[pierrefar]

I built (and sold) cli.gs, one of the 13 final complaint services. The system that evolved was effectively strict border controls along with frequent police checks:

1. When a new URL shortening request is received, the requester is checked and the destination is also checked. If both pass, the new short URL is returned.

2. When a short URL forwarding request was received (i.e. the bulk of the traffic), the destination is checked again at a configurable probability. If the destination is now deemed malicious, it is disabled on the spot and a message is shown. In times of spam attacks, the checking probability would be set to 100%.

I blogged about this when it launched and started evolving:

http://blog.cli.gs/news/new-anti-spam-and-anti-malware-featu...

http://blog.cli.gs/news/more-anti-spam-and-anti-malware-prot...