Security researchers have discovered undetectable Linux Malware that utilizes unknown techniques to remain under the radar and targets publicly accessible Docker servers hosting common cloud platforms, including AWS, Azure, and Alibaba Cloud.

When user organizations migrate off-premises more of their business infrastructure, cybercriminals are becoming increasingly inspired to target cloud environments built on Linux, including Docker servers with misconfigured API ports.

And while crypto-jacking schemes comprise some of the more traditional varieties of such Linux-based malware attacks, researchers have just revealed the discovery of a Docker container attack that distributes a malicious “completely undetectable” backdoor that exploits the Dogecoin cryptocurrency blockchain for dynamic generation of C2 domains.

What is a Docker Server?

Docker is a platform designed to facilitate the development, deployment, and running of applications using containers. Containers allow a developer to assemble and deploy an application as one, with all the parts it needs, such as libraries and other dependencies. Through this, the developer may rest assured, thanks to the container, that the program can run on every other Linux machine irrespective of any modified settings that the computer might have that may vary from the computer used to write and check the code.

Docker is sort of like a virtual machine, in a way. But unlike a virtual machine, instead of building a full virtual operating system, Docker allows applications to use the same Linux kernel as the system they are running on and needs only applications to be shipped with items that are not already running on the host. This provides a significant boost in efficiency and reduces application size.

The Docker Servers attack

The vulnerability is aimed at misconfigured containerized cloud environments. The attackers search for and manipulate publicly available Docker API ports to mount their containers and execute the malware on the infrastructure of the victims. During this attack, the attackers spawn and remove several containers.

Any container created during the attack is based on an alpine image mounted with curl. The picture can be seen on the Docker website. The picture is not malicious, but to conduct malicious acts is being exploited. Curl commands are executed by using an image that includes the curl program as soon as the container is up and running.

The benefit of using an image that is open to the public is that the attacker does not need to hide it on Docker Hub or other hosting solutions. The attackers may then use an existing image and execute their logic and malware on top of it.

As mentioned above, however, attackers may build any container that they will use as a container escape method to execute code from the hosting machine. The technique is based on constructing a new container that is achieved by posting a request for a ‘build’ API. The application body includes container configuration parameters. One of the parameters is bind which allows the user to configure which file or directory to mount into a container on the host machine.

Containers that are created during the attack are configured to bind /tmpXXXXXX to the hosting server’s root directory. It ensures that any file on the server filesystem can be accessed and even changed from inside the container, with the appropriate user permissions.

Ngrok is a service that provides secure tunnels to communicate with the public internet from local servers. The attacker exploits Ngrok to create short-lived unique URLs and uses them to access payloads during the attack by transferring them to the image based on curl. The downloaded payload is stored in the file directory /tmpXXXXXX.

Using the attached configuration, the host cron function can be managed by the attacker. The attacker modifies the cron of the host to execute every minute of the downloaded payload. We have observed two types of payloads: one is a script for network scanners and the other is a script for downloaders.

The network scanner checks the ports associated with Redis, Docker, SSH, and HTTP using zmap, zgrap, and jq.

Using a list of hardcoded IP address ranges, which often belong to cloud servers such as AWS and local cloud providers in international regions (we’ve seen providers from China, Austria, and the UK), the script collects the information and uploads it to another Ngrok URL.

The downloader script is responsible for downloading and installing various malware binaries, which are also one of many well-known crypto miners. We found that a fully undetected portion of malware can be installed too. We have called this malware Doki and in the next section, we will include a technical review.

The attacker has full control over the design of the container that he generates and the files that fall into the container. The attacker can escape from the container he created by using valid API commands and executing any code from inside the server itself.

Increase in Targeting Instances Docker servers Attacks

Moreover, while the C&C framework for Doki malware is something clever and innovative, the real challenge here is the relentless attacks on Docker servers. Docker servers have been steadily attacked by malware operators over the last few months, and in particular by crypto-mining gangs.

Cyber Security firms have documented several different crypto-mining campaigns just over the past month that targeted misconfigured Docker APIs to install new Linux servers where they run crypto-mining malware to make a profit using resources for the victim.

It includes Palo Alto Networks results, and two Aqua studies[1, 2]. In addition, the cyber-security company Trend Micro also posted on a series of attacks in which hackers attacked Docker servers to mount DDoS malware, an unusual case where hackers did not opt for a crypto-mining payload.

All in all, the conclusion here is that businesses using Docker as their cloud platform for virtualization need to ensure that the API of the management interface is not exposed to the internet —a slight misconfiguration that enables third parties to access their Docker installation.

The only way to protect what you’ve worked hard to build is to be vigilant when it comes to cybersecurity. If you’d like to know more about how your business can benefit from managed services, just give us a call, we are here to help.

Undetectable Linux Malware Targeting Docker Servers With Exposed APIs ... stay under the radar and targets publicly accessible Docker servers hostings.