When visiting a web page with public-facing user input fields, we’re encouraged to submit content that alters what subsequent visitors see on that web page. For example, if we want to comment on an article we just read on a news site, we can simply type a string of text into a clearly labeled “comment” field, and once we’re done, we can click “enter” or “submit” to incorporate that information into the web page’s display. When the next visitor to this news site opens and reads that same article, their browser will interpret updated information from the web server that includes our comment, and that visitor can now read our comment along with the original article.

Most of the time, the content of the comments we post is exactly what the web application expects (in this case, something to the effect of plain text with a few punctuation characters here and there). If the website has high security standards, it won’t blindly trust that users are entering the exact form of content it expects – it will attempt to validate and clean that input to enforce its content limitations. If, however, the application does not properly validate and sanitize the input we’re allowed to submit, a user (threat actor) with malicious intent can easily induce the server to display insecure content – which they fully control – for subsequent web page visitors. This threat actor can, for example, enter malicious scripts into comment fields that other users’ client-side browsers automatically interpret and execute, allowing them to infiltrate and compromise those users’ devices.

This scenario represents one of several ways in which a common form of cyber-attack – widely referred to as Cross-Site Scripting (XSS) – can be carried out through a vulnerable website or web application. Once malicious scripts are executed by a client-side user’s browser, there’s little the affected user can do; the attacker can now carry out criminal actions such as stealing the user’s personal information, hijacking their session, redirecting them to malicious websites with virus or malware payloads, and much more. When this occurs, the vulnerable website or web application bears responsibility for allowing this breach to affect a patron of their service, and the ensuing loss of trust can significantly impact their future business prospects.

How Does Cross-Site Scripting Work?

To initiate an XSS attack, a threat actor must first identify a website which has poor input validation and sanitization measures. Targets in real-life XSS breaches have included a variety of popular sites including prominent news sites (exploiting insecure comment sections as outlined in the example above), email hosts, social media sites, and even ecommerce platforms.

When an attacker identifies an XSS vulnerability, they can enter a malicious script into a user-input field and induce subsequent client-side browsers to execute that script. They can also inject scripts using malicious URLs, HTTP headers and insecure Cookies. These attacks can impact more than one user at a time, and in particularly devastating cases, they can be extremely difficult for those users to recover from.

What Makes a Website Vulnerable to Cross-Site Scripting?

A website is vulnerable to Cross-Site Scripting attacks when it does not properly scrutinize user input or attempt to neutralize potential threats before it displays user-supplied content for subsequent web page viewers.

A web page must first validate the input it receives against the input it expects; for example, if a particular input field expects only integers, it should not process and display string values or special characters, and it must not allow incorrect content to persist to the database.

The web page must also sanitize user supplied content, which means removing malicious elements from the original user input. If, for example, an attacker attempts to initiate an XSS attack by supplying a malicious script enclosed within HTML tags, the web application should ensure these tags are completely removed from the text input so user-supplied HTML cannot be executed in the next visitor’s browser.

Preventing XSS Attacks with Cloudmersive APIs

The XSS Protection iteration of the Cloudmersive Security API defends web applications against XSS attacks by detecting and subsequently normalizing (removing) threats from an input string. The API response provides a Boolean determining if the input string contained an XSS attack, and below that, it supplies a string containing the original (potentially threatening) input along with a string containing the Normalized Result after the threat is rendered inert.

For more information on the XSS Protection iteration of the Cloudmersive Security API, please do not hesitate to reach out to a member of our Sales Team.