Content Security Policy or CSP is a built-in browser technology which **helps protect from attacks such as cross-site scripting (XSS)**. It lists and describes paths and sources, from which the browser can safely load resources. The resources may include images, frames, javascript and more. Here is an example of allowing resource from the local domain (self) to be loaded and executed in-line and allow string code executing functions like `eval`, `setTimeout` or `setInterval:`
Content Security Policy is implemented via **response headers** or **meta elements of the HTML page**. The browser follows the received policy and actively blocks violations as they are detected.
CSP works by restricting the origins that active and passive content can be loaded from. It can additionally restrict certain aspects of active content such as the execution of inline javascript, and the use of `eval()`.
* **script-src**: This directive specifies allowed sources for JavaScript. This includes not only URLs loaded directly into elements, but also things like inline script event handlers (onclick) and XSLT stylesheets which can trigger script execution. 
* **default-src**: This directive defines the policy for fetching resources by default. When fetch directives are absent in CSP header the browser follows this directive by default. 
* **Child-src**: This directive defines allowed resources for web workers and embedded frame contents. 
* **connect-src**: This directive restricts URLs to load using interfaces like fetch, websocket, XMLHttpRequest 
* **frame-src**: This directive restricts URLs to which frames can be called out. 
* **frame-ancestors**: This directive specifies the sources that can embed the current page. This directive applies to , , , and tags. This directive can't be used in tags and applies only to non-HTML resources. 
* **base-uri**: It defines allowed URLs which can be loaded using element. 
* **form-action**: This directive lists valid endpoints for submission from tags. 
* **plugin-types**: It defines limits the kinds of mime types a page may invoke. 
* **upgrade-insecure-requests**: This directive instructs browsers to rewrite URL schemes, changing HTTP to HTTPS. This directive can be useful for websites with large numbers of old URL's that need to be rewritten. 
* **sandbox**: sandbox directive enables a sandbox for the requested resource similar to the sandbox attribute. It applies restrictions to a page's actions including preventing popups, preventing the execution of plugins and scripts, and enforcing a same-origin policy.
* **unsafe-eval**: This allows the use of eval() and similar methods for creating code from strings. This is not a safe practice to include this source in any directive. For the same reason it is named as unsafe.
* **unsafe-hashes**: This allows to enable specific inline event handlers.
* **unsafe-inline**: This allows the use of inline resources, such as inline elements, javascript: URLs, inline event handlers, and inline elements. Again this is not recommended for security reasons.
* **nonce**: A whitelist for specific inline scripts using a cryptographic nonce (number used once). The server must generate a unique nonce value each time it transmits a policy.
* **sha256-\<hash>**: Whitelist scripts with an specific sha256 hash
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](https://github.com/Polydet/polyglot-database)).
Scenarios like this where `script-src` is set to `self` and a particular domain which is whitelisted 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.
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`
If the **base-uri** directive is missing you can abuse it to perform a [**dangling markup injection**](dangling-markup-html-scriptless-injection.md).
Moreover, if the **page is loading a script using a relative path** (like `/js/app.js`) using a **Nonce**, you can abuse the **base****tag** to make it **load** the script from **your own server achieving a XSS.**\
****If the vulnerable page is loaded with **httpS**, make use a httpS url in the base.
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:
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](https://github.com/cure53/XSSChallengeWiki/wiki/H5SC-Minichallenge-3:-%22Sh\*t,-it's-CSP!%22) repo.
`'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):
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 **execute**a 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/](https://www.secjuice.com/hiding-javascript-in-png-csp-bypass/)
This time you can make the victim **load** a page in **your control** via **XSS** with a `<iframe`. 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:
Imagine a situation where a **page is redirecting** to a different **page with a secret depending** on the **user**. For example the user **admin** accessing **redirectme.domain1.com** is redirected to: **adminsecret321.domain2.com** and you can cause a XSS to the admin.\
The CSP violation is an instant leak. All that needs to be done is to load an iframe pointing to `https://redirectme.domain1.com` and listen to `securitypolicyviolation` event which contains `blockedURI` property containing the domain of the blocked URI. That is because the `https://redirectme.domain1.com` (allowed by CSP) redirects to `https://adminsecret321.domain2.com` (**blocked by CSP**). This makes use of undefined behavior of how to handle iframes with CSP. Chrome and Firefox behave differently regarding this.
When you know the characters that may compose the secret subdomain, you can also use a binary search and check when the CSP blocked the resource and when not creating different forbidden domains in the CSP (in this case the secret can be in the form doc-X-XXXX.secdrivencontent.dev)
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:
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](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)
