I've had 2 mac users hit my site today, and both their browser user agents were:
Mozilla/5.0 (000000000; 0; 00000 000 00 0 000000; 00000) DDDDDDDDDDDDDDDDDDDD DDDDDDD DDDD DDDDDD DDDDDDDDDDDDD DDDDDDDDDDDDDDD
instead of what I would generally expect. I contacted one of them and had them try FireFox and Chrome as well - same result. So clearly something (a proxy, most likely?) is stripping their useragent and replacing it with junk. They're not aware of any software running on their Macs which would do this.
Has anyone else seen this before? Any ideas what does this and how widespread it is?