The best way to find flaws is to perform a security review of the code and search for all places where input from an HTTP request could possibly make its way into the HTML output. Nessus, Nikto, and some other available tools can help scan a website for these flaws, but can only scratch the surface.
If one part of a website is vulnerable, there is a high likelihood that there are other problems as well. An attacker can steal cookie data via Javascript even when document.
If we need to hide against web application filters we may try to encode string characters, e. We may encode our script in base64 and place it in META tag. This way we get rid of alert totally. More information about this method can be found in RFC Cross-site scripting attacks may occur anywhere that possibly malicious users are allowed to post unregulated material to a trusted website for the consumption of other valid users. The most common example can be found in bulletin-board websites which provide web based mailing list-style functionality.
The code in this example operates correctly if eid contains only standard alphanumeric text. If eid has a value that includes meta-characters or source code, then the code will be executed by the web browser as it displays the HTTP response.
Initially, this might not appear to be much of a vulnerability. After all, why would someone enter a URL that causes malicious code to run on their own computer? The real danger is that an attacker will create the malicious URL, then use e-mail or social engineering tricks to lure victims into visiting a link to the URL.
When victims click the link, they unwittingly reflect the malicious content through the vulnerable web application back to their own computers. The attacker can use injected scripts to change the content of the website or even redirect the browser to another web page, for example, one that contains malicious code.
Consequences of the ability to execute JavaScript on a web page may not seem dire at first. Most web browsers run JavaScript in a very tightly controlled environment. However, JavaScript can still be dangerous if misused as part of malicious content:. The above, in combination with social engineering, allow criminals to pull off advanced attacks including cookie theft, planting trojans, keylogging, phishing, and identity theft.
XSS vulnerabilities provide the perfect ground to escalate attacks to more serious ones. You can read more about them in an article titled Types of XSS. For step one to be possible, the vulnerable website needs to directly include user input in its pages. There are also variants of XSS attacks where the attacker lures the user to visit a URL using social engineering and the payload is part of the link that the user clicks.
The following is a snippet of server-side pseudocode that is used to display the most recent comment on a web page:. The above script simply takes the latest comment from a database and includes it in an HTML page. It assumes that the comment printed out consists of only text and contains no HTML tags or other code.
It is vulnerable to XSS, because an attacker could submit a comment that contains a malicious payload, for example:. Most often, the victim does not realize it and is unable to prevent such an attack. Criminals often use XSS to steal cookies. This allows them to impersonate the victim. The attacker can send the cookie to their own server in many ways. To learn more about how XSS attacks are conducted, you can refer to an article titled A comprehensive tutorial on cross-site scripting.
The following is a list of common XSS attack vectors that an attacker could use to compromise the security of a website or web application through an XSS attack. A script tag can reference external JavaScript code or you can embed the code within the script tag itself. JavaScript event attributes such as onload and onerror can be used in many different tags. This is a very popular XSS attack vector. However, IFrames are still very effective for pulling off phishing attacks.
This ability to exploit commonly used platforms makes XSS attacks both dangerous and widespread. By exploiting XSS vulnerabilities, an attacker can perform malicious actions, such as:. Attackers can trick users into entering credentials on a fake form, which provides all the information to the attacker.
Once they obtain user credentials, attacks could use them to commit identity theft or financial fraud. Stored cross-site scripting Persistent XSS. Stored XSS occurs when user-supplied input is stored and then rendered within a web page. Typical entry points for stored XSS include message forums, blog comments, user profiles, and username fields. An attacker typically exploits this vulnerability by injecting XSS payloads on popular pages of a site or passing a link to a victim, tricking them into viewing the page that contains the stored XSS payload.
Reflected cross-site scripting Non-persistent XSS. In this case, the attacker's payload has to be a part of the request sent to the webserver. Attackers use malicious links, phishing emails, and other social engineering techniques to trick the victim into making a request to the server. Reflected XSS is not a persistent attack, so the attacker needs to deliver the payload to each victim.
These attacks are often made using social networks. In reflected and stored cross-site scripting attacks, you can see the vulnerability payload in the response page, but in DOM-based cross-site scripting, the attack's HTML source code and response will be the same, i. It can only be observed on runtime or by investigating the DOM of the page. This makes it even more difficult to detect for Web Application Firewalls WAFs and security engineers who analyze server logs because they never see the attack.
The embedded tags become a permanent feature of the page, causing the browser to include them with the rest of the source code every time the page is opened. The attacker adds a comment, along these lines: Great value item. After that, each time the page is accessed, the HTML tag in the comment will activate a JavaScript file, which is hosted on another site and can steal visitors' session cookies.
Meanwhile, the visitor, who may not have even scrolled down to the comments section, remains unaware that the attack took place. Unlike a reflected attack, where the script is activated after a link is clicked, a stored attack only requires that the victim visit the compromised web page. This increases the reach of the attack, jeopardizing all visitors no matter how cautious they are.
To avoid falling victim to an XSS attack, individual users should:. We use cookies to make your experience of our websites better. By using and further navigating this website you accept this.
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