This malware category is related to server-side malicious scripts which are commonly used as backdoors. The typical example of such backdoors are various File Managers, Web Shells, tools for bypassing admin login or various one-purpose scripts allowing the attacker to upload and run another type of malicious scripts. The payload is PHP based, thus intended for server-side use and the payload is executed directly on the server, while the site is loaded. Only the payload result (such as Web Shell environment) is visible in the browser, not the malicious code itself. It's very common, that backdoors don't have any visible signs in the site code and it's impossible to detect them by accessing the infected site from outside. Server level analysis is necessary in case of infection by this type of malware.
file_get_contents() function is used by malware to download remote code which can be executed or directly displayed. This way, malware authors are able to create just really small backdoor which has only this file_get_contents functionality implemented and the payload itself can be downloaded and executed from the remote source.
Any PHP based web site (often through outdated WordPress, Joomla, osCommerce, Magento, Drupal and stolen passwords).
Cleanup is done by deleting the malicious file, which can be found in your system by searching for the dump code below inside your files. Reviewing access logs for non-expected HTTP POSTs can point out the possible infected files.
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Really simple, one line backdoor example. The main payload is loaded using the file_get_contents() function: