Use PHP programming to prevent XSS cross-site scripting attacks

Many forums in China have cross-site scripting vulnerabilities. There are also many such examples in foreign countries, even Google, but they were fixed in early December. (Editor's note: for cross-site scripting attacks, refer to "XSS cross-site scripting attack details"). Cross-site attacks can be easily constructed and are very concealed and difficult to detect (usually jump back to the original page immediately after information is stolen ). Many forums in China have cross-site scripting vulnerabilities. There are also many such examples in foreign countries, even Google, but they were fixed in early December. (Editor's note: for cross-site scripting attacks, refer to "XSS cross-site scripting attack details"). Cross-site attacks can be easily constructed and are very concealed and difficult to detect (usually jump back to the original page immediately after information is stolen ).

Here we will not explain how to attack (or ask me), but mainly discuss how to prevent it. First, cross-site scripting attacks are caused by the absence of strict filtering of user input. Therefore, we must block possible dangers before all data enters our website and database. Htmlentities () can be used for illegal HTML code, including single and double quotation marks ().

<? Php
$ Str = "A 'quote' is <B> bold </B> ";

// Outputs: A 'quote' is <B> bold </B>
Echo htmlentities ($ str );

// Outputs: A 'quote' is <B> bold </B>
Echo htmlentities ($ str, ENT_QUOTES );
?>
In this way, invalid scripts can be invalidated.

But note that htmlentities () is encoded as a ISO-8859-1 by default, and if your illegal script code is another, it may not be filtered out, while the browser can recognize and execute. To solve this problem, I need to test several sites first.

Here is a function for filtering out invalid scripts:

Function RemoveXSS ($ val ){
// Remove all non-printable characters. CR (0a) and LF (0b) and TAB (9) are allowed
// This prevents some character re-spacing such as <java \ 0 script>
// Note that you have to handle splits with \ n, \ r, and \ t later since they * are * allowed in some inputs
$ Val = preg_replace ('/([\ x00-\ x08] [\ x0b-\ x0c] [\ x0e-\ x20])/', '', $ val );

// Straight replacements, the user shoshould never need these since they're normal characters
// This prevents like _ # X6Cert ('xss')>
$ Search = 'abcdefghijklmnopqrstuvwxy ';
$ Search. = 'abcdefghijklmnopqrstuvwxy ';
$ Search. = '2017! @ # $ % ^ &*()';
$ Search. = '~ '";:? /= {} []-_ | \'\\';
For ($ I = 0; $ I <strlen ($ search); $ I ){
//;? Matches the;, which is optional
// 0 {0, 7} matches any padded zeros, which are optional and go up to 8 chars

// @ Search for the hex values
$ Val = preg_replace ('/(& # [x | X] 0 {0, 8}'. dechex (ord ($ search [$ I]). ';?) /I ', $ search [$ I], $ val); // with;
// @ 0 {0, 7} matches '0' zero to seven times
$ Val = preg_replace ('/({0, 8}'. ord ($ search [$ I]). ';?) /', $ Search [$ I], $ val); // with;
}

// Now the only remaining whitespace attacks are \ t, \ n, and \ r
$ Ra1 = Array ('javascript ', 'vbscript', 'expression', 'applet', 'meta', 'xml', 'Blink ', 'link ', 'style', 'script', 'Embed ', 'object', 'iframe', 'frameset', 'ilayer', 'lay', 'bgsound ', 'title', 'base ');
$ Ra2 = Array ('onabport', 'onactivate', 'onafterprint ', 'onafterupdate', 'onbeforeactivate', 'onbeforecopy', 'onbeforecut', 'onbeforedeactivate ', onbeforeeditfocus, onbeforepaste, onbeforeprint, onbeforeunload, onbeforeupdate, onblur, onbounce, oncellchange, onchange, onclick ', oncontextmenu, oncontrolselect, oncopy, oncut, ondataavailable, ondatasetchanged, ondatasetcomplete, ondblclick, ondeactivate, ondrag ', 'ondragend', 'ondragenter', 'ondragleave ', 'ondragover', 'ondragstart', 'ondrop', 'onerrorupdat', 'onfilterchang', 'onfinish ', 'oncore', 'onfocusin', 'onfocusout ', 'onhelp', 'onkeylow', 'onkeypress', 'onkeyup', 'onlayoutcomplete', 'onload', 'onlosecapture ', onmousedown, onmouseenter, onmouseleave, onmousemove, onmouseout, onmouseover, onmouseup, onmousewheel, onmove, onmoveend ', 'onmovestart', 'onpaste ', 'onpropertychang', 'onreadystatechang', 'onreset', 'onresizeend', 'onresizestart', 'onrowenter', 'onrowexit ', 'onrowsdelete', 'onrowsinserted', 'onscroll ', 'onselect', 'onselectionchang', 'onselectstart', 'onstart', 'onstop', 'onsubmit ', 'onload ');
$ Ra = array_merge ($ ra1, $ ra2 );

$ Found = true; // keep replacing as long as the previous round replaced something
While ($ found = true ){
$ Val_before = $ val;
For ($ I = 0; $ I <sizeof ($ ra); $ I ){
$ Pattern = '/';
For ($ j = 0; $ j <strlen ($ ra [$ I]); $ j ){
If ($ j> 0 ){
$ Pattern. = '(';
$ Pattern. = '(& # [x | X] 0 {0, 8} ([9] [a] [B]);?)? ';
$ Pattern. = '| ({0, 8} ([9] [10] [13]);?)? ';
$ Pattern. = ')? ';
}
$ Pattern. = $ ra [$ I] [$ j];
}
$ Pattern. = '/I ';
$ Replacement = substr ($ ra [$ I], 0, 2 ). '<x> '. substr ($ ra [$ I], 2); // add in <> to nerf the tag
$ Val = preg_replace ($ pattern, $ replacement, $ val); // filter out the hex tags
If ($ val_before ==$ val ){
// No replacements were made, so exit the loop
$ Found = false;
}
}
}
}