Cross Site Scripting

Cross-Site Scripting (XSS) attacks are a type of injection, in which malicious scripts are injected into otherwise benign and trusted web sites. XSS attacks occur when an attacker uses a web application to send malicious code, generally in the form of a browser side script, to a different end user. Flaws that allow these attacks to succeed are quite widespread and occur anywhere a web application uses input from a user within the output it generates without validating or encoding it.

In order to run malicious JavaScript code in a victim’s browser, an attacker must first find a way to inject a payload into a web page that the victim visits. Of course, an attacker could use social engineering techniques to convince a user to visit a vulnerable page with an injected JavaScript payload.

In order for an XSS attack to take place the vulnerable website needs to directly include user input in its pages. An attacker can then insert a string that will be used within the web page and treated as code by the victim’s browser.

Types of Cross-Site Scripting Attacks

Reflected XSS Attack

In a Reflected XSS attack, untrusted input sent to a web application is immediately included in the application’s output, i.e. it is reflected from the server back to the browser in the same request. Reflection can occur with error messages, search engine submissions, comment previews, etc. This form of attack can be mounted by persuading a user to click a link or submit a form of the attacker’s choosing. Getting a user to click untrusted links may require a bit of persuasion and involve emailing the target, mounting a UI Redress attack, or using a URL Shortener service to disguise the URL. Social services are particularly vulnerable to shortened URLs since they are commonplace in that setting.

Stored XSS Attack

A Stored XSS attack is when the payload for the attack is stored somewhere and retrieved as users view the targeted data. While a database is to be expected, other persistent storage mechanisms can include caches and logs which also store information for long periods of time. We’ve already learned about Log Injection attacks.

DOM-based XSS Attack

DOM-based XSS can be either reflected or stored and the differentiation lies in how the attack is targeted. Most attacks will strike at the immediate markup of a HTML document. However, HTML may also be manipulated by Javascript using the DOM. An injected payload, rendered safely in HTML, might still be capable of interfering with DOM operations in Javascript. There may also be security vulnerabilities in Javascript libraries or their usage which can also be targeted.

PHP Functions that can prevent cross-site scripting

htmlentities()
htmlspecialchars()
strip_tags()

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