Anti cross-site scripting (XSS) filter for Java web apps (original) (raw)

Here is a good and simple anti cross-site scripting (XSS) filter written for Java web applications. What it basically does is remove all suspicious strings from request parameters before returning them to the application. It’s an improvement over my previous post on the topic.

You should configure it as the first filter in your chain (web.xml) and it’s generally a good idea to let it catch every request made to your site.

The actual implementation consists of two classes, the actual filter is quite simple, it wraps the HTTP request object in a specialized HttpServletRequestWrapper that will perform our filtering.

public class XSSFilter implements Filter {

@Override
public void init(FilterConfig filterConfig) throws ServletException {
}

@Override
public void destroy() {
}

@Override
public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)
    throws IOException, ServletException {
    chain.doFilter(new XSSRequestWrapper((HttpServletRequest) request), response);
}

}

The wrapper overrides the getParameterValues(), getParameter() and getHeader() methods to execute the filtering before returning the desired field to the caller. The actual XSS checking and striping is performed in the stripXSS() private method.

import java.util.regex.Pattern; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletRequestWrapper;

public class XSSRequestWrapper extends HttpServletRequestWrapper {

public XSSRequestWrapper(HttpServletRequest servletRequest) {
    super(servletRequest);
}

@Override
public String[] getParameterValues(String parameter) {
    String[] values = super.getParameterValues(parameter);

    if (values == null) {
        return null;
    }

    int count = values.length;
    String[] encodedValues = new String[count];
    for (int i = 0; i < count; i++) {
        encodedValues[i] = stripXSS(values[i]);
    }

    return encodedValues;
}

@Override
public String getParameter(String parameter) {
    String value = super.getParameter(parameter);

    return stripXSS(value);
}

@Override
public String getHeader(String name) {
    String value = super.getHeader(name);
    return stripXSS(value);
}

private String stripXSS(String value) {
    if (value != null) {
        // NOTE: It's highly recommended to use the ESAPI library and uncomment the following line to
        // avoid encoded attacks.
        // value = ESAPI.encoder().canonicalize(value);

        // Avoid null characters
        value = value.replaceAll("", "");

        // Avoid anything between script tags
        Pattern scriptPattern = Pattern.compile("<script>(.*?)</script>", Pattern.CASE_INSENSITIVE);
        value = scriptPattern.matcher(value).replaceAll("");

        // Avoid anything in a src='...' type of expression
        scriptPattern = Pattern.compile("src[\r\n]*=[\r\n]*\\\'(.*?)\\\'", Pattern.CASE_INSENSITIVE | Pattern.MULTILINE | Pattern.DOTALL);
        value = scriptPattern.matcher(value).replaceAll("");

        scriptPattern = Pattern.compile("src[\r\n]*=[\r\n]*\\\"(.*?)\\\"", Pattern.CASE_INSENSITIVE | Pattern.MULTILINE | Pattern.DOTALL);
        value = scriptPattern.matcher(value).replaceAll("");

        // Remove any lonesome </script> tag
        scriptPattern = Pattern.compile("</script>", Pattern.CASE_INSENSITIVE);
        value = scriptPattern.matcher(value).replaceAll("");

        // Remove any lonesome <script ...> tag
        scriptPattern = Pattern.compile("<script(.*?)>", Pattern.CASE_INSENSITIVE | Pattern.MULTILINE | Pattern.DOTALL);
        value = scriptPattern.matcher(value).replaceAll("");

        // Avoid eval(...) expressions
        scriptPattern = Pattern.compile("eval\\((.*?)\\)", Pattern.CASE_INSENSITIVE | Pattern.MULTILINE | Pattern.DOTALL);
        value = scriptPattern.matcher(value).replaceAll("");

        // Avoid expression(...) expressions
        scriptPattern = Pattern.compile("expression\\((.*?)\\)", Pattern.CASE_INSENSITIVE | Pattern.MULTILINE | Pattern.DOTALL);
        value = scriptPattern.matcher(value).replaceAll("");

        // Avoid javascript:... expressions
        scriptPattern = Pattern.compile("javascript:", Pattern.CASE_INSENSITIVE);
        value = scriptPattern.matcher(value).replaceAll("");

        // Avoid vbscript:... expressions
        scriptPattern = Pattern.compile("vbscript:", Pattern.CASE_INSENSITIVE);
        value = scriptPattern.matcher(value).replaceAll("");

        // Avoid onload= expressions
        scriptPattern = Pattern.compile("onload(.*?)=", Pattern.CASE_INSENSITIVE | Pattern.MULTILINE | Pattern.DOTALL);
        value = scriptPattern.matcher(value).replaceAll("");
    }
    return value;
}

}

Notice the comment about the ESAPI library, I strongly recommend you check it out and try to include it in your projects.

If you want to dig deeper on the topic I suggest you check out the OWASP page about XSS and RSnake’s XSS (Cross Site Scripting) Cheat Sheet.

Reference: Stronger anti cross-site scripting (XSS) filter for Java web apps from our JCG partner Ricardo Zuasti at the Ricardo Zuasti’s blog blog.