Cross-frame scripting (XFS)
What is cross-frame scripting?
In a cross-frame scripting (XFS) attack, the attacker intercepts sensitive information provided by the user by displaying a legitimate site in an inline frame and capturing keystrokes. XFS attacks resemble clickjacking attacks and are sometimes considered a type of clickjacking.
Note that a cross-frame scripting attack is only possible if the victim uses an outdated web browser. The attack also needs to be preceded by social engineering, so it is difficult to perform successfully. For these reasons, XFS is perceived as a minor cybersecurity threat.
How does cross-frame scripting work?
For a successful XFS attack, all the following conditions must be met:
- The target web page must accept valuable user input (for example, login credentials or other sensitive information).
- The victim must use a browser version with browser bugs that make it vulnerable to XFS (for example, Microsoft Internet Explorer 6).
- The victim has to visit the attacker-controlled web page following a social engineering ploy, such as a phishing attack using an open redirect.
Nowadays, it is very unlikely for all these conditions to be met at the same time, especially since only very old browser versions are vulnerable to such attacks. To have any chance of success, an XFS attack must be specifically targeted, for example, at an organization that still uses old browsers. But if all the conditions for a successful attack are met, the following chain of events occurs:
- The victim visits the malicious web page, which then loads the legitimate page ( such as a login page) in an HTML iframe element. The frame element has its borders removed and is maximized to fill the entire page, so the victim thinks they are visiting the legitimate site.
Cross-frame scripting and the Same-Origin Policy
Even older browsers such as Internet Explorer 6 already employed the Same-Origin Policy (which was originally introduced in Netscape Navigator 2 in 1995), but they did not use it for cross-frame communication. This is why XFS attacks are still possible if victims are using browsers with early SOP implementations such as IE6 and Firefox 2.
Cross-frame scripting vs. cross-site scripting
How to detect cross-frame scripting?
Since a cross-frame scripting attack is aimed at a specific user and a particular version of that user’s browser, there is no way to detect any vulnerabilities in the web application itself, simply because there is no such thing as cross-frame scripting vulnerabilities in web applications. There are also no misconfigurations that make web servers vulnerable to XFS.
How to prevent cross-frame scripting?
Frequently asked questions
What is cross-frame scripting?
Cross-frame scripting (XFS) is a cyberattack technique where the malicious hacker targets a specific user with social engineering, tricks them into visiting a malicious site, includes a legitimate site in an invisible browser frame, and then captures sensitive information typed by the victim. This type of attack is most often used to steal login credentials.
Read an article about cross-frame scripting attacks on our blog.
How dangerous is cross-frame scripting?
Cross-frame scripting is considered a very minor issue simply because it requires the victim to use an old, vulnerable browser version, such as Internet Explorer 6 on Windows XP. The attack is not possible in modern browsers because they all fully enforce the Same-Origin Policy.
Learn about Same-Origin Policy and how it protects web browser users.
How to prevent XFS attacks?
Find out more about methods used to protect against clickjacking.
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Written by: Tomasz Andrzej Nidecki, reviewed by: Benjamin Daniel Mussler