Simon Willison’s Weblog

11th August 2025 - Link Blog

Chromium Docs: The Rule Of 2. Alex Russell pointed me to this principle in the Chromium security documentation as similar to my description of the lethal trifecta. First added in 2019, the Chromium guideline states:

When you write code to parse, evaluate, or otherwise handle untrustworthy inputs from the Internet — which is almost everything we do in a web browser! — we like to follow a simple rule to make sure it's safe enough to do so. The Rule Of 2 is: Pick no more than 2 of

• untrustworthy inputs;
• unsafe implementation language; and
• high privilege.

Chromium uses this design pattern to help try to avoid the high severity memory safety bugs that come when untrustworthy inputs are handled by code running at high privilege.

Chrome Security Team will generally not approve landing a CL or new feature that involves all 3 of untrustworthy inputs, unsafe language, and high privilege. To solve this problem, you need to get rid of at least 1 of those 3 things.