A SQL Server DBA reported many login failures in his SQL logs - variously quoted as “hundreds per day” and “50+ in a minute”. These failures were not apparent to system users. Two user communities were accessing this data - one using trusted connections the other using SQL Server Authentication. Only the Trusted Connection group were affected by this issue. The connection string included the clause <Trusted_Connections=Yes>, this should have been <Trusted_Connection=Yes>. Once the connection string had been corrected the log-in failures stopped.

Example of repeated log-in failures from SQL logs:

2012-04-18 09:19:44.50 Logon       Login failed for user 'smruser'. [CLIENT: 159.15.250.xxx]
2012-04-18 09:19:44.53 Logon       Error: 18456, Severity: 14, State: 5.
2012-04-18 09:19:44.53 Logon       Login failed for user ‘yyyy.xxxx’. [CLIENT: 159.15.250.xxx]
2012-04-18 09:19:44.54 Logon       Error: 18456, Severity: 14, State: 5.
2012-04-18 09:19:44.54 Logon       Login failed for user 'smruser'. [CLIENT: 159.15.250.xxx]
2012-04-18 09:19:46.87 Logon       Error: 18456, Severity: 14, State: 5.
2012-04-18 09:19:46.87 Logon       Login failed for user ‘zzzz.yyyy’. [CLIENT: 159.15.250.xxxx]
2012-04-18 09:19:46.90 Logon       Error: 18456, Severity: 14, State: 5.