The "sa" account is installed with a default null password on (1) Microsoft SQL Server 2000, (2) SQL Server 7.0, and (3) Data Engine (MSDE) 1.0, including third party packages that use these products such as (4) Tumbleweed Secure Mail (MMS) (5) Compaq Insight Manager, and (6) Visio 2000, which allows remote attackers to gain privileges, as exploited by worms such as Voyager Alpha Force and Spida.
http://www.securityfocus.com/bid/4797
http://www.microsoft.com/security/security_bulletins/ms02020_sql.asp
http://www.kb.cert.org/vuls/id/635463
http://www.iss.net/security_center/static/1459.php
http://support.microsoft.com/default.aspx?scid=kb%3BEN-US%3Bq321081
http://support.microsoft.com/default.aspx?scid=kb%3B%5BLN%5D%3BQ313418
http://security-archive.merton.ox.ac.uk/bugtraq-200008/0233.html
http://online.securityfocus.com/archive/1/273639
http://marc.info/?l=bugtraq&m=96644570412692&w=2