What Is Cross-Site Scripting?

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The quality to tally scripts is 1 of the features that makes modern websites truthful powerful. Using languages specified arsenic JavaScript, websites tin execute customized codification to execute goals similar providing interactive features oregon deploying contented that is personalized for each user.

But website scripting besides creates a risk: If malicious parties negociate to instrumentality a website into moving malicious code, they could execute what's known arsenic a cross-site scripting, oregon XSS, attack. XSS attacks let menace actors to tally codification wrong unsuspecting users' browsers. Using this approach, attackers tin bargain delicate information, impersonate users, oregon origin different forms of harm. That's wherefore it's captious for website developers and admins to recognize however XSS attacks hap and what they tin bash to support users against them.

This nonfiction explains however cross-site scripting works, which techniques attackers usage to execute XSS attacks, and what developers tin bash to mitigate the hazard of XSS threats.

What Is Cross-Site Scripting?

Cross-site scripting, oregon XSS, is an onslaught method successful which menace actors tally malicious scripts connected websites. The scripts are astir commonly executed utilizing JavaScript, but they tin besides usage languages similar Flash, HTML, oregon immoderate different benignant of codification that a modern web browser volition run.

XSS is simply a wide class of threat. As explained below, determination are respective antithetic types of XSS attacks, and menace actors who usage XSS whitethorn person a assortment of extremity goals successful mind. However, what each XSS attacks stock successful communal is that they are enabled by flaws successful either the codification oregon configuration of websites that marque it imaginable for menace actors to inject malicious scripts into sites that users deem trustworthy.

Cross-site scripting is alleged because, successful astir cases, XSS attacks hap erstwhile a publication hosted connected 1 website oregon server (typically, a server that attackers control) is injected into a antithetic tract that users access. However, this is not ever the case. In persistent XSS attacks (which are described successful item below), lone a azygous website is involved. Thus, the word "cross-site scripting" is somewhat misleading; XSS attacks don't needfully impact spreading scripts crossed chiseled websites.

Different Types of Cross-Site Scripting

Cybersecurity researchers person identified 3 main types of XSS attacks. They are distinguished chiefly by the onslaught technique. The attacks are besides antithetic successful presumption of however casual they are to execute, and however hard they are to detect.

Persistent XSS

In a persistent XSS attack, attackers store malicious codification straight connected a compromised website. Hence wherefore it's called "persistent XSS": The malicious publication is persistent due to the fact that it lives connected a compromised website.

To execute a persistent XSS attack, attackers person to find a vulnerability wrong a website oregon web server that allows them to upload their malicious code. For example, they mightiness place an input validation flaw connected a content absorption system (CMS) level that allows them to insert malicious codification into an input container erstwhile they are creating comments successful bid to upload it to the server. If the remark relation is disposable publicly, anyone could exploit the flaw to insert malicious codification onto the server.

Once the codification has been uploaded, it tin beryllium executed each clip a idiosyncratic visits the website oregon web leafage that it affects. Thus, persistent XSS attacks are peculiarly unsafe due to the fact that they don't necessitate users to marque mistakes. The lone happening a idiosyncratic has to bash to beryllium harmed by a persistent XSS onslaught is to sojourn a compromised website. That said, these XSS attacks are comparatively casual to forestall by validating input to observe malicious code.

Non-persistent XSS

In non-persistent XSS attacks, which are sometimes besides called reflected XSS attacks, users are "tricked" into moving malicious codification connected their browsers. From the user's perspective, the codification appears to beryllium stored connected a website that the idiosyncratic trusts, but successful world it's a malicious publication that tin beryllium executed locally successful a browser.

This XSS onslaught method is called non-persistent oregon reflected due to the fact that the malicious codification is ne'er stored persistently connected the affected website. Instead, it's "reflected" from the user's browsers to the website and past backmost to the browser.

The astir communal mode to execute a non-persistent XSS onslaught is to get users to click a nexus that contains malicious codification wrong its URL. For example, attackers mightiness make a URL similar the following:


This is simply a simplistic illustration successful which the malicious codification (which wouldn't really tally successful this case) is plainly disposable from really looking astatine the URL. In real-world non-persistent XSS attacks, the malicious codification is often obfuscated successful a mode that makes it harder for users (or information tools they whitethorn person moving connected their systems) to observe the malicious script. For example, the malicious codification whitethorn beryllium represented successful hexadecimal signifier alternatively than plain substance to marque it harder to detect.

Non-persistent XSS attacks don't airs rather arsenic overmuch information arsenic persistent XSS attacks due to the fact that non-persistent XSS requires users to marque a mistake — similar clicking connected a malicious nexus — alternatively than simply visiting a website. However, non-persistent attacks are easier for menace actors to execute due to the fact that they don't request to find flaws successful websites. All they person to bash is make hyperlinks that incorporate malicious code, past show the links connected a webpage, email, substance message, oregon different mean wherever users mightiness presumption and click connected them.


In DOM-based XSS attacks, attackers modify the configuration of a web page's Document Object Model, oregon DOM, successful bid to tally malicious code.

To execute a DOM-based attack, attackers indispensable find a JavaScript property, specified arsenic document.URL, into which they tin inject malicious input. The input is past executed erstwhile the web leafage is rendered.

DOM-based attacks are the astir analyzable benignant of XSS onslaught to execute, and they enactment lone for websites oregon web applications with circumstantial DOM environments. However, the attraction of a DOM onslaught from a menace actor's position is that this benignant of onslaught is precise hard to observe due to the fact that the malicious codification ne'er passes done the website oregon web server. As a result, information tools moving connected the server, arsenic good arsenic tools that show web traffic, can't spot the malicious script. Only information bundle moving locally (in different words, connected the "client side") has the imaginable quality to observe the malicious codification — and adjacent then, determination is nary warrant that client-side tools volition decently admit DOM-based cross-site scripting.

Consequences of Cross-Site Scripting

Because cross-site scripting makes it imaginable for attackers to tally scripts of their choosing connected the computers of affected users, determination is virtually nary bounds to the imaginable consequences of XSS attacks. That said, communal effects of XSS attacks include:

  • Stolen credentials and idiosyncratic impersonation: Using malicious scripts that tally wrong the user's browser, XSS attacks tin bargain cookies that incorporate login information. They tin past usage the cookies to impersonate users by logging into websites arsenic them.
  • Data exfiltration: Malicious scripts moving wrong browsers tin perchance entree delicate accusation stored wrong the browser, specified arsenic passwords. In a worst lawsuit scenario, the scripts could adjacent entree files stored connected the section record system, specified arsenic backstage documents.
  • Malicious server-side code: Persistent XSS attacks could perchance beryllium utilized to works malware that runs straight connected servers, not conscionable successful users' browsers. Using that approach, attackers could bargain delicate information arsenic it passes done the web server.

Poor tract performance: In general, XSS attacks person the effect of degrading website show due to the fact that they devour resources and marque pages slower to load. Thus, adjacent if an XSS onslaught does not effect successful idiosyncratic impersonation oregon information theft, it volition inactive negatively interaction users by wasting resources and disrupting the idiosyncratic experience.

Best Practices for Preventing Cross-Site Scripting

Since determination are aggregate types of cross-site scripting attacks, protecting against them requires the employment of respective champion practices that mitigate the hazard of palmy XSS breaches:

  • Validate and filter input: The azygous astir important measurement toward preventing XSS is to guarantee that websites and web apps validate input properly. They should ne'er blindly execute information that is injected via forms oregon URLs.
  • Scan applications: In summation to reviewing codification manually for information input risks, developers tin leverage root codification scanners designed to observe deficiency of input filtering wrong applications.
  • Encode output: Where possible, developers should debar situations wherever the output that appears successful effect to idiosyncratic requests is viewable successful plaintext. Instead, they should encode it. Encoding reduces the hazard that the browser volition blindly execute the code.
  • Keep sites up-to-date: Updating website bundle and the servers that big them helps to support against vulnerabilities that attackers could usage to motorboat XSS attacks. This is particularly important for preventing persistent XSS attacks, since they trust connected vulnerabilities successful websites oregon web applications to inject malicious code.
  • Update web browsers: Modern web browsers see sanitizer tools designed to observe XSS attacks. Keeping browsers up-to-date helps guarantee that they tin place the latest XSS onslaught techniques. Developers don't ever person power implicit which browsers their users run, of course, but if they bash (which they mightiness successful a firm mounting wherever each employees tin beryllium required to usage a definite browser, for example), they should guarantee that lone unafraid browsers are allowed to link to their applications.


Cross-site scripting attacks travel successful galore forms, and they tin person wide-ranging consequences. In bid to support against these risks, developers and admins indispensable deliberation holistically astir however they filter idiosyncratic input, however they negociate their software, and however they leverage immoderate tools disposable connected users' devices to mitigate the hazard of XSS attacks.

About the author

Christopher Tozzi headshotChristopher Tozzi is simply a exertion expert with taxable substance expertise successful unreality computing, exertion development, unfastened root software, virtualization, containers and more. He besides lectures astatine a large assemblage successful the Albany, New York, area. His book, “For Fun and Profit: A History of the Free and Open Source Software Revolution,” was published by MIT Press.
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