Killed by Proxy: Analyzing Client-end TLS Interception Software

To filter SSL/TLS-protected traffic, some antivirus and parental-control applications interpose a TLS proxy in the middle of the host’s communications. We set out to analyze such proxies as there are known problems in other (more matured) TLS processing engines, such as browsers and common TLS libraries. Compared to regular proxies, client-end TLS proxies impose several unique constraints, and must be analyzed for additional attack vectors; e.g., proxies may trust their own root certificates for externally-delivered content and rely on a custom trusted CA store (bypassing OS/browser stores). Covering existing and new attack vectors, we design an integrated framework to analyze such client-end TLS proxies. Using the framework, we perform a thorough analysis of eight antivirus and four parentalcontrol applications for Windows that act as TLS proxies, along with two additional products that only import a root certificate. Our systematic analysis uncovered that several of these tools severely affect TLS security on their host machines. In particular, we found that four products are vulnerable to full server impersonation under an active man-in-the-middle (MITM) attack out-of-the-box, and two more if TLS filtering is enabled. Several of these tools also mislead browsers into believing that a TLS connection is more secure than it actually is, by e.g., artificially upgrading a server’s TLS version at the client. Our work is intended to highlight new risks introduced by TLS interception tools, which are possibly used by millions of users.