In a corporate proprietary code base this is REALLY easy. Just commit a bug. Happens every day, everywhere. Just that normally these are mistakes. You can easily mask a deliberately inserted exploitable bug as a mistake. Make the code a bit convoluted, leave out a crucial corner case from your test, slip this in in a moment when your code reviewers have time pressure and/or are stressed somehow, and even if you are caught, the plausible deniability is convincing. How many eyes will see it after the fact? None.