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Web skimming is a common class of attacks generally aimed at online shoppers. The principle is quite simple: malicious code is injected into the compromised site, which collects and sends user-entered data to a cybercriminal resource. If the attack is successful, the cybercriminals gain access to shoppers’ payment information.

To make the data flow to a third-party resource less visible, fraudsters often register domains resembling the names of popular web services, and in particular, Google Analytics (google-anatytics[.]com, google-analytcsapi[.]com, google-analytc[.]com, google-anaiytlcs[.]com, google-analytics[.]top, google-analytics[.]cm, google-analytics[.]to, google-analytics-js[.]com, googlc-analytics[.]com, etc.). But attack of this kind were also found to sometimes use the authentic service.

To harvest data about visitors using Google Analytics, the site owner must configure the tracking parameters in their account on analytics.google.com, get the tracking ID (trackingId, a string like this: UA-XXXX-Y), and insert it into the web pages together with the tracking code (a special snippet of code). Several tracking codes can rub shoulders on one site, sending data about visitors to different Analytics accounts.

Recently, we identified several cases where this service was misused: attackers injected malicious code into sites, which collected all the data entered by users, and then sent it via Analytics. As a result, the attackers could access the stolen data in their Google Analytics account. We found about two dozen infected sites worldwide. The victims included stores in Europe and North and South America selling digital equipment, cosmetics, food products, spare parts etc.

The screenshot below shows how the infection looks — malicious code with the attacker’s tracking code and tracking ID

The attacker tries to hide their malicious activity using a classic anti-debugging technique. Screenshot 2 shows code for checking whether Developer mode is enabled in the visitor’s browser. The code in the screenshot above is executed only if the result is negative.

Curiously, the attackers left themselves a loophole — the option to monitor the script in Debug mode. If the browser’s local storage (localStorage) contains the value ‘debug_mode’==’11’, the malicious code will spring into life even with the developer tools open, and will go as far as to write comments to the console in clumsy English with errors. In screenshot 3, the line with the ‘debug_mode’ check follows the implementation of the RC4 encryption algorithm (used to encrypt the harvested data before sending it).

If the anti-debugging is passed, the script collects everything anyone inputs on the site (as well as information about the user who entered the data: IP address, UserAgent, time zone). The collected data is encrypted and sent using the Google Analytics Measurement Protocol. The collection and sending process is shown in screenshot 4.
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