How to fix Improper Neutralization of Script-Related HTML Tags in a Web Page (Basic XSS) with error message?

We use web control adapter in our login page. Recently we run VeraCode on our web application. In following function, we got CWE80, Improper Neutralization of Script-Related HTML Tags in a Web Page (Basic XSS), on the line

rev.ErrorMessage = msg;

Following is the function in the WebControlAdapterExtender class.

static public void WriteRegularExpressionValidator(HtmlTextWriter writer, RegularExpressionValidator rev, string className, string controlToValidate, string msg, string expression)
        {
            if (rev != null)
            {
                rev.CssClass = className;
                rev.ControlToValidate = controlToValidate;
                rev.ErrorMessage = msg;
                rev.ValidationExpression = expression;
                rev.RenderControl(writer);
            }
        }

Does anyone have any suggestion how to fix this?

The problem is that 'msg' is being passed down to your function, but there is no neutralization of this before it gets used - the string gets uses 'as-is' so could contain scripts that cause harm. There is a good description explaining this and why it is a problem: http://www.veracode.com/images/pdf/top5mostprevalent.pdf

I've not used this myself, but I think ErrorMessage gets rendered and displayed in the event of an error. Because this will get rendered on the final page if 'msg' was a naughty snippet of code you are exposing yourself and your users to a security vulnerability.

Have a read of the tips on this cheat sheet: https://www.owasp.org/index.php/XSS_%28Cross_Site_Scripting%29_Prevention_Cheat_Sheet

You should be able to use HtmlEncode to make this safe HttpUtility.HtmlEncode(unencoded);

rev.ErrorMessage = System.web.HttpUtility.HtmlEncode(msg);