What are Backdoor Attacks and How to Mitigate Them

A backdoor is a kind of malware that negates normal authentication procedures to access a system. This results in the grant of remote access to resources within an application like file servers and databases, letting perpetrators remotely issue system commands and update malware. 

A backdoor can be installed by taking advantage of vulnerable components in a web application. It will not be easy to detect when installed because files will be highly obfuscated. Webserver backdoors are used for some malicious activities like data theft, server hijacking, website defacing, infecting site visitors, and launching distributed denial of service (DDoS) attacks. 

Installing Backdoor Trojan

Prevalent installation methods can lead to backdoor exploit through remote file inclusion (RFI). Typically, perpetrators identify targets using scanners that locate websites having unpatched or outdated components that allow file injection. Then, a successful scanner abuses the vulnerability to install the backdoor on the underlying server. 

Backdoor trojan injection is usually performed in a two-step process to bypass security rules, preventing the upload of files above a particular size. The process includes installing a dropper, which is a small file meant to retrieve a larger file from a remote location. Then, the next stage includes downloading and installing the backdoor script on the server. 

The Challenge of Removing Backdoor Shell

Backdoors are not easy to weed out when installed. Traditionally, detection includes the use of software scanners to look for known malware signatures in a server file system. But, this process is prone. Backdoor shell files are nearly always masked by using alias names and code obfuscation. 

To complete detection further, a lot of applications are made on external frameworks that use third-party plugins. In some instances, these are laden with built-in backdoors or vulnerabilities. Scanners that depend on signature-based rules are unlikely to detect the concealed code in such frameworks. Even if a backdoor is detected, it cannot be removed from an application using typical mitigation methods. 

Backdoor Shell Attacks Mitigation

To mitigate backdoor shell attacks a combination of methods can be used to prevent backdoor installation and detect and quarantine existing backdoor shells. Web application firewalls use a combination of default and user-friendly security rules to prevent RFI attacks from compromising an application. If the webserver is already compromised before onboarding, a reliable backdoor protection solution can be employed to detect and remove shells from the file system. The solution works by intercepting connection requests to malicious shells.