Cross-Site Scripting: When Your Own Website Attacks Your Customers

A customer logs into your website, does their usual browsing, and without realising it, hands their session over to somebody else entirely. No malware, no phishing email, nothing suspicious in their inbox. The attack came from your own site, hidden in a comment box or a search field that nobody thought to lock down properly, and by the time anything looks wrong, the damage is already done.

What cross-site scripting actually does

Cross-site scripting, usually shortened to XSS, happens when an attacker sneaks malicious code into a webpage through an input field your site fails to check properly. That code then runs in the browser of anyone who views the page, often without any obvious sign that something has gone wrong. It might steal login cookies, redirect visitors to a fake page, or quietly log what they type into a form that looks entirely genuine. Some variants store themselves permanently in a database, waiting for every future visitor to trigger them; others fire only once, tucked into a single crafted link.

The tricky part is that the vulnerability sits in trusted territory. Visitors have no reason to distrust your domain, so their browser happily executes whatever script has been planted there, treating it exactly like any other legitimate part of the page. A thorough round of web application pen testing is the most reliable way to find these flaws before someone else does, because it tests every field, form, and parameter the way a real attacker would, rather than relying on the site simply looking fine to the Exposed eye.

The business cost of a script nobody noticed

The damage from XSS rarely stays technical for long. A customer whose account gets hijacked through your checkout page will not care that the flaw was three lines of unvalidated code. They will care that their card details or personal information ended up somewhere they should not have, and they will tell other people about it. That single incident can undo months of marketing spend built around trust and reliability, and it tends to surface at the worst possible moment, often when a customer complains publicly before your own team even knows there is a problem.

William Fieldhouse has seen this pattern often enough to sum it up in one line.

“I have tested sites where a single comment field, left unchecked since launch, was the only thing standing between a customer and a stolen session cookie, and nobody in the business even knew that field existed anymore”

— William Fieldhouse, Director of Aardwolf Security Ltd

That is the uncomfortable truth about XSS. It thrives in the parts of a website nobody looks at twice: an old contact form, a search bar bolted on years ago, a third-party widget nobody remembers installing. Each one is a potential doorway, and most businesses have no idea how many doors are actually open, largely because the site has grown piecemeal over several years with different developers touching different corners of it.

Closing the gap before customers find it for you

Fixing cross-site scripting is not usually expensive once it is found, but finding it requires someone who knows exactly where to look and how attackers think. Regular testing, proper input validation, and a habit of reviewing older pages rather than only new ones will catch most of what matters, along with a sensible content security policy that limits what a rogue script could actually do even if one slipped through. If you would like a clear picture of where your own site stands, get in touch with Aardwolf Security for a penetration testing quote and find out what an attacker would see before they do.