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.

Thanks, Tavis.

This bug is subject to a 90 day disclosure deadline. If 90 days elapse
without a broadly available patch, then the bug report will automatically
become visible to the public.