Best regex to catch XSS (Cross-site Scripting) attack (in Java)?

Solution 1:

Don't do this with regular expressions. Remember, you're not protecting just against valid HTML; you're protecting against the DOM that web browsers create. Browsers can be tricked into producing valid DOM from invalid HTML quite easily.

For example, see this list of obfuscated XSS attacks. Are you prepared to tailor a regex to prevent this real world attack on Yahoo and Hotmail on IE6/7/8?

<HTML><BODY>
<?xml:namespace prefix="t" ns="urn:schemas-microsoft-com:time">
<?import namespace="t" implementation="#default#time2">
<t:set attributeName="innerHTML" to="XSS&lt;SCRIPT DEFER&gt;alert(&quot;XSS&quot;)&lt;/SCRIPT&gt;">
</BODY></HTML>

How about this attack that works on IE6?

<TABLE BACKGROUND="javascript:alert('XSS')">

How about attacks that are not listed on this site? The problem with Jeff's approach is that it's not a whitelist, as claimed. As someone on that page adeptly notes:

The problem with it, is that the html must be clean. There are cases where you can pass in hacked html, and it won't match it, in which case it'll return the hacked html string as it won't match anything to replace. This isn't strictly whitelisting.

I would suggest a purpose built tool like AntiSamy. It works by actually parsing the HTML, and then traversing the DOM and removing anything that's not in the configurable whitelist. The major difference is the ability to gracefully handle malformed HTML.

The best part is that it actually unit tests for all the XSS attacks on the above site. Besides, what could be easier than this API call:

public String toSafeHtml(String html) throws ScanException, PolicyException {

    Policy policy = Policy.getInstance(POLICY_FILE);
    AntiSamy antiSamy = new AntiSamy();
    CleanResults cleanResults = antiSamy.scan(html, policy);
    return cleanResults.getCleanHTML().trim();
}

Solution 2:

I extracted from NoScript best Anti-XSS addon, here is its Regex: Work flawless:

<[^\w<>]*(?:[^<>"'\s]*:)?[^\w<>]*(?:\W*s\W*c\W*r\W*i\W*p\W*t|\W*f\W*o\W*r\W*m|\W*s\W*t\W*y\W*l\W*e|\W*s\W*v\W*g|\W*m\W*a\W*r\W*q\W*u\W*e\W*e|(?:\W*l\W*i\W*n\W*k|\W*o\W*b\W*j\W*e\W*c\W*t|\W*e\W*m\W*b\W*e\W*d|\W*a\W*p\W*p\W*l\W*e\W*t|\W*p\W*a\W*r\W*a\W*m|\W*i?\W*f\W*r\W*a\W*m\W*e|\W*b\W*a\W*s\W*e|\W*b\W*o\W*d\W*y|\W*m\W*e\W*t\W*a|\W*i\W*m\W*a?\W*g\W*e?|\W*v\W*i\W*d\W*e\W*o|\W*a\W*u\W*d\W*i\W*o|\W*b\W*i\W*n\W*d\W*i\W*n\W*g\W*s|\W*s\W*e\W*t|\W*i\W*s\W*i\W*n\W*d\W*e\W*x|\W*a\W*n\W*i\W*m\W*a\W*t\W*e)[^>\w])|(?:<\w[\s\S]*[\s\0\/]|['"])(?:formaction|style|background|src|lowsrc|ping|on(?:d(?:e(?:vice(?:(?:orienta|mo)tion|proximity|found|light)|livery(?:success|error)|activate)|r(?:ag(?:e(?:n(?:ter|d)|xit)|(?:gestur|leav)e|start|drop|over)?|op)|i(?:s(?:c(?:hargingtimechange|onnect(?:ing|ed))|abled)|aling)|ata(?:setc(?:omplete|hanged)|(?:availabl|chang)e|error)|urationchange|ownloading|blclick)|Moz(?:M(?:agnifyGesture(?:Update|Start)?|ouse(?:PixelScroll|Hittest))|S(?:wipeGesture(?:Update|Start|End)?|crolledAreaChanged)|(?:(?:Press)?TapGestur|BeforeResiz)e|EdgeUI(?:C(?:omplet|ancel)|Start)ed|RotateGesture(?:Update|Start)?|A(?:udioAvailable|fterPaint))|c(?:o(?:m(?:p(?:osition(?:update|start|end)|lete)|mand(?:update)?)|n(?:t(?:rolselect|extmenu)|nect(?:ing|ed))|py)|a(?:(?:llschang|ch)ed|nplay(?:through)?|rdstatechange)|h(?:(?:arging(?:time)?ch)?ange|ecking)|(?:fstate|ell)change|u(?:echange|t)|l(?:ick|ose))|m(?:o(?:z(?:pointerlock(?:change|error)|(?:orientation|time)change|fullscreen(?:change|error)|network(?:down|up)load)|use(?:(?:lea|mo)ve|o(?:ver|ut)|enter|wheel|down|up)|ve(?:start|end)?)|essage|ark)|s(?:t(?:a(?:t(?:uschanged|echange)|lled|rt)|k(?:sessione|comma)nd|op)|e(?:ek(?:complete|ing|ed)|(?:lec(?:tstar)?)?t|n(?:ding|t))|u(?:ccess|spend|bmit)|peech(?:start|end)|ound(?:start|end)|croll|how)|b(?:e(?:for(?:e(?:(?:scriptexecu|activa)te|u(?:nload|pdate)|p(?:aste|rint)|c(?:opy|ut)|editfocus)|deactivate)|gin(?:Event)?)|oun(?:dary|ce)|l(?:ocked|ur)|roadcast|usy)|a(?:n(?:imation(?:iteration|start|end)|tennastatechange)|fter(?:(?:scriptexecu|upda)te|print)|udio(?:process|start|end)|d(?:apteradded|dtrack)|ctivate|lerting|bort)|DOM(?:Node(?:Inserted(?:IntoDocument)?|Removed(?:FromDocument)?)|(?:CharacterData|Subtree)Modified|A(?:ttrModified|ctivate)|Focus(?:Out|In)|MouseScroll)|r(?:e(?:s(?:u(?:m(?:ing|e)|lt)|ize|et)|adystatechange|pea(?:tEven)?t|movetrack|trieving|ceived)|ow(?:s(?:inserted|delete)|e(?:nter|xit))|atechange)|p(?:op(?:up(?:hid(?:den|ing)|show(?:ing|n))|state)|a(?:ge(?:hide|show)|(?:st|us)e|int)|ro(?:pertychange|gress)|lay(?:ing)?)|t(?:ouch(?:(?:lea|mo)ve|en(?:ter|d)|cancel|start)|ime(?:update|out)|ransitionend|ext)|u(?:s(?:erproximity|sdreceived)|p(?:gradeneeded|dateready)|n(?:derflow|load))|f(?:o(?:rm(?:change|input)|cus(?:out|in)?)|i(?:lterchange|nish)|ailed)|l(?:o(?:ad(?:e(?:d(?:meta)?data|nd)|start)?|secapture)|evelchange|y)|g(?:amepad(?:(?:dis)?connected|button(?:down|up)|axismove)|et)|e(?:n(?:d(?:Event|ed)?|abled|ter)|rror(?:update)?|mptied|xit)|i(?:cc(?:cardlockerror|infochange)|n(?:coming|valid|put))|o(?:(?:(?:ff|n)lin|bsolet)e|verflow(?:changed)?|pen)|SVG(?:(?:Unl|L)oad|Resize|Scroll|Abort|Error|Zoom)|h(?:e(?:adphoneschange|l[dp])|ashchange|olding)|v(?:o(?:lum|ic)e|ersion)change|w(?:a(?:it|rn)ing|heel)|key(?:press|down|up)|(?:AppComman|Loa)d|no(?:update|match)|Request|zoom))[\s\0]*=

Test: http://regex101.com/r/rV7zK8

I think it block 99% XSS because it is a part of NoScript, a addon that get updated regularly

Solution 3:

I'm not to convinced that using a regular expression is the best way for finding all suspect code. Regular expressions are quite easy to trick specially when dealing with broken HTML. For example, the regular expression listed in the Sanitize HTML link will fail to remove all 'a' elements that have an attribute between the element name and the attribute 'href':

< a alt="xss injection" href="http://www.malicous.com/bad.php" >

A more robust way of removing malicious code is to rely on a XML Parser that can handle all kind of HTML documents (Tidy, TagSoup, etc) and to select the elements to remove with an XPath expression. Once the HTML document is parsed into a DOM document the elements to revome can be found easily and safely. This is even easy to do with XSLT.

Best regex to catch XSS (Cross-site Scripting) attack (in Java)?

Solution 1:

Solution 2:

Solution 3:

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