Browser fingerprinting is one of the many tactics used by criminals using phishing to bypass security checks and thus extend the usefulness of malicious attack campaigns.

While legitimate organizations have been using browser fingerprinting to uniquely identify web browsers for the past 15 years, it is now also routinely exploited by cybercriminals: a recent study shows that one in four phishing actors use some form of this technique.
The Director of Operational Intelligence of the Fortra’s PhishLabs, Kevin Cryan, explains in the study that browser fingerprinting uses a variety of client-side checks to establish browser identities, which can then be used to detect bots or other unwanted site visits. In this context, numerous pieces of data can be collected as part of fingerprinting, such as: time zone, language settings, IP address, cookie settings, screen resolution, browser privacy or user-agent string.
Many legitimate providers use browser fingerprinting to detect bots misusing their services and other suspicious activity, but phishing site authors have also realized its advantages and are using the technique to avoid automated systems that may flag their website as phishing. By implementing their own browser fingerprinting controls by loading their site’s content, cybercriminals can hide phishing content in real time.
For example, Fortra has observed that threat actors use browser fingerprinting to bypass Google‘s ad review process. Since Google ‘s review process is semi-automated, the implementation of browser fingerprint checks allowed the criminals to identify when the server was seeing their ad destinations compared to a normal user. If the threat actor suspected Google activity, benign content was displayed. This resulted in phishing reports being rejected by Google because malicious content could not be detected.
The bot fight mode of Cloudflare is an example of a legitimate provider that uses browser fingerprinting techniques to identify and block bots. Whenever a website loads with bot fight mode, the following JavaScript is executed and sends the results to Cloudflare. Depending on the results, a captcha will be presented or blocked.
If the JavaScript is decoded, security teams will see that someone is investigating and can infer from the strings displayed that it is requesting numerous browser properties and testing to see the results.
Once the JavaScript is finished, it generates a fingerprint and sends all the information to the phishing site where the results are analysed by the server. Depending on what it determines, benign or malicious content will be displayed.
This fingerprint contains all browser properties, including information about screen dimensions, operating system, GPU hardware, time zone and many other data points. All this information combined can make it very easy to determine whether the browser is real or an emulator.
In the past, cybercriminals could easily avoid detection by exploiting an intermediate server and changing its UserAgent. However, browser fingerprinting is very effective in identifying these automated systems, allowing authors to modify the content of their site based on the results. Understanding the browser properties being collected by criminals through fingerprinting is critical for security teams to avoid suspicion.
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