Discovered by Paul Rascagneres.
Cisco Talos has discovered two similar vulnerabilities in the ProtonVPN and NordVPN VPN clients. The vulnerabilities allow attackers to execute code as an administrator on Microsoft Windows operating systems from a standard user. The vulnerabilities were assigned to the CVE IDs TALOS-2018-0622 / CVE-2018-3952 (NordVPN) and TALOS-2018-0679 / CVE-2018-4010 (ProntonVPN).
The vulnerabilities are similar to a bug previously discovered by VerSprite in April 2018: CVE-2018-10169. That same month, both clients released similar patches to fix this flaw. However, we identified a way to bypass that patch. Despite the fix, it is still possible to execute code as an administrator on the system. The details section later on in this post will explain the first patch, why it was not successful, and how the editors finally fixed the problem.
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