Researchers discovered a new evasion technique used by attackers in client-side web skimmer to bypass their conceal their fraudulent activity.
Recently, a security researcher revealed a steganography-based credit card skimmer in which they found that the attackers uploads or modifies an existing image and appends the JS code.
With this Steganography technique, attackers hiding malicious code inside of the picture files, and it is a great way to go undetected the malformed files, and it has mainly used to target the eCommerce sites.
There is one chance to figure out the file whether it is malformed are not is by analysing the additional data found at the end of the normal file.
Skimmers Found in Steganographic Images
Researchers from Malwarebytes analyzed the sample malformed steganographic image in a hex editor, and find the extra data was added after the final segment.
By analyzing the strings such as onestepcheckout or authorizenet, research confirmed that this is the credit-card skimming code.
There are several following artifacts researchers observed from the web skimmer.
- Skimmer code loaded from an external resource (script tag with src attribute)
- Exfiltration of the stolen data (HTTP GET or HTTP POST requests with encoded data)
Attackers cleverly using a WebSocket, a communication protocol to conceal a connection to a server controlled by the criminals over a WebSocket.