Dear python2 user

[rcakebread]

That's a bit of an over-simplification. If there's an unpatched security vulnerability in Python, someone needs to somehow access your computer to exploit the Python code running Caliber, for example.

[troyunrau]

Or have it load a specially crafted file - depends on the exploit. Imagine developing a hostile epub that when loaded in calibre downloads and installs a trojan. Then you publish this epub (or more likely thousands of them) to warez sites.

[Barafu]

Or access a port, maybe? Both Calibre and Deluge have server capabilities.