Virtual private network (VPN) service providers ProtonVPN and NordVPN have made another attempt to patch a potentially serious privilege escalation vulnerability that they first tried to address a few months ago.

Fabius Watson of VerSprite Security discovered in March that the Windows versions of the ProtonVPN and NordVPN applications were affected by a vulnerability that could have been abused to execute arbitrary code with elevated privileges. The vendors released patches in April.

However, Cisco researchers discovered that the initial patch could be easily bypassed, triggering a new round of updates from ProtonVPN and NordVPN.

The flaw, initially tracked as CVE-2018-10169, allowed an attacker with low privileges to execute arbitrary code with elevated permissions by making changes to the OpenVPN configuration file. Specifically, an attacker could have added a parameter such as “plugin” or “script-security” to the configuration file and the file specified through these parameters would get executed by OpenVPN with admin privileges.

Both ProtonVPN and NordVPN attempted to resolve the issue by ensuring that the “plugin,” “script-security,” “up” or “down” strings could not be added to the configuration file – all of these parameters allow code or command execution through the VPN program.

However, Cisco researchers discovered that simply adding these parameters in quotation marks in the configuration file bypassed the patch. The company has published a simple proof-of-concept (PoC) exploit that shows how the vulnerability can be exploited to execute Notepad in Windows.

ProtonVPN and NordVPN have now released new fixes, which should be much more effective. They now prevent users with limited privileges from making any kinds of modifications to the configuration files.

The vulnerability is tracked as CVE-2018-3952 (NordVPN) and CVE-2018-4010 (ProtonVPN), and it has been classified as “high severity” for both applications. NordVPN released a patch on August 8, but ProtonVPN made the second fix available only in early September.

“The new patches developed by the editors are different. For ProtonVPN, they put the OpenVPN configuration file in the installation directory, and a standard user cannot modify it. Thus, we cannot add the malicious string in it. For NordVPN, the editor decided to use an XML model to generate an OpenVPN configuration file. A standard user cannot edit the template,” Cisco said in a blog post.

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