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Cross-site scripting, also known as XSS, is a cyberattack that happens when a hacker injects malicious code into a legitimate website. Learn where XSS attacks come from and how they work, then find out how to protect yourself against all types of online threats with a top-tier security solution like Avast One.
XSS attacks are a type of code injection attack, a broad category of cybercrimes. To perform one of these injection attacks, a hacker adds — injects — their own code into a vulnerable program. Some attacks completely alter the program’s execution, while others add additional results, such as downloading malware onto a victim’s computer or stealing personal data.
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In computer programming, a script is a series of instructions that tells a program what to do. Hackers can exploit vulnerabilities in a website’s code to inject their own malicious scripts that change the website’s behavior in some way.
Your browser can’t tell the difference between the website’s legitimate code and anything that has been injected by a cybercriminal. Your browser automatically executes all the scripts on a given website, so it will run the hacker’s code along with everything else.
XSS lets hackers turn legitimate websites into traps. Normal browsing turns dangerous when you land on a site that’s infected with hidden malicious scripts. Even worse, XSS means the hacker doesn’t have to attack each victim directly — they can sit back and let the infected websites do all the work — reaching far more people simultaneously.
Behind every website is a bunch of code written in a markup language called HTML. Computer programs, such as web applications, use markup languages to distinguish code from normal text.
A web application is a remotely-stored program delivered to you on your computer via your internet browser. Common examples include webmail services and e-stores. As you feed input into the web app, it interprets and acts on your requests back on its server, then relays the results to you in your browser. The difference between web applications and native apps is that the latter are “traditional” programs that are installed and run fully on your device.
Avast One helps to stop XSS in its tracks, so you’re never at risk of giving away your personal info to a hacker. It also provides 24/7 protection against malicious links, infected email attachments, and all the other online threats out there.
There are three basic categories of XSS attacks. The first two — reflected and stored — involve code manipulation on the web application server, while DOM-based XSS attacks are a newer category that function exclusively on the victim’s device.
Reflected XSS is the simplest and most common of the three types of XSS attacks. It’s the easiest to execute, and (as is usually the case in these situations) it’s also the easiest for you to detect and avoid. In fact, it’s the only type that you can reasonably hope to detect on your own, without the aid of antivirus software. Also known as non-persistent attacks, reflected XSS requires victims to click malicious links already primed with the hacker’s script. The victim’s browser sends the loaded request to the website’s server, and it’s then “reflected” back to the victim via an infected page. The hacker doesn’t have to breach the website itself, but they do need to convince potential victims to click their infected links. It’s a trade-off between technical wizardry and feats of social engineering.
Whereas reflected XSS attacks must be actively initiated every time via a malicious link, stored XSS overcomes this limitation. Stored XSS allows hackers to permanently inject their scripts directly into a website, infecting each user who visits it. That’s why they’re also known as persistent cross-site scripting attacks. Stored XSS attacks are more complicated than their reflected counterparts, because a hacker must identify a website with:
Enough traffic to be worth attacking, and
A security vulnerability that can be targeted with stored XSS attacks.
But if a hacker can pull it off, they’re able to affect a much wider range of potential victims — anyone who views an infected site will have the malicious scripts delivered to their browser. It’s for this reason that stored XSS is nearly impossible to detect. There aren’t any warning signs to raise suspicions until it’s too late.
By design, scripts are automated procedures that don’t need any input from you to execute. This automation makes most XSS attacks impossible to detect, since your browser won’t know which scripts are legitimate and which have been injected by a hacker. Reflected XSS attacks are an exception to the rule — it’s possible to avoid these as described above. Otherwise, it’s the responsibility of a website’s admin to find and plug any XSS vulnerabilities before they can be exploited to target users.
With reflected cross-site scripting, the attacker’s challenge is to trick the victim into clicking a poisoned link. But that also gives you a chance to sidestep this type of XSS attack, unlike the other two types. You can avoid reflected XSS attacks by following one of the internet’s most hallowed best practices: don’t click suspicious links.
Ignore links that you find in:
Website comment sections, forums, and message boards.
Emails and instant messages from unknown senders.
Social media posts from people you don’t know.
Suspicious messages from your contacts that don’t sound like them — they may have been hacked.
But even those tips aren’t foolproof. For example, hackers can use spoofing to make it seem as though their communications are being sent to you from trusted contacts. Or, they can trick you into clicking with effective social engineering chicanery. Always keep your guard up when it comes to the internet.
XSS attacks are notoriously difficult to detect, and you can’t rely on your browser to keep you safe. Take an active stand against cybercriminals by using a powerful security solution designed to detect and prevent threats of all stripes.
Avast One is a comprehensive cybersecurity tool trusted by hundreds of millions of people all around the world. By continually monitoring your device and network connection for any hints of trouble, Avast safeguards your valuable data against even the sneakiest hacking techniques.