A JavaScript skimmer identified earlier this year uses dynamic loading to avoid detection by static malware scanners, Visa warns.
Referred to as Baka, the e-commerce skimmer was first discovered in February 2020, but has already impacted several merchant websites across numerous global regions.
The skimmer is basic, containing the expected components and functionality of such a kit, namely an administration panel, an exfiltration gateway, and a skimming script generator, but has an advanced design, suggesting that it is the work of a skilled developer, Visa notes in a security alert.
Baka features a unique loader, loads dynamically, and obfuscates the malicious code with unique encryption parameters for each of its victims.
To further avoid detection and analysis, the skimmer removes itself from memory when detecting attempts for dynamic analysis using Developer Tools, as well as when the targeted data has been successfully exfiltrated, Visa Payment Fraud Disruption (PFD) says.
“The Baka loader works by dynamically adding a script tag to the current page. The new script tag loads a remote JavaScript file, the URL of which is stored encrypted in the loader script. The attacker can change the URL for each victim,” Visa explains.
The malicious skimming code is fetched and executed when a user visits a merchant’s checkout page. The decrypted payload resembles code used to dynamically load pages.
Visa also discovered that the code would skim the targeted fields every 100 milliseconds, and that the attacker can specify which fields to be targeted for each victim. The code checks if the skimmer found any data, also every 100 milliseconds.
If data is found, the function calls for data exfiltration and a check is performed every 3 seconds to determine whether the script should send data to the exfiltration gateway.
Once the data has been exfiltrated, a clean-up function removes the entire skimming code from memory, for detection evasion.
“To further prevent detection, Baka uses an XOR cipher to encrypt hard-coded values and obfuscate the skimming code delivered by the C2. While the use of an XOR cipher is not new, this is the first time Visa has observed its use in JavaScript skimming malware. The developer of this malware kit uses the same cipher function in the loader and the skimmer,” Visa says.
In November last year, Visa published information on another JavaScript skimmer, called Pipka. Just as Baka, the skimmer had the ability to remove itself after execution, to hinder detection.
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