Security, explained by xkcd
xkcd explains hard security ideas better than most textbooks, in a single panel. These are the ones I keep coming back to.
Supply chain

The entire modern stack, balanced on one unpaid maintainer in Nebraska.

Design

Every boundary you add is also a new thing to escape.

Crypto

Still the clearest explanation of Heartbleed anyone has written.

Encryption is rarely the weak part. Key management is.
Auth


correct horse battery staple.
Classics

Your real threat model might just be a $5 wrench.

Little Bobby Tables. Sanitize your inputs.
Other comics I love: PhD Comics, Webcomic Name, and CommitStrip.