Format String in Aspnet with Basic Auth

A format string vulnerability occurs when user-controlled input is used directly in functions that interpret format specifiers, such as printf-style functions or .NET string formatting methods. In an ASP.NET application using HTTP Basic Authentication, the client sends credentials in the Authorization header as Basic base64(username:password). If the server decodes this value and incorporates it into a logging or diagnostic routine using an unsafe format string, an attacker can supply crafted credentials containing format specifiers like %s, %x, or %n.

Consider an endpoint that decodes the Basic Auth header and logs the credentials with a vulnerable pattern:

string authHeader = Request.Headers["Authorization"];
if (authHeader != null && authHeader.StartsWith("Basic "))
{
    var base64 = authHeader.Substring("Basic ".Length).Trim();
    var credentialBytes = Convert.FromBase64String(base64);
    string decoded = Encoding.UTF8.GetString(credentialBytes); // username:password
    // Unsafe: user-controlled 'decoded' used as format string
    string logMessage = string.Format(decoded, arg1, arg2);
    logger.Info(logMessage);
}

An attacker can set the username to %s%0a%0aSECRET=%s, causing the format string to read stack memory or write to memory via %n, potentially exposing secrets or leading to further exploitation. Because Basic Auth credentials are often reused across environments, leaked memory contents may reveal session tokens or internal pointers. The combination of trusting external input (the credentials) and using it as a format string violates the principle of treating all user input as untrusted.

In the context of middleBrick’s 12 security checks, this would be surfaced under Input Validation and Data Exposure. The scanner does not assume the server’s intent; it detects patterns where untrusted data flows into formatting routines and reports the risk with severity and remediation guidance. Even if the application does not directly use user input as a format string, concatenating credentials into error messages or logs without proper sanitization can create information disclosure that compounds the impact of weak authentication mechanisms.

To detect this during a scan, middleBrick sends probes that include format specifiers in the Basic Auth header and inspects runtime behavior and any reflected output. Because the scan is unauthenticated and runs in 5–15 seconds, it can identify whether the endpoint reflects or logs credentials in an unsafe manner without requiring credentials or access to source code.

Remediation focuses on two principles: never treat Basic Auth credentials as format string input, and handle them as opaque binary data. Do not decode and format them using user-controlled format strings. Instead, parse the username and password separately using a robust Base64 decoder and avoid any further interpolation into format strings.

Safe handling example in ASP.NET Core:

var authHeader = Request.Headers["Authorization"].ToString();
if (!string.IsNullOrEmpty(authHeader) && authHeader.StartsWith("Basic ", StringComparison.OrdinalIgnoreCase))
{
    var base64 = authHeader.Substring("Basic ".Length).Trim();
    byte[] credentialBytes;
    try
    {
        credentialBytes = Convert.FromBase64String(base64);
    }
    catch (FormatException)
    {
        // Malformed header, reject request
        return Unauthorized();
    }
    string decoded = Encoding.UTF8.GetString(credentialBytes);
    var separatorIndex = decoded.IndexOf(':');
    if (separatorIndex < 0)
    {
        // Invalid format, reject request
        return Unauthorized();
    }
    string username = decoded.Substring(0, separatorIndex);
    string password = decoded.Substring(separatorIndex + 1);
    // Use username and password separately for authentication logic
    // Do NOT include raw credentials in format strings or logs
    if (ValidateUser(username, password))
    {
        // Proceed with authenticated context
    }
    else
    {
        return Unauthorized();
    }
}

Key mitigations:

• Do not pass the decoded credential string to string.Format or any function that interprets format specifiers.
• If logging is required, log only non-sensitive metadata (e.g., request ID, endpoint) and exclude the raw credentials. If you must log credentials for debugging, sanitize them or use structured logging that treats the values as data, not format templates.
• Validate the Base64 decoding step and reject malformed headers to avoid exceptions that could lead to information disclosure.
• Use constant-time comparison for authentication checks where possible to reduce timing side channels, although this does not directly mitigate format string issues.

For applications that rely on third-party libraries or legacy code, audit any usage of credentials in logging, exception messages, or dynamic string construction. The middleBrick CLI can be used to scan endpoints and verify whether such patterns exist by running middlebrick scan <url> against the authenticated surface after addressing authentication bypass risks.