Backdoor Attacks: How Hackers Exploit Systems and How to Stop Them

A backdoor is a hidden vulnerability or a method of bypassing a website’s normal authentication procedures that allow access to that system. A backdoor attack is a type of cyber attack in which a hacker gains unauthorized access to that system. 

Although they are technically a virus and a kind of Trojan attack, backdoors aren’t designed to be destructive. They don’t purposefully cause damage to the targeted site. Rather, they provide access to the site, which attackers can use to do whatever kind of damage they want. 

Three types of “backdoors” 

There are actually three types of backdoors, and not all of them are malicious. Hackers can access the code of a website and add a backdoor that didn’t previously exist, or they can inappropriately access and exploit an existing backdoor’s vulnerability. 

Backdoors generally fall into one of three categories. 

1: Government-mandated backdoors

The law requires network developers to include these backdoors to provide lawful interception of data. When a law enforcement or government agency gets a court order or a warrant for information from a website, these backdoors allow quick and reliable access. 

Developers know about these backdoors, and they are protected like any other part of the design code. Many national governments around the world require these backdoors.

2: Network service backdoors

In some circumstances, an individual or group may get temporary access to a backdoor so that they can assist in the set-up, updates, maintenance, or repairs of networks.

A great example is the telecommunications industry, which the law requires to operate with “The Five Nines” in mind. That means that they must provide network connectivity to their customers 99.999% of the time. In order to do so, they sometimes need to allow temporary access to support staff so that they can resolve issues with as little disruption as possible. 

When people get backdoor access to a network, the security team watches them closely and logs their every keystroke.

3. Malicious backdoors

This is the type of backdoor that is concerning to internet users, web developers, network administrators, and more. 

A malicious backdoor exists to steal data, take advantage of system vulnerabilities, and potentially access, delete, or alter private consumer or industry data. 

What is the purpose of a malicious backdoor attack? 

These backdoors are incredibly useful for hackers and cybercriminals, because they are difficult to detect and provide the attacker with remote control of the site through VNC (Virtual Network Computing).

Some of the ways that hackers can use these backdoors include: 

• Remotely accessing and controlling a victim’s system without being detected
• Allowing hackers to remotely execute commands, install additional malware, access or exfiltrate data, or take full control over the compromised system
• Taking control of webcams, microphones, and other surveillance tools for spying and invasion of privacy
• Capturing keystrokes and logging user credentials for other systems
• Pivoting laterally through the network, thus gaining elevated privileges and broader access
• Establishing covert command and control channels to issue instructions for later attacks
• Modifying, disabling, or erasing security controls
• Deleting backups 
• Eliminating the site’s defenses
• Bricking, disrupting, or destroying systems by abusing backdoor privileges, thus rendering devices inoperable

This is not a comprehensive list, but it should give you a good idea of how damaging backdoor access can be.

Where are backdoors created? 

Hackers can create backdoors in a variety of places. They choose where to embed their backdoor based on where it will be the least conspicuous, and therefore, less likely to be detected. 

These locations include:

• Websites
• Domain controllers
• Cloud services
• Network devices
• Servers
• User endpoints
• Mobile devices
• Services and processes
• Firmware and hardware

How attackers create backdoor access 

Hackers can create a malicious backdoor by hacking a piece of code, or they can exploit poor coding practices or undisclosed manufacturer backdoors. 

They can be installed through malware, rootkits, or simply by taking advantage of unpatched vulnerabilities in any system. Advanced persistent threat (APT) groups often use backdoors as part of a multi-stage attack. 

Backdoor attackers try to mask their presence, and they design their backdoors to evade detection by security software. Sometimes, only a specific command or very specific circumstances will reveal them. 

Common backdoor techniques include stealthy remote access trojans (RATs), web shells on web servers, debug interfaces that have been left open, undocumented accounts, and covert communication channels. 

How developers can prevent backdoor attacks

Developers are responsible for providing safety to a number of groups: their employers, people browsing their website or application, and clients and users who share their data with the web application. 

That means that one of their responsibilities is to prevent attackers from utilizing existing backdoors or adding their own to a vulnerable piece of coding. 

These are some of the ways that developers and coders should strive to keep their applications from providing backdoor access to cyber criminals: 

• Follow secure coding practices: Adhere to guidelines like the OWASP Top 10 to avoid introducing vulnerabilities that could become backdoors. Validate inputs, sanitize outputs, use encryption properly, etc.
• Use the principle of least privilege: Only allow services/users the minimum necessary access and permissions. Limit damage from any single account compromise.
• Perform regular auditing and static analysis: Continuously inspect codebases and systems for any abnormalities that could indicate backdoors.
• Track dependencies: Use tools to identify all third party libraries/components and check for known vulnerabilities. Update and patch frequently.
• Practice system hardening: Disable or remove unneeded services, ports, accounts to reduce attack surface. Whitelist allowed connections.
• Use logging and monitoring: Log extensive activity and monitor to detect anomalies that could signify backdoor usage.
• Adopt multi-factor authentication: Require strong MFA such as 2FA (2-factor authentication) to make stolen credentials useless for backdoor access.
• Separate development from production: Isolate development and testing environments from production systems.
• Implement network segmentation: Use VLANs, subnets, firewall rules to constrain backdoors from spreading across networks.
• Be timely in your patching: Install relevant security updates on all systems to eliminate backdoor risks from unfixed flaws.
• Require employee training: Educate all personnel on backdoor threats and how to avoid introducing or propagating them.

How to remove a suspicious backdoor: 11 steps 

When you suspect that there has been suspicious backdoor activity in your web application, you need to act quickly to ensure the security of your work. 

This 11-step process will help you determine what you should do when you suspect that a cyber criminal has added backdoor to your system.

Take note that many of these strategies for removing a backdoor from a system require a complete rebuild – from scratch. Although patches can sometimes be effective, it is more likely that you will need to start over with a variety of systems in order to get rid of the malicious backdoor. 

1. Reinstall the operating system and software applications from a clean, trusted source. This will wipe any backdoors at the software level.
2. Restore systems from a known good backup made before the backdoor was installed. Don’t use compromised backups.
3. Replace compromised firmware like BIOS/UEFI with a clean version from the vendor. You should flash firmware updates.
4. If you suspect hardware backdoors, replace affected hardware with brand new components.
5. Change all passwords, keys, and credentials that could have been accessed via the backdoor.
6. Scan for malware and rootkits using up-to-date tools. Quarantine or delete any infections found.
7. Block suspicious outbound connections, IP addresses, domain names that attackers may use.
8. Monitor system and network activity closely for signs of reinfection. Attackers may try to regain access.
9. Where possible, reinstall or reconfigure network devices that could have backdoors.
10. Consider hiring a forensic investigator to thoroughly inspect systems and identify potential backdoors.
11. For critical systems, have a vendor security team validate that you’ve eliminated all backdoors.

Preventing and responding to backdoors 

Backdoors provide cybercriminals with stealthy and persistent access to systems and data, making them a serious threat to individuals, businesses, and governments alike. 

While not all backdoors are illicit, malicious actors can insert backdoors practically anywhere. They’re designed to evade detection through obfuscation techniques. Defending against backdoor attacks requires constant vigilance through preventative coding practices, system hardening, access control, and monitoring for anomalies. 

If you suspect a backdoor, act swiftly to remove it by reinstalling software, replacing potentially compromised components, changing credentials, and inspecting systems thoroughly. 

With diligence and layered security, organizations can protect themselves against the menace of backdoor attacks that target their critical systems and sensitive data. While backdoors will continue to pose a risk, following security best practices provides the greatest chance of detecting and mitigating these covert and dangerous threats.