The attackers, who Sonatype researchers linked to the SylexSquad malware-as-a-service (MaaS) gang in Spain, gave their programme a not-so-subtle name: "reverse-shell." Reverse shells are programmes that are often used by hackers to run commands remotely and receive data from targeted machines.
"I think what's quite funny about this is that it's just so blatant," says Dan Conn, developer advocate at Sonatype. "Perhaps SylexSquad were advertising themselves, or they simply didn't care about being caught."
Inside the'reverse-shell' Data-Heisting Malware
Sonatype researchers were taken aback when they discovered a package dubbed "reverse-shell" on a public forum. "Why would someone name a malicious package in such a blatantly obvious way?" the researchers pondered in their blog article for Malware Monthly.
In actuality, the programme turned out to be much more than a reverse shell. This was revealed when the researchers studied one of its files, "WindowsDefender.py." WindowsDefender.py contains several routines with apparent names, such as get_login_data(), get_web_history(),get_downloads(),get_cookies(),get_credit_cards(),ImageGrab.grab().
According to the theme, the hackers had not gone to great lengths to conceal their intentions: this was malware designed to steal information.
"With no obfuscation, [this] appears to be a Discord bot that executes commands and performs actions on the infected machine," according to the analysis. "The malware can retrieve cookies, take screenshots, run shell commands, steal browsing history, and send all this data to the attacker's Discord channel."
More information can be found in another file called "setup.py." There were multiple Spanish-language instructions here to "Clone GitHub repository and execute file," "replace with URL of your GitHub repository," and "path where you want to clone the repo" — indicating that reverse-shell was a MaaS product.
Further investigation revealed several "Made by SylexSquad" tags sprinkled throughout the code, some of which was minimally obfuscated. The researchers discovered that SylexSquad was formerly a hacking marketplace running on the Sellix e-commerce platform in 2022. It has subsequently been decommissioned.
Publishing so publicly to a public repo could have been a deliberate attempt by the organisation to draw attention to their product. "How do we know about groups like Anonymous, LulzSec, or Killnet?" Conn inquires rhetorically. "It's because they get a bad reputation."
However, PyPI is considerably more valuable to them than that.
Why Do Hackers Use Public Repositories?
According to Sonatype, the SylexSquad attackers aren't the only miscreants using forums like PyPI and GitHub, and there are a variety of reasons for their audacity.
"Hosting malicious files on a public repository provides bad actors more control over them," the researchers explained in their blog. "It gives them the power of deleting, upgrading, or even doing version control of the payload."
Among other benefits, "it allows the malware to be shared a lot more widely," Conn elaborates, "and it might actually trip up, in particular, a lot of antivirus software that uses generic signatures — like, actual bytes — to store whether something is malicious or not."
In other words, rather of sending malware upfront, which antivirus scanners may detect fast, hackers can just provide a link to their harmful code elsewhere: "By providing a link to a GitHub, they're perhaps evading that check,"" he says.
To avoid becoming a hotspot for hackers, public repositories have protection safeguards in place. Even the finest scanners and moderators are not perfect, and they cannot be everywhere at the same time.
"Hackers take certain measures like encoding or otherwise obfuscating the code they host, to make it a little bit more difficult for automated engines to pick up," Juan Aguirre, security researcher at Sonatype, points out. SylexSquad encoded its malicious software as numbers in this example, utilising easily reversible ASCII codes for each character.
Sonatype reported the package to the PyPI maintainers, and it was removed. But "it's just a game of cat and mouse," Aguirre says. "Someone catches them and they just run to the next spot."
Aguirre sees this tale as part of a larger issue with open source software: as long as malware developers find use in public repositories, organisations must be conscious of the types of packages they may be picking up.
"It's important to understand what it is that you're running," he concludes. "This is a great case for that. You have to have a bill of materials, you've got to know what you're doing, and what dependencies you're using. If you're just blindly installing things and grabbing code you see, things like this could very easily get into your system."
