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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 resources being allowed 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 from where 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 the CSP header the browser follows this directive by default.
* **frame-ancestors**: This directive specifies the sources that can embed the current page. This directive applies to [`<frame>`](https://developer.mozilla.org/en-US/docs/Web/HTML/Element/frame), [`<iframe>`](https://developer.mozilla.org/en-US/docs/Web/HTML/Element/iframe), [`<object>`](https://developer.mozilla.org/en-US/docs/Web/HTML/Element/object), [`<embed>`](https://developer.mozilla.org/en-US/docs/Web/HTML/Element/embed), or [`<applet>`](https://developer.mozilla.org/en-US/docs/Web/HTML/Element/applet). This directive can't be used in tags and applies only to non-HTML resources.
* **plugin-types**: It defines limits on 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 URLs 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 unsafe.
* **unsafe-hashes**: This allows to enable of 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
* **strict-dynamic**: It allows the browser to load and execute new JavaScript tags in the DOM from any script source that has previously been whitelisted by a "nonce" or "hash" value.
If you can somehow make an **allowed JS code created a new script tag** in the DOM with your JS code, because an allowed script is creating it, the **new script tag will be allowed to be executed**.
Moreover, even if you could upload a **JS code inside** a file using an extension accepted by the server (like: _script.png_) this won't be enough because some servers like apache server **select 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)).
#### Payloads using Angular + a library with functions that return the `window` object ([check out this post](https://blog.huli.tw/2022/09/01/en/angularjs-csp-bypass-cdnjs/)):
{% hint style="info" %}
The post shows that you could **load** all **libraries** from `cdn.cloudflare.com` (or any other allowed JS libraries repo), execute all added functions from each library, and check **which functions from which libraries return the `window` object**.
According to [**this CTF writeup**](https://blog-huli-tw.translate.goog/2023/07/28/google-zer0pts-imaginary-ctf-2023-writeup/?\_x\_tr\_sl=es&\_x\_tr\_tl=en&\_x\_tr\_hl=es&\_x\_tr\_pto=wapp#noteninja-3-solves) you can abuse [https://www.google.com/recaptcha/](https://www.google.com/recaptcha/) inside a CSP to execute arbitrary JS code bypassing the CSP:
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.
As described in the [following post](https://sensepost.com/blog/2023/dress-code-the-talk/#bypasses), there are many third party domains, that might be allowed somewhere in the CSP, can be abused to either exfiltrate data or execute JavaScript code. Some of these third-parties are:
| Google Firebase | *.firebaseapp.com | Exfil, Exec |
If you find any of the allowed domains in the CSP of your target, chances are that you might be able to bypass the CSP by registering on the third-party service and, either exfiltrate data to that service or to execute code.
You should be able to exfiltrate data, similarly as it has always be done with [Google Analytics](https://www.humansecurity.com/tech-engineering-blog/exfiltrating-users-private-data-using-google-analytics-to-bypass-csp)/[Google Tag Manager](https://blog.deteact.com/csp-bypass/). In this case, you follow these general steps:
1. Create a new "Facebook Login" app and select "Website".
1. Go to "Settings -> Basic" and get your "App ID"
1. In the target site you want to exfiltrate data from, you can exfiltrate data by directly using the Facebook SDK gadget "fbq" through a "customEvent" and the data payload.
1. Go to your App "Event Manager" and select the application you created (note the event manager could be found in an URL similar to this: https://www.facebook.com/events_manager2/list/pixel/[app-id]/test_events
1. Select the tab "Test Events" to see the events being sent out by "your" web site.
Then, on the victim side, you execute the following code to initialize the Facebook tracking pixel to point to the attacker's Facebook developer account app-id and issue a custom event like this:
As for the other seven third-party domains specified in the previous table, there are many other ways you can abuse them. Refer to the previously [blog post](https://sensepost.com/blog/2023/dress-codethe-talk/#bypasses) for additional explanations about other third-party abuses.
In addition to the aforementioned redirection to bypass path restrictions, there is another technique called Relative Path Overwrite (RPO) that can be used on some servers.
For example, if CSP allows the path `https://example.com/scripts/react/`, it can be bypassed as follows:
The browser will ultimately load `https://example.com/scripts/angular/angular.js`.
This works because for the browser, you are loading a file named `..%2fangular%2fangular.js` located under `https://example.com/scripts/react/`, which is compliant with CSP.
However, for certain servers, when receiving the request, they will decode it, effectively requesting `https://example.com/scripts/react/../angular/angular.js`, which is equivalent to `https://example.com/scripts/angular/angular.js`.
By **exploiting this inconsistency in URL interpretation between the browser and the server, the path rules can be bypassed**.
The solution is to not treat `%2f` as `/` on the server-side, ensuring consistent interpretation between the browser and the server to avoid this issue.
Moreover, if the **page is loading a script using a relative path** (like `<script src="/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.**\
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 classes. For more details visit this awesome [git](https://github.com/cure53/XSSChallengeWiki/wiki/H5SC-Minichallenge-3:-%22Sh\*t,-it's-CSP!%22) repo.
Other JSONP arbitrary execution endpoints can be found in [**here**](https://github.com/zigoo0/JSONBee/blob/master/jsonp.txt) (some of them were deleted or fixed)
What happens when CSP encounters server-side redirection? If the redirection leads to a different origin that is not allowed, it will still fail.
However, according to the description in [CSP spec 4.2.2.3. Paths and Redirects](https://www.w3.org/TR/CSP2/#source-list-paths-and-redirects), if the redirection leads to a different path, it can bypass the original restrictions.
If CSP is set to `https://www.google.com/a/b/c/d`, since the path is considered, both `/test` and `/a/test` scripts will be blocked by CSP.
However, the final `http://localhost:5555/301` will be **redirected on the server-side to `https://www.google.com/complete/search?client=chrome&q=123&jsonp=alert(1)//`**. Since it is a redirection, the **path is not considered**, and the **script can be loaded**, thus bypassing the path restriction.
With this redirection, even if the path is specified completely, it will still be bypassed.
Therefore, the best solution is to ensure that the website does not have any open redirect vulnerabilities and that there are no domains that can be exploited in the CSP rules.
`'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 by exfiltrating the data via images (in this occasion the XSS abuses a CSRF where a page accessible by the bot contains an 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 loading 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/)
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:
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](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**.\
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 `<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:
This attack would imply some social engineering where the attacker **convinces the user to drag and drop a link over the bookmarklet of the browser**. This bookmarklet would contain **malicious javascript** code that when drag\&dropped or clicked would be executed in the context of the current web window, **bypassing CSP and allowing to steal sensitive information** such as cookies or tokens.
For more information [**check the original report here**](https://socradar.io/csp-bypass-unveiled-the-hidden-threat-of-bookmarklets/).
In [**this CTF writeup**](https://github.com/google/google-ctf/tree/master/2023/web-biohazard/solution), CSP is bypassed by injecting inside an allowed iframe a more restrictive CSP that disallowed to load a specific JS file that, then, via **prototype pollution** or **dom clobbering** allowed to **abuse a different script to load an arbitrary script**.
You can **restrict a CSP of an Iframe** with the **`csp`** attribute:
In [**this CTF writeup**](https://github.com/aszx87410/ctf-writeups/issues/48), it was possible via **HTML injection** to **restrict** more a **CSP** so a script preventing CSTI was disabled and therefore the **vulnerability became exploitable.**\
CSP can be made more restrictive using **HTML meta tags** and inline scripts can disabled **removing** the **entry** allowing their **nonce** and **enable specific inline script via sha**:
### JS exfiltration with Content-Security-Policy-Report-Only
If you can manage to make the server responds with the header **`Content-Security-Policy-Report-Only`** with a **value controlled by you** (maybe because of a CRLF), you could make it point your server and if you **wraps** the **JS content** you want to exfiltrate with **`<script>`** and because highly probable `unsafe-inline` isn't allowed by the CSP, this will **trigger a CSP error** and part of the script (containing the sensitive info) will be sent to the server from `Content-Security-Policy-Report-Only`.
For an example [**check this CTF writeup**](https://github.com/maple3142/My-CTF-Challenges/tree/master/TSJ%20CTF%202022/Nim%20Notes).
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 an XSS to the admin.\
**Also pages that are redirected aren't allowed by the security policy, but the page that redirects is.**
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)
PHP is known for **buffering the response to 4096** bytes by default. Therefore, if PHP is showing a warning, by providing **enough data inside warnings**, the **response** will be **sent****before** the **CSP header**, causing the header to be ignored.\
Idea from [**this writeup**](https://hackmd.io/@terjanq/justCTF2020-writeups#Baby-CSP-web-6-solves-406-points).
### Rewrite Error Page
From [**this writeup**](https://blog.ssrf.kr/69) it looks like it was possible to bypass a CSP protection by loading an error page (potentially without CSP) and rewriting its content.
SOME is a technique that abuses an XSS (or highly limited XSS) **in an endpoint of a page** to **abuse****other endpoints of the same origin.** This is done by loading the vulnerable endpoint from an attacker page and then refreshing the attacker page to the real endpoint in the same origin you want to abuse. This way the **vulnerable endpoint** can use the **`opener`** object in the **payload** to **access the DOM** of the **real endpoint to abuse**. For more information check:
Moreover, **wordpress** has a **JSONP** endpoint in `/wp-json/wp/v2/users/1?_jsonp=data` that will **reflect** the **data** sent in the output (with the limitation of only letter, numbers and dots).
An attacker can abuse that endpoint to **generate a SOME attack** against WordPress and **embed** it inside `<script s`rc=`/wp-json/wp/v2/users/1?_jsonp=some_attack></script>` note that this **script** will be **loaded** because it's **allowed by 'self'**. Moreover, and because WordPress is installed, an attacker might abuse the **SOME attack** through the **vulnerable****callback** endpoint that **bypasses the CSP** to give more privileges to a user, install a new plugin...\
For more information about how to perform this attack check [https://octagon.net/blog/2022/05/29/bypass-csp-using-wordpress-by-abusing-same-origin-method-execution/](https://octagon.net/blog/2022/05/29/bypass-csp-using-wordpress-by-abusing-same-origin-method-execution/)
If there is a strict CSP that doesn't allow you to **interact with external servers**, there are some things you can always do to exfiltrate the information.
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