Nobody writes it all
Developers do not build date handling, image resizing or payment forms from scratch. They install a package: a reusable piece of code published by someone else.
A typical modern web app installs a few dozen packages directly. Those packages install packages of their own, and so on. The real total is routinely over a thousand.
- Your coderoughly 2 percent
- Packages you chosea few dozen
- Packages those pulled inoften over a thousand
Most vulnerabilities live in the bottom band, which nobody picked.
You wrote a fraction of your product. Strangers wrote the rest.