Abstract:
STARTTLS is a mechanism that allows upgrading plain connections to encrypted connections via TLS. It is frequently used in email communications. The transition from communicating in plain to a communication protected by TLS offers additional attacking vectors to those of TLS itself. A secure connection requires correct implementations of the participating server as well as of the client. It is necessary to test STARTTLS implementations to detect vulnerabilities related to STARTTLS and avoid those implementation mistakes for future releases.
With TLS-Scanner there already exists a framework that allows to automatically scan TLS client and server instances for vulnerabilities. We extend the TLS-Scanner project for STARTTLS related test suites, for both client- and server implementations. We use these test suites to evaluate implementations of email servers and well-known email clients Thunderbird and Mutt.