Cross-site scripting (XSS) has been a known vulnerability since 1999. Yet in 2026, it remains a major cause of concern for AppSec leaders, software engineers, and developers alike. Injection attacks (including XSS) ranked fifth on OWASP's 2025 list of top 10 web app vulnerabilities, and the IBM 2026 X-Force Threat Intelligence Index reported a 44% increase in attacks via the exploitation of public-facing applications (including XSS). The question “What is a cross-site scripting (XSS) vulnerability, and how do you manage it?" inevitably pops up at some point, showing how significant this vulnerability really is. Understanding what cross-site scripting is and how it works can help security experts protect against XSS attacks.
Cross-site scripting is a security vulnerability in which an attacker can inject malicious code into a trusted website. The attacker can then exploit users using the legitimate website as a cover. With XSS, the main site is often not compromised, but the version users are exposed to can become a gateway to exploitation. Attackers exploit users' trust in a website's safety to infiltrate their systems.
Vulnerabilities occur when a web application incorporates data from users or other untrusted sources into its responses without sufficient validation or encoding.
Although XSS is more than 25 years old, it remains a major threat in 2026, and its persistence stems from a couple of factors. The chief reason XSS persists as a major threat is that web platforms allow scripts to execute within documents containing user-controlled data. The risks of injection-type vulnerabilities, such as XSS, will persist as long as data and executable code can coexist in the same space.
Other reasons include a wider attack surface due to more compAIlex frontend frameworks, an increase in AI-generated code, and inconsistent security practices amongst developers. To learn more about what are the top cybersecurity threats in 2026 and how to mitigate them, read our blog.
XSS attacks are often executed in two steps or stages. The first is a way for injecting malicious code, often known as a payload, into a web page that the target visits. To accomplish this, the intended website must enable in-page user input. Once the attacker verifies that user input is allowed, they deploy their malicious code on the page, which is treated as part of the page's source code when a user visits. The second step is getting the victim to visit the page with the altered code. Attackers achieve this through social engineering tactics or a phishing attack to direct the victim to the webpage.
There are three different types of XSS attacks: stored XSS, reflected XSS, and DOM-based XSS.
Also known as persistent XSS, this is the most dangerous type of XSS and is executed in a straightforward way. It takes advantage of an application or page without input validation; thus, when an attacker injects their payload, the malicious code is stored permanently or ‘persisted’ in a location like a database on the page. A practical example of this would be the comment areas of blogs or forum posts, where an attacker could include a malicious script.
What this means is that any user who visits that infected page will execute the malicious code.
Reflected XSS is the more common type of XSS. In this scenario, the harmful code isn't embedded within a page; instead, an attacker creates a malicious URL or form input that is echoed back in the web application's response. This XSS attack leverages the victim’s innocent or coerced participation.
DOM-based XSS is a sophisticated variant of XSS that occurs exclusively when a web application incorporates user-provided data into the DOM. Rather than allowing the web server to handle the malicious payload, client-side scripts on the page actively manage data sourced from elements such as the URL fragment or query parameters. If scripts process untrusted data without proper sanitization, there’s a risk of executing attacker-controlled code.
XSS attacks typically begin with an attacker performing automated scanning and fuzzing to discover an injection point, followed by crafting a payload. When the web application fails to sanitize, encode, or properly validate user input, malicious code can take root and wreak havoc.
XSS vulnerabilities can be very damaging to users and pages alike. Damage can range from session hijacking and theft of private data and login credentials to user impersonation, malware delivery, and more. The key risk of XSS is how it can turn legitimate pages into user traps.
Protecting against XSS requires both practical layered defenses and strict coding practices. These should include validating all user and external input and encoding all output data before rendering. A well-defined content security policy should be deployed to restrict what types of scripts can be executed. Finally, an automated layer of dynamic and static security should be deployed through the development stage.
Secure coding training plays an invaluable role in protecting against XSS. This is not because it fills in for technical controls, but because it cuts to the issue from the root. While scanners can catch vulnerabilities after deployment, the risk of vulnerabilities coming up at all falls with trained developers. Security Journey's approach is built on this premise: hands-on labs in live, full applications where developers view entire source code, intercept requests, and patch XSS vulnerabilities however they choose, not multiple-choice exercises or isolated code snippets.
In order to manage XSS, developers need professional training on how to stop it at its source. PCI-DSS 4.0 requirements 6.2.2–6.2.4 and NIST SSDF both mandate developer-specific secure coding education. For this reason, awareness isn't enough, and neither is scanning after the fact.
Security Journey provides role-based learning paths and security knowledge assessments that aid security leaders in identifying where skill gaps exist and how to close them. There is also a security champion program that helps single out team members who will champion safe and secure development ethics within their teams. Book a free demo to see how training from Security Journey can help your team build safeguards against attacks.