Weaponized Free Download Manager for Linux Steals System Data

In recent years, Linux systems gained prominence among diverse threat actors, with more than 260,000 unique samples emerging in H1 2023.

In the case of Linux, threat actors can run multiple campaigns without being detected for years, and maintain long-term existence on the compromised systems.

Cybersecurity researchers at Kaspersky Lab recently detected that threat actors are weaponizing the Free Download manager for Linux to steal system data and passwords.

While investigating the suspicious domains, cybersecurity analysts uncovered a persistent and long-lasting attack. Here below, we have mentioned the set of domains analyzed by security experts-

• 2c9bf1811ff428ef9ec999cc7544b43950947b0f.u.fdmpkg[.]org
• c6d76b1748b67fbc21ab493281dd1c7a558e3047.u.fdmpkg[.]org
• 0727bedf5c1f85f58337798a63812aa986448473.u.fdmpkg[.]org
• c3a05f0dac05669765800471abc1fdaba15e3360.u.fdmpkg[.]org

Weaponized Free Download Manager

These domains raise alarm for security researchers, hinting at potential malware using the domain-generation algorithms for C2 communication, and that’s why analysts primarily focused on the following domain:-

The domain under scrutiny features the “deb.fdmpkg[.]org” subdomain that leads to the following webpage:-

The subdomain claims to host the ‘Free Download Manager’ Debian repository, but we found a malicious package at “https://deb.fdmpkg[.]org/freedownloadmanager.deb”. 

This package deploys infected scripts and creates persistence with a cron task running /var/tmp/crond every 10 minutes.

The infected package installed a January 24, 2020 version of Free Download Manager. The postinst script, in Russian and Ukrainian, references malware improvements and activist messages, with the following dates:- 

• 20200126 (January 26, 2020)
• 20200127 (January 27, 2020)

After installation, the package triggers /var/tmp/crond on startup via cron, serving as a self-contained backdoor using statically linked dietlibc for Linux API access. It contacts a secondary C2 server via DNS request to <hex-encoded 20-byte string>.u.fdmpkg[.]org.

Now after parsing the DNS response, the backdoor establishes a reverse shell via SSL or TCP with the secondary C2 server, using /var/tmp/bs for SSL or self-made for TCP.

Discovering the crond backdoor’s reverse shell, experts tested it in a malware analysis sandbox, finding it delivered a Bash stealer. This stealer gathers system info like:-

• Browsing history
• Saved passwords
• Crypto wallet files
• Cloud service credentials

Here below we have mentioned the cloud services that are targeted primarily:-

• AWS
• Google Cloud
• Oracle Cloud Infrastructure
• Azure

The stealer retrieves an uploader binary from the C2 server, stores it in /var/tmp/atd, and employs it to send stolen data to the infrastructure that is under the control of attackers.

Origins

Moreover, security analysts found Linux installation tutorials for Free Download Manager on YouTube videos.

After tracing the Free Download Manager package’s infection source, researchers linked the discovered implants to Bew backdoor and Exim mail server vulnerability.

Despite the malware’s long history and noisy implants, the malicious Free Download Manager package went undetected for over three years, affecting victims worldwide.

However, besides this, it’s been confirmed by the security analysts that this campaign is inactive at the moment, but, recommended users to equip their Linux machines with robust security solutions.

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