VoIP phones using Digium's software have been targeted to drop a web shell on their servers as part of an attack campaign designed to exfiltrate data by downloading and executing additional payloads.

"The malware installs multilayer obfuscated PHP backdoors to the web server's file system, downloads new payloads for execution, and schedules recurring tasks to re-infect the host system," Palo Alto Networks Unit 42 said in a Friday report.

The unusual activity is said to have commenced in mid-December 2021 and targets Asterisk, a widely used software implementation of a private branch exchange (PBX) that runs on the open-source Elastix Unified Communications Server.

Unit 42 said the intrusions share similarities with the INJ3CTOR3 campaign that Israeli cybersecurity firm Check Point disclosed in November 2020, alluding to the possibility that they could be a "resurgence" of the previous attacks.

Coinciding with the sudden surge is the public disclosure in December 2021 of a now-patched remote code execution flaw in FreePBX, a web-based open source GUI that's used to control and manage Asterisk. Tracked as CVE-2021-45461, the issue is rated 9.8 out of 10 for severity.

The attacks commence with retrieving an initial dropper shell script from a remote server, which, in turn, is orchestrated to install the PHP web shell in different locations in the file system as well as create two root user accounts to maintain remote access.

It further creates a scheduled task that runs every minute and fetches a remote copy of the shell script from the attacker-controlled domain for execution.

Besides taking measures to cover its tracks, the malware is also equipped to run arbitrary commands, ultimately allowing the hackers to take control of the system, steal information, while also maintaining a backdoor to the compromised hosts.

"The strategy of implanting web shells in vulnerable servers is not a new tactic for malicious actors," the researchers said, adding it's a "common approach malware authors take to launch exploits or run commands remotely."