Backdoors are pieces of code that allow attackers to bypass authentication, maintain their access to the server and reinfect files. Some of those malicious files can be as simple as a single line of code, allowing the execution of remote code, or complex algorithms, providing different functions to the attacker.
Malware creators are almost always trying to hide their code to make the detection and analysis as difficulty as possible. One of such hiding methods is renaming the file to have an image extension, or appending php code to an existing image and then loading the image as code from a different loader component.
Severity
HIGH