A backdoor is a method, often secret, of bypassing normal authentication or encryption in a computer system, a product, or an embedded device (e.g. a home router), or its embodiment, e.g. as part of a cryptosystem, an algorithm, a chipset, or a “homunculus computer” —a tiny computer-within-a-computer (such as that found in Intel’s AMT technology). Backdoors are often used for securing remote access to a computer, or obtaining access to plaintext in cryptographic systems. The backdoor may be used to gain access to passwords, delete data on hard drives, or transfer information within the cloud. By Wikipedia
Nowadays, with the widespread use of WordPress, anyone can have an information media, and there are many malicious programs that are installed by falsifying a program on a server called a “WEB backdoor” aimed at the website. Has been confirmed on the site of and is raging around the world.