Web Security: Using Content Security Policy against Cross-Site Scripting, Part 1

Cross-site scripting (XSS) remains a serious threat, even though the most commonly used front-end frameworks include many security features out of the box. Frameworks like React or Angular provide mechanisms to mitigate risk by default, but improper implementation or inclusion of external libraries can lead to security vulnerabilities. So developers should not rely solely on these tools, but should continue to code in a security-conscious manner and take additional defensive measures to effectively prevent XSS.

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Attackers are constantly discovering new ways to identify XSS vulnerabilities and insert malicious code into trusted websites. Due to the ongoing threat, development and security teams must regularly update their best practices and continually implement advanced security mechanisms such as Content Security Policy (CSP).

CSP plays a central role in protection against XSS because it dictates to browsers exactly what external content they are allowed to load. This article demonstrates the use of CSP as part of a layered security strategy to prevent XSS attacks and secure web applications.

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Malicious code goes unnoticed with cross-site scripting

have cross-site scripting A common security vulnerability in web applicationsThis makes it possible to embed malicious code – usually in the form of JavaScript – into the pages of a trusted website. When the page is accessed, the browser executes the injected code regardless. This attack is based on the browser’s inability to distinguish between legitimate scripts from the server and malicious scripts from the attackers.

An attack typically goes like this:

1. Identifying vulnerabilities: The attacker specifically looks for opportunities to insert malicious code into a web application. This often happens through input fields, for example for comments, usernames or search queries.
2. Injecting code: If an attacker finds a vulnerability, he injects malicious code into the web application. This can be done directly through input into forms or indirectly through links that contain code in the URL and are sent to unsuspecting users.
3. Execution of malicious code: If an unknowing victim accesses the site, malicious code runs in their browser. The attacker can then, among other things, change the displayed content, redirect the browser to a malicious website, or access cookies to impersonate the victim.

Depending on where the malicious code is stored and executed, a distinction is made between server-side XSS and client-side XSS. In the former, the malicious code is stored on the server and displayed to every user who accesses the affected page. Client-side XSS is an attack through changes to the DOM (Document Object Model) of the page in the victim’s browser, which does not require the client to obtain the affected data from the server.

The following example assumes that a web application has an XSS vulnerability. To exploit such a vulnerability, an attacker can use different payloads. The following code snippet lists some of the different options:

//index.html 
 
 

 
 

 
 Directly execute maliciously injected JavaScript code.

The attack finally tries to end 
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