How Cross-Frame Scripting Works
In combination with a suitable browser bug, this may allow the attacker to obtain login credentials and other sensitive information from the unsuspecting user who is interacting with the framed legitimate site as usual. From this point of view, XFS can be considered a variety of clickjacking.
The Difference Between Cross-Frame Scripting and Cross-Site Scripting
Before we get into the technical details, let’s clear up the naming confusion. Despite the similar name, cross-frame scripting has nothing to do with cross-site scripting (XSS). There is a lot of unclear or misleading information online, even on the OWASP site, so to be clear:
- Cross-frame scripting (XFS) means snooping on the user after tricking them to visit a malicious site that contains an iframe with a legitimate page.
That said, if the embedded page is vulnerable to an XSS attack, the two can be combined – more on that later.
When XFS Is Possible
However, specific browser bugs might make it possible for a parent frame to access a child frame loaded from a different origin. If a vulnerable browser version is used to open a specially prepared malicious website (typically after clicking a phishing link), the attacker may snoop on the user’s actions. Note that a successful XFS attack requires all of the following to be true:
- The user receives and opens a malicious URL.
- The user’s browser has a buggy implementation of same-origin policy.
- The legitimate website that lures the user allows itself to be embedded in an iframe.
In this day and age, it is highly unlikely that all three conditions will be met. While users are still quite happy to click phishing links, finding someone who uses a browser with a suitable bug (such as some versions of Internet Explorer) would be a tall order. It is also common practice nowadays to prevent sites from being loaded into frames. All things considered, XFS can be regarded as a relatively minor web application security threat.
What Data Can Be Leaked
If successful, an XFS attack may extract key events and other browser events that provide information about the browser user’s activity on the framed page. Login pages are obvious targets for extracting credentials, but other pages can also be targeted to steal personal details and other confidential information. By manipulating the visible frame, attackers might also use XFS to perform clickjacking – or worse.
Can You Combine XFS and XSS?
meta redirect injection, the iframe version has the advantage that the user only ever sees the URL of the legitimate page in the address bar, making the attack harder to spot.
Preventing Cross-Frame Scripting
Even though cross-frame scripting vulnerabilities are not a major threat (unless you can find a reliable way to exploit an old IE bug or new bugs come along), controlling framing permissions in general is a basic web hardening practice to prevent clickjacking and similar attacks. Framebusting code was the legacy way to prevent your page from being framed, but many such countermeasures could be bypassed by a determined attacker. HTTP security headers are a much better way, with the X-Frame-Options header and the Content Security Policy
frame-ancestors directive available to control framing behavior.
Checking your sites with Invicti is always a good idea, as it provides best-practice recommendations if it finds missing or misconfigured HTTP headers, including framing controls. And, as always, you should use a modern browser and update it regularly to minimize the risk of critical bugs. That way, you can at least be sure that something as fundamental as same-origin policy is always enforced as expected.
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