“Highly capable” hackers root corporate networks by exploiting firewall 0

Volexity, the security firm that discovered the zero-day attacks, said that it’s currently unable to tie the attackers to any previously known groups. However, based on the resources required and the organizations targeted, they are “highly capable” and likely backed by a nation-state. So far, only a single threat group—which Volexity tracks as UTA0218—is known to be leveraging the vulnerability in limited attacks. The company warned that as new groups learn of the vulnerability, CVE-2024-3400 is likely to come under mass exploitation, just as recent zero-days affecting products from the likes of Ivanti, Atlassian, Citrix, and Progress have in recent months.

“As with previous public disclosures of vulnerabilities in these kinds of devices, Volexity assesses that it is likely a spike in exploitation will be observed over the next few days by UTA0218 and potentially other threat actors who may develop exploits for this vulnerability,” company researchers wrote Friday. “This spike in activity will be driven by the urgency of this window of access closing due to mitigations and patches being deployed. It is therefore imperative that organizations act quickly to deploy recommended mitigations and perform compromise reviews of their devices to check whether further internal investigation of their networks is required.”

The earliest attacks Volexity has seen took place on March 26 in what company researchers suspect was UTA0218 testing the vulnerability by placing zero-byte files on firewall devices to validate exploitability. On April 7, the researchers observed the group trying unsuccessfully to install a backdoor on a customer’s firewall. Three days later, the group’s attacks were successfully deploying malicious payloads. Since then, the threat group has deployed custom, never-before-seen post-exploitation malware. The backdoor, which is written in the Python language, allows the attackers to use specially crafted network requests to execute additional commands on hacked devices.

One of the files recovered by Volexity was named update.py, which UTA0218 unsuccessfully tried to download and execute using the CVE-2024-3400 zero-day. Company researchers provided the following analysis of the backdoor, which they track under the name Upstyle:

The purpose of the update.py script is to deploy a backdoor to the following path: /usr/lib/python3.6/site-packages/system.pth. The backdoor, written in Python, starts by an import and its main content is stored as a base64 encoded blob. The .pth extension is used to append additional paths to a Python module. Starting with the release of Python 3.5, lines in .pth files beginning with the text “import” followed by a space or a tab, are executed as described in the official documentation. Therefore, by creating this file, each time any other code on the device attempts to import the module, the malicious code is executed.

The commands to be executed are forged by the attacker by requesting a non-existent web page which contains the specific pattern. The backdoor’s purpose is to then parse the web server error log (/var/log/pan/sslvpn_ngx_error.log) looking for the pattern, and to parse and decode data added to the non-existent URI, executing the command contained within. The command output is then appended to a CSS file which is a legitimate part of the firewall (/var/appweb/sslvpndocs/global-protect/portal/css/bootstrap.min.css).

After the command’s execution is complete and the output has been written, the log entry that was originally read and contained the command is removed from the sslvpn_ngx_error.log file. Fifteen seconds after execution, the original version of bootstrap.min.css is also restored to its previous state. The access and modified timestamps are also restored for both files.

The following image illustrates how CVE-2024-3400 and Upstyle fit together: