Login Failures Bloating SQL Server Log

By Ginger Daniel | Beginner

Mar 16

–By Ginger Keys

After migrating a client’s SQL instances to VMs from physical servers, I noticed the following week that the SQL Server Log files had tons of failed login attempts from what looked like an application. These attempts were happening several times per second, and had totally bloated the SQL Server logs.


This is not desirable because 1) obviously something should not be attempting and failing to connect that much, and 2) it makes it super cumbersome to monitor more critical messages in the SQL logs when so many login errors are bloating the log. Too many failed login attempts could mean that the application has not been set with the correct permissions to access the databases needed, or it could be an indication of a hacking attempt.

I ran the script below to determine how many failed login attempts had occurred in the last 7 days, and the count came back with over 3 million failed attempts!

— Create a temporary table

CREATE TABLE #FailedLogins

(Logdate DATETIME,

Process VARCHAR(20),

Text VARCHAR(4000))


— Insert data from SQL Server Log into temp table

INSERT INTO #FailedLogins

EXEC xp_readerrorlog 1, 1, N’Login Failed’, N’LoginName’,‘20170309’,‘20170316’


— For more info about xp_readerrorlog click here https://sqlandme.com/2012/01/25/sql-server-reading-errorlog-with-xp_readerrorlog/

  –Count number of occurrences

SELECT COUNT(Text) AS LoginFailures, Text

FROM #FailedLogins



–Drop temp table

DROP TABLE #FailedLogins


As it turns out it was a legacy system moved from the old environment that needed to be adjusted to connect to the new environment. I didn’t exactly get to put on my superwoman cape and solve the problem, but at least I was able to identify the issue and direct it to the appropriate IT staff to correct.

As part of your everyday duties as a DBA it is prudent to monitor your SQL Server Logs to catch hacking attempts or malicious activity with your instance, and also to look for errors that could be occurring. By default, SQL contains 7 error log files (one current, and six archives). Error logs contain critical information about your SQL server. A new log is created each time the SQL service starts, and the oldest archived log gets deleted. For security reasons it is best practice to keep a large number of error logs, depending on the space you have available. Someone with access to your SQL server could execute sp_cycle_errorlog and regenerate the logs at a rate that could ‘delete’ their malicious activity or cover up the fact that they have gained access to your system. If you have highly sensitive data or stringent auditing requirements you can increase the number of retained error logs up to 99, just be sure to monitor your disk space.

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