Cross-site Scripting (XSS) Affecting elgg/elgg package, versions >=4.0.0-beta.1, <4.0.6<3.3.24

elgg/elgg is a development framework used for creating socially aware web applications.

Affected versions of this package are vulnerable to Cross-site Scripting (XSS) due to missing sanitization of the user input into the address request parameter. This behavior allows authenticated users to report a page to administrator, and instead of inserting the page address it is possible to insert a malicious script.

POC:

1 => Having your Elgg instance, create two different accounts (administrator and normal user);

2 => Logged in as the normal user, go to the Blogs page, and click on any blog (if there is none, you can create one yourself);
3 => Once in the blog page, click on the three dots in the right side of the window, and select the option "Report this". A form will be opened, so that you can specify what exactly is the problem with the page you are reporting;
4 => If you are using a proxy tool, enable the intercept functionality, and then going back to Elgg, click on "Report this". If you are going to use the browser Dev Tools, just open it on the network tab;
5 => Seeing the request that is being sent to /elgg/action/reportedcontent/add, change the address parameter to the script below, and then forward the request. If you are using Dev Tools, maybe you are going to need to edit the original request and re-send it.
javascript:alert(${document.domain}\n${document.cookie})
6 => Now log in as the administrator, an go to the reports page, which can be found on
/admin/administer_utilities/reportedcontent 
7 => In this page, you probably are going to see at first the report you sent containing a script, click on the reference to this report;
8 => After that, an alert will be popped out containing the domain where the code is coming from, and also the session cookies, which can be used as an authentication method for anyone who has it.

A cross-site scripting attack occurs when the attacker tricks a legitimate web-based application or site to accept a request as originating from a trusted source.

This is done by escaping the context of the web application; the web application then delivers that data to its users along with other trusted dynamic content, without validating it. The browser unknowingly executes malicious script on the client side (through client-side languages; usually JavaScript or HTML) in order to perform actions that are otherwise typically blocked by the browser’s Same Origin Policy.

Injecting malicious code is the most prevalent manner by which XSS is exploited; for this reason, escaping characters in order to prevent this manipulation is the top method for securing code against this vulnerability.

Escaping means that the application is coded to mark key characters, and particularly key characters included in user input, to prevent those characters from being interpreted in a dangerous context. For example, in HTML, < can be coded as &lt; and > can be coded as &gt; in order to be interpreted and displayed as themselves in text, while within the code itself, they are used for HTML tags. If malicious content is injected into an application that escapes special characters and that malicious content uses < and > as HTML tags, those characters are nonetheless not interpreted as HTML tags by the browser if they’ve been correctly escaped in the application code and in this way the attempted attack is diverted.

The most prominent use of XSS is to steal cookies (source: OWASP HttpOnly) and hijack user sessions, but XSS exploits have been used to expose sensitive information, enable access to privileged services and functionality and deliver malware.

Types of attacks

There are a few methods by which XSS can be manipulated:

Type	Origin	Description
Stored	Server	The malicious code is inserted in the application (usually as a link) by the attacker. The code is activated every time a user clicks the link.
Reflected	Server	The attacker delivers a malicious link externally from the vulnerable web site application to a user. When clicked, malicious code is sent to the vulnerable web site, which reflects the attack back to the user’s browser.
DOM-based	Client	The attacker forces the user’s browser to render a malicious page. The data in the page itself delivers the cross-site scripting data.
Mutated		The attacker injects code that appears safe, but is then rewritten and modified by the browser, while parsing the markup. An example is rebalancing unclosed quotation marks or even adding quotation marks to unquoted parameters.

Affected environments

The following environments are susceptible to an XSS attack:

• Web servers
• Application servers
• Web application environments

How to prevent

This section describes the top best practices designed to specifically protect your code:

• Sanitize data input in an HTTP request before reflecting it back, ensuring all data is validated, filtered or escaped before echoing anything back to the user, such as the values of query parameters during searches.
• Convert special characters such as ?, &, /, <, > and spaces to their respective HTML or URL encoded equivalents.
• Give users the option to disable client-side scripts.
• Redirect invalid requests.
• Detect simultaneous logins, including those from two separate IP addresses, and invalidate those sessions.
• Use and enforce a Content Security Policy (source: Wikipedia) to disable any features that might be manipulated for an XSS attack.
• Read the documentation for any of the libraries referenced in your code to understand which elements allow for embedded HTML.

• GitHub Commit
• Release Note