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.
Proof of Concept: