Responsible disclosure - Our encounter with Monero mining

On the 18^th of March, we noticed some unusual activity on one of our servers we use for build automation. Further investigation revealed that an outside party had deployed a Monero miner. The server was immediately taken offline. There was no customer data stored on the server and we have since replaced all our private keys and secrets that might have been potentially compromised.

After careful and thorough investigation of the incident, we decided to publish this announcement and share this news with our customers and users, hoping that it might serve as a lesson for others.

At NXLog, we take security seriously. This is our first security incident since the company was founded. Let’s look at some of the details of this incident.

On March 18^th, our monitoring system alerted us about some atypical, high CPU usage on one of our servers. The server was used to run automated tests on the software we build, as well as build automation. We rely on Docker containers for running these workloads, however, the Docker containers responsible for the resource utilization were not familiar to us:

Upon closer inspection, it turned out that these Docker containers had been started in order to run a Monero (XMR) mining tool. Monero is a privacy-focused cryptocurrency that provides anonymity and makes it difficult to trace the transactions. Because of this anonymity, Monero has became popular on the Darknet for illicit activities like this.

When this became evident, we immediately took the server offline and copied the data for forensics to analyze how this happened. Since we are a logging company, the first thing we looked at was the logs that were collected by the Docker containers.

The containers were started to execute the following:

The file named d is fetched from a server that is most likely a compromised WordPress site hosted on Google Cloud. This is a simple shell script that is responsible for fetching the dk86 malware from yet another WordPress site hosted by DigitalOcean:

The malware is fetched and executed as both 64-bit and 32-bit versions to ensure that at least one of them would run, depending on the hardware. Here are their checksums:

Here are the set of commands responsible for executing the payload running the miner:

The payload trace was fetched from a Chinese server hosted on Alibaba Cloud. This is actually a statically-linked binary of the xmrig Monero miner built on Alpine Linux.

The file has been previously reported, e.g., by Malwarebazaar with the following checksums:

Further investigatation into how it was even possible for the malware to be deployed on our server revealed the root cause. It was an error in our iptables firewall:

Due to a DNS resolution failure for a name we used in a rule, the ports were left unprotected after the server was rebooted. Unfortunately, this left the Docker daemon port exposed that was configured to listen on a TCP socket, which by default, requires neither authentication nor encryption. This was yet another oversight on our part since we should not have allowed a Docker API to be listening on an usecured port, even on the local network. On top of this, the Docker daemon and libvirt both manage iptables rules directly and insert FORWARD rules with a higher priority over a custom chain making it even more difficult to protect with the packet filter. PaloAlto Networks has published a nice article about how unsecured Docker deamons are exploited in the wild.

Since the Docker daemon also allows access to the host which could have made it possible for a targeted attack to penetrate further, we have decided to replace and revoke all secrets such as API keys, signing keys and certificates, ssh keys, vpn keys, and auth tokens. There was no customer data stored on this server or other servers where such data could have been accessed. Since we were well prepared for an incident, we managed to rebuild the server from scratch relatively quickly. Within two working days, we were able to resume all operations. The downtime only partially affected our QA and development teams.

Based on our investigation and analysis which have now been completed, this incident was a result of a botnet attack with the purpose of deploying crypto mining software to generate profit for the attackers. It was not a targeted attack to access company data, source code, or other confidential information.

This incident presented a great opportunity for us to revisit our DevOps practices and place even more emphasis on security. In a fast paced IT world where our DevOps team tries to take advantage of the latest technologies, including containers, orchestration and automation tools, monitoring systems and more, it has become increasingly difficult to ensure that everything is secure and immune to human error.

We practice what we preach, and as such, centralizing logs plays a vital role in such a situation. Apart from basic Linux system logs the NXLog Enterprise Edition supports collecting Docker logs, Linux kernel audit logs from the host as well as file integrity monitoring. These resources provide detailed information on how to collect the logs needed for analyzing such incidents. The team at Utpycs has written a blog post about Detecting Docker container malware using osquery. NXLog already supports fetching logs from osquery and we will consider deploying this across our fleet as well.

While increasing our perimeter security is an obvious choice, possible insider threats should be seriously considered, especially with a remote-only workforce that cannot be easily locked behind a corporate firewall. Migrating towards the zero trust security model, and improving secret management would also allow us to better mitigate such an attack. While this has been the first, we cannot be sure this was the last incident to occur no matter how hard we try to prevent these from happening in the future. With proper mitigation techniques, and with the ability to recover quickly, we need to ensure that our organization can remain resilient.

Botond Botyánszki, Founder and CEO of NXLog