Crooks hide software skimmer inside CSS files

Pierluigi Paganini December 09, 2020

Security researchers have uncovered a new technique to inject a software skimmer onto websites, the malware hides in CSS files.

Security researchers have uncovered a new technique used by threat actors to inject a software skimmer onto websites, the attackers hide the malware in CSS files.

Security experts have analyzed multiple Magecart attack techniques over the past months, threat actors compromised websites by hiding malicious code in multiple components of the sites, including live chat windows, images, and favicons.

Now researchers from security firm Sansec have spotted Magecart attacks that hide the software skimmer cascading style sheets (CSS) files.

Cascading Style Sheets (CSS) is a style sheet language used for describing the presentation of a document written in a markup language such as HTML. CSS is designed to enable the separation of presentation and content, including layout, colors, and fonts.

One of the most recent features supported by CSS language allows it to load and run JavaScript code from within a CSS rule. Unfortunately, this new feature is now being abused by threat actors to compromise websites.

Software skimmer CSS

The code used by the attackers is a common keystroke logger, since the company disclosed its discovery the code has been taken offline.

“We found a handful of victim stores with this injection method,” SanSec founder Willem de Groot told ZDNet.

“However, the infrastructure has been in place since September and was previously used for several dozen more traditional attacks. This CSS disguise looks like a recent experiment.”

Researchers recommend shoppers to use virtual cards designed for one-time payments to avoid being victims of these attacks.

Virtual debit cards allow to securely complete a transaction, even if the attackers are able to steal payment card details, the card data is useless once the transaction is completed or the card is expired.

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Pierluigi Paganini

(SecurityAffairs – hacking, software skimmer)

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