Avast will render the commonName of X.509 certificates into an HTMLLayout frame when your MITM proxy detects a bad signature. Unbelievably, this means CN="<h1>really?!?!?</h1>" actually works, and is pretty simple to convert into remote code execution. To verify this bug, I've attached a demo certificate for you. Please find attached key.pem, cert.pem and cert.der. Run this command to serve it from a machine with openssl: $ sudo openssl s_server -key key.pem -cert cert.pem -accept 443 Then visit that https server from a machine with Avast installed. Click the message that appears to demonstrate launching calc.exe. PoC: https://code.google.com/p/google-security-research/issues/detail?id=546 Thanks, Tavis.