January 25, 2003 (Saturday)
- The Internet was attacked by very high traffic caused by a self-replicating software worm program called "SQL Slammer". This attacked Microsoft SQL servers, causing them to spray the Internet with more copies of the worm program. This was made possible by a security vulnerability in Microsoft SQL Server. The worm attack had the side effect of causing a distributed denial-of-service attack on the rest of the Internet.
- The New Democratic Party in Canada picks Toronto city councillor Jack Layton as its new leader, replacing Alexa McDonough, at its convention in Toronto. Layton wins on the first ballot, beating veteran MP Bill Blaikie.