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

by Ricardo Zuasti on July 2nd, 2012 | Filed in: Enterprise Java Tags: Security, XSS

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.

01public class XSSFilter implements Filter {

02 

03    @Override

04    public void init(FilterConfig filterConfig) throws ServletException {

05    }

06 

07    @Override

08    public void destroy() {

09    }

10 

11    @Override

12    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)

13        throws IOException, ServletException {

14        chain.doFilter(new XSSRequestWrapper((HttpServletRequest) request), response);

15    }

16 

17}

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.

01import java.util.regex.Pattern;

02import javax.servlet.http.HttpServletRequest;

03import javax.servlet.http.HttpServletRequestWrapper;

04 

05public class XSSRequestWrapper extends HttpServletRequestWrapper {

06 

07    public XSSRequestWrapper(HttpServletRequest servletRequest) {

08        super(servletRequest);

09    }

10 

11    @Override

12    public String[] getParameterValues(String parameter) {

13        String[] values = super.getParameterValues(parameter);

14 

15        if (values == null) {

16            return null;

17        }

18 

19        int count = values.length;

20        String[] encodedValues = new String[count];

21        for (int i = 0; i < count; i++) {

22            encodedValues[i] = stripXSS(values[i]);

23        }

24 

25        return encodedValues;

26    }

27 

28    @Override

29    public String getParameter(String parameter) {

30        String value = super.getParameter(parameter);

31 

32        return stripXSS(value);

33    }

34 

35    @Override

36    public String getHeader(String name) {

37        String value = super.getHeader(name);

38        return stripXSS(value);

39    }

40 

41    private String stripXSS(String value) {

42        if (value != null) {

43            // NOTE: It's highly recommended to use the ESAPI library and uncomment the following line to

44            // avoid encoded attacks.

45            // value = ESAPI.encoder().canonicalize(value);

46 

47            // Avoid null characters

48            value = value.replaceAll("", "");

49 

50            // Avoid anything between script tags

51            Pattern scriptPattern = Pattern.compile("<script>(.*?)</script>", Pattern.CASE_INSENSITIVE);

52            value = scriptPattern.matcher(value).replaceAll("");

53 

54            // Avoid anything in a src='...' type of expression

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

56            value = scriptPattern.matcher(value).replaceAll("");

57 

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

59            value = scriptPattern.matcher(value).replaceAll("");

60 

61            // Remove any lonesome </script> tag

62            scriptPattern = Pattern.compile("</script>", Pattern.CASE_INSENSITIVE);

63            value = scriptPattern.matcher(value).replaceAll("");

64 

65            // Remove any lonesome <script ...> tag

66            scriptPattern = Pattern.compile("<script(.*?)>", Pattern.CASE_INSENSITIVE | Pattern.MULTILINE | Pattern.DOTALL);

67            value = scriptPattern.matcher(value).replaceAll("");

68 

69            // Avoid eval(...) expressions

70            scriptPattern = Pattern.compile("eval\\((.*?)\\)", Pattern.CASE_INSENSITIVE | Pattern.MULTILINE | Pattern.DOTALL);

71            value = scriptPattern.matcher(value).replaceAll("");

72 

73            // Avoid expression(...) expressions

74            scriptPattern = Pattern.compile("expression\\((.*?)\\)", Pattern.CASE_INSENSITIVE | Pattern.MULTILINE | Pattern.DOTALL);

75            value = scriptPattern.matcher(value).replaceAll("");

76 

77            // Avoid javascript:... expressions

78            scriptPattern = Pattern.compile("javascript:", Pattern.CASE_INSENSITIVE);

79            value = scriptPattern.matcher(value).replaceAll("");

80 

81            // Avoid vbscript:... expressions

82            scriptPattern = Pattern.compile("vbscript:", Pattern.CASE_INSENSITIVE);

83            value = scriptPattern.matcher(value).replaceAll("");

84 

85            // Avoid onload= expressions

86            scriptPattern = Pattern.compile("onload(.*?)=", Pattern.CASE_INSENSITIVE | Pattern.MULTILINE | Pattern.DOTALL);

87            value = scriptPattern.matcher(value).replaceAll("");

88        }

89        return value;

90    }

91}

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.