package com.josephoconnell.html;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.regex.Matcher;
import java.util.regex.Pattern;
/**
*
* HTML filtering utility for protecting against XSS (Cross Site Scripting).
*
* This code is licensed under a Creative Commons Attribution-ShareAlike 2.5 License
* http://creativecommons.org/licenses/by-sa/2.5/
*
* This code is a Java port of the original work in PHP by Cal Hendersen.
* http://code.iamcal.com/php/lib_filter/
*
* The trickiest part of the translation was handling the differences in regex handling
* between PHP and Java. These resources were helpful in the process:
*
* http://java.sun.com/j2se/1.4.2/docs/api/java/util/regex/Pattern.html
* http://us2.php.net/manual/en/reference.pcre.pattern.modifiers.php
* http://www.regular-expressions.info/modifiers.html
*
* A note on naming conventions: instance variables are prefixed with a "v"; global
* constants are in all caps.
*
* Sample use:
* String input = ...
* String clean = new HTMLInputFilter().filter( input );
*
* If you find bugs or have suggestions on improvement (especially regarding
* perfomance), please contact me at the email below. The latest version of this
* source can be found at
*
* http://josephoconnell.com/java/xss-html-filter/
*
* @author Joseph O'Connell <joe.oconnell at gmail dot com>
* @version 1.0
*/
public class HTMLInputFilter
{
/**
* flag determining whether to try to make tags when presented with "unbalanced"
* angle brackets (e.g. "<b text </b>" becomes "<b> text </b>"). If set to false,
* unbalanced angle brackets will be html escaped.
*/
protected static final boolean ALWAYS_MAKE_TAGS = true;
/**
* flag determing whether comments are allowed in input String.
*/
protected static final boolean STRIP_COMMENTS = true;
/** regex flag union representing /si modifiers in php **/
protected static final int REGEX_FLAGS_SI = Pattern.CASE_INSENSITIVE | Pattern.DOTALL;
/** set of allowed html elements, along with allowed attributes for each element **/
protected Map<String,List<String>> vAllowed;
/** counts of open tags for each (allowable) html element **/
protected Map<String,Integer> vTagCounts;
/** html elements which must always be self-closing (e.g. "<img />") **/
protected String[] vSelfClosingTags;
/** html elements which must always have separate opening and closing tags (e.g. "<b></b>") **/
protected String[] vNeedClosingTags;
/** attributes which should be checked for valid protocols **/
protected String[] vProtocolAtts;
/** allowed protocols **/
protected String[] vAllowedProtocols;
/** tags which should be removed if they contain no content (e.g. "<b></b>" or "<b />") **/
protected String[] vRemoveBlanks;
/** entities allowed within html markup **/
protected String[] vAllowedEntities;
protected boolean vDebug;
public HTMLInputFilter()
{
this(false);
}
public HTMLInputFilter( boolean debug )
{
vDebug = debug;
vAllowed = new HashMap<String,List<String>>();
vTagCounts = new HashMap<String,Integer>();
ArrayList<String> a_atts = new ArrayList<String>();
a_atts.add( "href" );
a_atts.add( "target" );
vAllowed.put( "a", a_atts );
ArrayList<String> img_atts = new ArrayList<String>();
img_atts.add( "src" );
img_atts.add( "width" );
img_atts.add( "height" );
img_atts.add( "alt" );
vAllowed.put( "img", img_atts );
ArrayList<String> no_atts = new ArrayList<String>();
vAllowed.put( "b", no_atts );
vAllowed.put( "strong", no_atts );
vAllowed.put( "i", no_atts );
vAllowed.put( "em", no_atts );
vSelfClosingTags = new String[] { "img" };
vNeedClosingTags = new String[] { "a", "b", "strong", "i", "em" };
vAllowedProtocols = new String[] { "http", "mailto" }; // no ftp.
vProtocolAtts = new String[] { "src", "href" };
vRemoveBlanks = new String[] { "a", "b", "strong", "i", "em" };
vAllowedEntities = new String[] { "amp", "gt", "lt", "quot" };
}
protected void reset()
{
vTagCounts = new HashMap<String,Integer>();
}
protected void debug( String msg )
{
if (vDebug)
System.out.println( msg );
}
//---------------------------------------------------------------
// my versions of some PHP library functions
public static String chr( int decimal )
{
return String.valueOf( (char) decimal );
}
public static String htmlSpecialChars( String s )
{
s = s.replaceAll( "&", "&" );
s = s.replaceAll( "\"", """ );
s = s.replaceAll( "<", "<" );
s = s.replaceAll( ">", ">" );
return s;
}
//---------------------------------------------------------------
/**
* given a user submitted input String, filter out any invalid or restricted
* html.
*
* @param input text (i.e. submitted by a user) than may contain html
* @return "clean" version of input, with only valid, whitelisted html elements allowed
*/
public synchronized String filter( String input )
{
reset();
String s = input;
debug( "************************************************" );
debug( " INPUT: " + input );
s = escapeComments(s);
debug( " escapeComments: " + s );
s = balanceHTML(s);
debug( " balanceHTML: " + s );
s = checkTags(s);
debug( " checkTags: " + s );
s = processRemoveBlanks(s);
debug( "processRemoveBlanks: " + s );
s = validateEntities(s);
debug( " validateEntites: " + s );
debug( "************************************************\n\n" );
return s;
}
protected String escapeComments( String s )
{
Pattern p = Pattern.compile( "<!--(.*?)-->", Pattern.DOTALL );
Matcher m = p.matcher( s );
StringBuffer buf = new StringBuffer();
if (m.find()) {
String match = m.group( 1 ); //(.*?)
m.appendReplacement( buf, "<!--" + htmlSpecialChars( match ) + "-->" );
}
m.appendTail( buf );
return buf.toString();
}
protected String balanceHTML( String s )
{
if (ALWAYS_MAKE_TAGS)
{
//
// try and form html
//
s = regexReplace("^>", "", s);
s = regexReplace("<([^>]*?)(?=<|$)", "<$1>", s);
s = regexReplace("(^|>)([^<]*?)(?=>)", "$1<$2", s);
}
else
{
//
// escape stray brackets
//
s = regexReplace("<([^>]*?)(?=<|$)", "<$1", s);
s = regexReplace("(^|>)([^<]*?)(?=>)", "$1$2><", s);
//
// the last regexp causes '<>' entities to appear
// (we need to do a lookahead assertion so that the last bracket can
// be used in the next pass of the regexp)
//
s = s.replaceAll("<>", "");
}
return s;
}
protected String checkTags( String s )
{
Pattern p = Pattern.compile( "<(.*?)>", Pattern.DOTALL );
Matcher m = p.matcher( s );
StringBuffer buf = new StringBuffer();
while (m.find()) {
String replaceStr = m.group( 1 );
replaceStr = processTag( replaceStr );
m.appendReplacement(buf, replaceStr);
}
m.appendTail(buf);
s = buf.toString();
// these get tallied in processTag
// (remember to reset before subsequent calls to filter method)
for( String key : vTagCounts.keySet())
{
for(int ii=0; ii<vTagCounts.get(key); ii++) {
s += "</" + key + ">";
}
}
return s;
}
protected String processRemoveBlanks( String s )
{
for( String tag : vRemoveBlanks )
{
s = regexReplace( "<" + tag + "(\\s[^>]*)?></" + tag + ">", "", s );
s = regexReplace( "<" + tag + "(\\s[^>]*)?/>", "", s );
}
return s;
}
protected String regexReplace( String regex_pattern, String replacement, String s )
{
Pattern p = Pattern.compile( regex_pattern );
Matcher m = p.matcher( s );
return m.replaceAll( replacement );
}
protected String processTag( String s )
{
// ending tags
Pattern p = Pattern.compile( "^/([a-z0-9]+)", REGEX_FLAGS_SI );
Matcher m = p.matcher( s );
if (m.find()) {
String name = m.group(1).toLowerCase();
if (vAllowed.containsKey( name )) {
if (!inArray(name, vSelfClosingTags)) {
if (vTagCounts.containsKey( name )) {
vTagCounts.put(name, vTagCounts.get(name)-1);
return "</" + name + ">";
}
}
}
}
// starting tags
p = Pattern.compile("^([a-z0-9]+)(.*?)(/?)$", REGEX_FLAGS_SI);
m = p.matcher( s );
if (m.find()) {
String name = m.group(1).toLowerCase();
String body = m.group(2);
String ending = m.group(3);
//debug( "in a starting tag, name='" + name + "'; body='" + body + "'; ending='" + ending + "'" );
if (vAllowed.containsKey( name )) {
String params = "";
Pattern p2 = Pattern.compile("([a-z0-9]+)=([\"'])(.*?)\\2", REGEX_FLAGS_SI);
Pattern p3 = Pattern.compile("([a-z0-9]+)(=)([^\"\\s']+)", REGEX_FLAGS_SI);
Matcher m2 = p2.matcher( body );
Matcher m3 = p3.matcher( body );
List<String> paramNames = new ArrayList<String>();
List<String> paramValues = new ArrayList<String>();
while (m2.find()) {
paramNames.add(m2.group(1)); //([a-z0-9]+)
paramValues.add(m2.group(3)); //(.*?)
}
while (m3.find()) {
paramNames.add(m3.group(1)); //([a-z0-9]+)
paramValues.add(m3.group(3)); //([^\"\\s']+)
}
String paramName, paramValue;
for( int ii=0; ii<paramNames.size(); ii++ ) {
paramName = paramNames.get(ii).toLowerCase();
paramValue = paramValues.get(ii);
//debug( "paramName='" + paramName + "'" );
//debug( "paramValue='" + paramValue + "'" );
//debug( "allowed? " + vAllowed.get( name ).contains( paramName ) );
if (vAllowed.get( name ).contains( paramName )) {
if (inArray( paramName, vProtocolAtts )) {
paramValue = processParamProtocol( paramValue );
}
params += " " + paramName + "=\"" + paramValue + "\"";
}
}
if (inArray( name, vSelfClosingTags )) {
ending = " /";
}
if (inArray( name, vNeedClosingTags )) {
ending = "";
}
if (ending == null || ending.length() < 1) {
if (vTagCounts.containsKey( name )) {
vTagCounts.put( name, vTagCounts.get(name)+1 );
} else {
vTagCounts.put( name, 1 );
}
} else {
ending = " /";
}
return "<" + name + params + ending + ">";
} else {
return "";
}
}
// comments
p = Pattern.compile( "^!--(.*)--$", REGEX_FLAGS_SI );
m = p.matcher( s );
if (m.find()) {
String comment = m.group();
if (STRIP_COMMENTS) {
return "";
} else {
return "<" + comment + ">";
}
}
return "";
}
protected String processParamProtocol( String s )
{
s = decodeEntities( s );
Pattern p = Pattern.compile( "^([^:]+):", REGEX_FLAGS_SI );
Matcher m = p.matcher( s );
if (m.find()) {
String protocol = m.group(1);
if (!inArray( protocol, vAllowedProtocols )) {
// bad protocol, turn into local anchor link instead
s = "#" + s.substring( protocol.length()+1, s.length() );
if (s.startsWith("#//")) s = "#" + s.substring( 3, s.length() );
}
}
return s;
}
protected String decodeEntities( String s )
{
StringBuffer buf = new StringBuffer();
Pattern p = Pattern.compile( "&#(\\d+);?" );
Matcher m = p.matcher( s );
while (m.find()) {
String match = m.group( 1 );
int decimal = Integer.decode( match ).intValue();
m.appendReplacement( buf, chr( decimal ) );
}
m.appendTail( buf );
s = buf.toString();
buf = new StringBuffer();
p = Pattern.compile( "&#x([0-9a-f]+);?");
m = p.matcher( s );
while (m.find()) {
String match = m.group( 1 );
int decimal = Integer.decode( match ).intValue();
m.appendReplacement( buf, chr( decimal ) );
}
m.appendTail( buf );
s = buf.toString();
buf = new StringBuffer();
p = Pattern.compile( "%([0-9a-f]{2});?");
m = p.matcher( s );
while (m.find()) {
String match = m.group( 1 );
int decimal = Integer.decode( match ).intValue();
m.appendReplacement( buf, chr( decimal ) );
}
m.appendTail( buf );
s = buf.toString();
s = validateEntities( s );
return s;
}
protected String validateEntities( String s )
{
// validate entities throughout the string
Pattern p = Pattern.compile( "&([^&;]*)(?=(;|&|$))" );
Matcher m = p.matcher( s );
if (m.find()) {
String one = m.group( 1 ); //([^&;]*)
String two = m.group( 2 ); //(?=(;|&|$))
s = checkEntity( one, two );
}
// validate quotes outside of tags
p = Pattern.compile( "(>|^)([^<]+?)(<|$)", Pattern.DOTALL );
m = p.matcher( s );
StringBuffer buf = new StringBuffer();
if (m.find()) {
String one = m.group( 1 ); //(>|^)
String two = m.group( 2 ); //([^<]+?)
String three = m.group( 3 ); //(<|$)
m.appendReplacement( buf, one + two.replaceAll( "\"", """ ) + three);
}
m.appendTail( buf );
return s;
}
protected String checkEntity( String preamble, String term )
{
if (!term.equals(";")) {
return "&" + preamble;
}
if ( isValidEntity( preamble ) ) {
return "&" + preamble;
}
return "&" + preamble;
}
protected boolean isValidEntity( String entity )
{
return inArray( entity, vAllowedEntities );
}
private boolean inArray( String s, String[] array )
{
for (String item : array)
if (item != null && item.equals(s))
return true;
return false;
}
}