Incidents

Beware of Backdoored Linux Mint ISOs

Background

Yesterday a blog post on “The Linux Mint Blog” caught our attention. Apparently criminals managed to compromise a vulnerable instance of WordPress which the project used to run their website. The attackers modified download links pointing to backdoored ISO files of Linux Mint 17.3 Cinnamon edition. This “should only impact people who downloaded this edition on February 20th”, the author of the blog stated.

We managed to get our hands on the malware embedded in the ISO images. Let’s have a quick look.

Malware used

The criminals used a simple backdoor, which is controlled via an unencrypted IRC connection. We found four hardcoded C&C addresses. At the time of writing only one of them was available. We saw approx. 50 connected clients just in this channel called “#mint”:

IRC channels and user count on malicious C&C server

The malware is capable of:

  • running several types of UDP and TCP flooding (used in DDoS attacks)
  • downloading arbitrary files to the victim’s machine
  • executing arbitrary commands on the machine

We’re detecting this type of malware as HEUR:Backdoor.Linux.Tsunami.bh.

According to user reports, the compromised ISO images come with the backdoor’s C-source code, located in /var/lib/man.cy, which is compiled on first startup to “apt-cache” and is then executed.

Activity

While monitoring the C&C channel, we saw the criminal sending several SMB-related commands like “smbtree -N” to the connected bots. Apparently the attacker tries to access SMB/CIFS shares available in the local network of the victims.

Conclusion

In order to detect this kind of attack, one should use PKI with strong cryptographic signatures to ensure the integrity of downloaded software.

Integrity-checks based on file hashes like MD5 or SHA256 are insecure if a project’s website is compromised, since the attacker could also adjust the checksums provided on the website.

IOC

Hostnames and IPs

  • updates[.]absentvodka[.]com
  • updates[.]mintylinux[.]com
  • eggstrawdinarry[.]mylittlerepo[.]com
  • linuxmint[.]kernel-org[.]org
  • 5.104.175.212

MD5s

  • d945f9b959f76afe24f3a804fe316806
  • 7d590864618866c225ede058f1ba61f0

Beware of Backdoored Linux Mint ISOs

Your email address will not be published. Required fields are marked *

 

Reports

Operation TunnelSnake

A newly discovered rootkit that we dub ‘Moriya’ is used by an unknown actor to deploy passive backdoors on public facing servers, facilitating the creation of a covert C&C communication channel through which they can be silently controlled. The victims are located in Africa, South and South-East Asia.

APT trends report Q1 2021

This report highlights significant events related to advanced persistent threat (APT) activity observed in Q1 2021. The summaries are based on our threat intelligence research and provide a representative snapshot of what we have published and discussed in greater detail in our private APT reports.

The leap of a Cycldek-related threat actor

The investigation described in this article started with one such file which caught our attention due to the various improvements it brought to this well-known infection vector.

Subscribe to our weekly e-mails

The hottest research right in your inbox