy_pass http://yafnet_backend;
}