XSS (Cross-Site Scripting) 🔴 Attack

An XSS (Cross-Site Scripting) attack is a vulnerability A web security attack that allows an attacker to inject malicious code (usually JavaScript) into web pages viewed by other users. Unlike other attacks, which target the server directly, XSS targets the client, i.e. the user's browser.

• TYPE : 🔴 Computer attack by injection client-side code
• DANGEROUS : 💀💀💀💀
• FREQUENCY : 💣💣
• EASE OF ERADICATION : 🧹🧹

 How it works

The XSS attack takes place in several stages:

1. Injection : The attacker injects a malicious script into a vulnerable web page. This can be done in a number of ways:

   • Reflected XSS : the script is injected via an HTTP request (for example, in a URL parameter). The server then returns this script in the HTML response, which is executed by the user's browser. This is the most common type of XSS.
   • Stored XSS : the script is permanently stored on the server (for example, in a database, a forum or a comment). Each time a user visits the page containing the script, it is executed.
   • XSS DOM (DOM-based XSS) : the malicious script manipulates the DOM (Document Object Model) of the user's browser, without the server being directly involved. Injection takes place via data from the client (URL, cookiesetc.).

2. Execution: When users visit the compromised web page, their browser executes the malicious script.

3. 💥 Consequences : The script can then perform various malicious actions:

   • Cookie theft : Cookies often contain authentication information. If they are stolen, the attacker can usurp the user's identity.
   • Redirection to a malicious site : the user is redirected to a site controlled by the attacker, who can then steal personal information or install malicious software.
   • Disfiguring the web page : the script can change the appearance of the web page, display misleading messages or steal information entered by the user (credit card numbers, passwords, etc.).
   • Keylogging : recording of the user's keystrokes, enabling sensitive information to be recovered.
   • Taking control of the user account : in some cases, the attacker can take complete control of the user's account.

👉 Examples

• A web forum allows users to post comments. An attacker injects an XSS script into a comment. Each time a user views the comment, the script is executed, stealing the user's session cookies.
• A web application displays the results of a search using the search term in the URL. An attacker modifies the URL to include an XSS script. When the user clicks on the modified link, the script is executed.

💉 Protection and remedies for XSS attacks

Several measures can be put in place to prevent XSS attacks:

• Data escape : Before displaying user-supplied data in a web page, it is crucial to "escape" it, i.e. to replace special HTML characters (for example, < by <, > by >) so that they are interpreted as text rather than code.
• Validation of entries : check and filter the data entered by the user to ensure that it does not contain malicious code.
• Use of a Content Security Policy (CSP) : CSP allows you to define a security policy that controls the resources that the browser is allowed to load.
• Use of secure frameworks and libraries : These tools often incorporate mechanisms to protect against XSS.
• Regular software updates : updates often correct security flaws, including XSS vulnerabilities.
• Use of HTTPOnly flag for cookies : this flag prevents JavaScript from accessing cookies, thus limiting the impact of cookie theft.