What is a Backdoor Attack?

Backdoor Attacks Overview

All cybersecurity breaches are bad for organizations that experience them. But they’re especially harmful when attackers enter your systems without your knowledge.

This is precisely what happens during a backdoor attack. Using this technique, threat actors leverage a hidden access point – which they typically create for themselves by planting malware into an application or server – that they can use to enter a system on an ongoing basis. From there, they can carry out additional attacks, often while eluding detection by the business that has been breached.

Because backdoor attacks can lead to ongoing breaches that are difficult to identify, defending against them is critical. Keep reading for guidance on this topic as we explain how backdoor attacks work, as well as how to avoid them and detect them if they do occur.

The Goal of Backdoor Attacks in Cybersecurity

From a threat actor’s perspective, backdoor attacks offer two key advantages. One is that they enable access without requiring attackers to devise a means of accessing or controlling a system without having to find a way to defeat its standard access controls. On robust applications that use techniques like multi-factor authentication (MFA) to block unauthorized access, circumventing these controls can be difficult. A backdoor attack offers another means of accessing a system that ignores its standard authentication framework entirely.

Second, backdoor attacks can be challenging to detect because they don’t involve the use of standard access controls. Often, software systems record logs of standard authentication events, making it possible to determine who has logged in, from where, and at what time. Using this information, engineers may be able to detect unusual access patterns that reflect an attack. But with a backdoor attack, this type of authentication data is typically not available because the attackers access the system using a backdoor, where there is no authentication logging in place to track their activity.

Backdoor Attack Risks

To date, the most serious backdoor attack ever to occur is arguably the SolarWinds software supply chain attack. In that incident, attackers planted malicious code inside a popular software monitoring tool. The code created a backdoor that allowed the attackers to access the systems of many organizations using the software – so in this case, the attack harmed not just SolarWinds, but also thousands of its customers. The financial fallout of the attack reached at least $18 million – and that was the cost just to SolarWinds. The figure doesn’t include financial repercussions for SolarWinds customers.

The SolarWinds attack, which started in 2019, drew widespread attention to the importance of securing the software supply chain, but backdoor attacks remain a prevalent threat. More recently, a major backdoor attack took place against XZ utils, a popular open source software package that is installed by default on many Linux systems. In this instance, too, a single backdoor gave attackers an entry point into the systems of a large number of organizations.

The bottom line: Despite widespread awareness of the threat of backdoor attacks, they remain a serious threat, and they can affect organizations not just in cases where attackers directly plant malicious code inside a company’s own codebases, but also due to security issues in the software supply chain, which can become a vector for distributing backdoors into the IT estates of organizations that use software created by third parties.

How Do Backdoor Attacks Work?

Backdoor attacks typically involve the following steps:

1. Implement a backdoor: First, attackers create or discover a backdoor within a software system. Typically, this happens because attackers deliberately inject malicious code into a system, although in some cases they simply find and take advantage of a preexisting backdoor – such as a remote entry point that a vendor created for maintenance purposes and that the vendor intended to keep secret.
2. Gain access: Using the backdoor, the attackers access the system without having their activities tracked through the system’s standard authentication framework.
3. Expand the attack: Once inside the system, attackers can steal or destroy sensitive data, or deploy additional malware that allows them to escalate the breach (by, for example, attacking other applications on the same server or network).

Types of Backdoor Attacks

Broadly speaking, backdoor attacks can be divided into three main categories.

#1. Backdoor trojans

The most common type of backdoor attack involves the use of trojans, meaning malicious software designed to appear legitimate. If, for example, security flaws in the software development lifecycle (SDLC) expose source code to unauthorized access, threat actors could plant a trojan inside an application, and then use the trojan to connect remotely to the application once it’s running.

#2. Rootkits

Rootkits are similar to trojans in that they are malicious software that can be injected into a legitimate system. However, whereas trojans run as standard “user space” applications, rootkits are typically embedded within a device’s operating system (or more specifically, within its kernel). This means that rather than creating a backdoor through a specific application or service, rootkits allow attackers to erect a backdoor at the operating system level.

Because rootkits are embedded into the operating system and typically become active as soon as a system boots, they can be especially difficult to detect.

#3. Hardware backdoors

In addition to creating a backdoor using software, threat actors can potentially create or discover a hardware backdoor – a remote entry point that is made available through code embedded into a hardware device, such as a server or network router.

Some hardware backdoors emerge because threat actors manage to infiltrate the hardware manufacturing process and inject malicious code into hardware before it ships. However, they can also create this type of backdoor by modifying a device’s firmware. Or, the hardware vendor itself may deliberately create a backdoor that it intends to use for support or maintenance purposes, but which threat actors also discover and abuse.

How to Detect Backdoor Attacks

Because backdoor attacks can happen in multiple ways, there is no single method that is effective at detecting all backdoor attacks. Instead, the best way to identify a backdoor attack is to use a variety of techniques, including:

• Tracking resource consumption patterns: Unusual spikes in CPU or memory consumption by applications or infrastructure may be a sign that an attacker has entered your system via a backdoor.
• Detecting the creation of unusual processes: These could be either the backdoor itself or an indication of efforts by attackers to deploy additional malware after gaining access through the backdoor.
• Monitoring network activity: Anomalous connections, such as the sustained flow of data between an unknown endpoint and an application, could indicate the existence of a backdoor.

Best Practices for Preventing and Mitigating Backdoor Attacks

The techniques we just described are helpful for detecting backdoor attacks. But even better than detection is preventing such attacks from occurring in the first place via practices like the following:

• Application monitoring: During the software development life cycle, teams should monitor applications and code repositories for unexpected changes, which could reflect efforts by attackers to inject malicious source code into an application.
• SCA scanning: Using Software Composition Analysis (SCA), you can validate the origins of third-party software within an application. This also helps ensure that none of the code you’re deploying contains malware.
• Software artifact management: Carefully managing software packages and artifacts (by, for example, using checksums to track binaries) helps prevent threat actors from substituting malicious binaries that contain backdoors for your legitimate applications.
• Hardware monitoring: Validating the origins of hardware devices and ensuring the physical security of devices helps prevent the installation of malicious firmware as a way of launching a hardware backdoor attack.
• Disabling unused network ports and services: In some cases, backdoor attacks may rely on certain ports or services being open, so blocking them provides additional protection.
• Kernel hardening: Kernel hardening frameworks, like SELinux, also provide some protection against backdoor attack risks. Although kernel hardening won’t completely prevent backdoor attacks, it can mitigate the damage they cause by restricting the actions and resources available to malicious users once they are inside a system.

Mitigating Backdoor Attacks with JFrog

The JFrog Software Supply Chain Platform provides the visibility businesses need to get ahead of backdoor attacks. By securely managing software packages and artifacts across all stages of the SDLC using Artifactory and scanning applications with Xray, you can validate that your code is actually the code it’s supposed to be – and that it wasn’t tampered with by attackers aiming to plant a backdoor.

Combined with other backdoor defense techniques, like runtime monitoring, JFrog gives you the peace of mind that comes with knowing your software systems are free of unintended backdoors.