One of the security recommendation inside SQL Server Security Best Practice white paper for guest user is that, to disable guest access in every database (expect “master”, “msdb” and “tempdb”) on SQL Server instance, and it should not be used in any circumstances. By default, guest user exists in all user and system databases. Having guest user enabled inside databases, leaves a security risks of unauthorized (or unaudited) access to the data because Guest user allows database access to logins who do not have associated users inside SQL Server databases. By disabling guest user access from the user databases will ensure, that member of PUBLIC server role may not be able to access user databases on SQL Server instance, unless they have access to the database explicitly.

I have written following script which you can use to list all databases with guest user access enabled:

USE [master]
GO
SET NOCOUNT ON
DECLARE  @First              [smallint]
        ,@Last               [smallint]
        ,@DBName             [varchar](200)
        ,@SQLCommand         [varchar](500)
        ,@DBWithGuestAccess  [nvarchar](4000)
IF OBJECT_ID('tempdb..#GuestUsersReport') IS NOT NULL
 DROP TABLE #GuestUsersReport
CREATE TABLE #GuestUsersReport ( [Database]  [varchar](256)
        ,[UserName]  [varchar](256)
        ,[HasDbAccess] [varchar](10))
DECLARE @DatabaseList TABLE ([RowNo] [smallint] identity (1, 1), [DBName] [varchar](200))
INSERT INTO @DatabaseList
SELECT [name] FROM [master]..[sysdatabases] WITH (NOLOCK)
WHERE [name] NOT IN ('master', 'tempdb', 'msdb') ORDER BY [name]
SELECT @First = MIN([RowNo]) FROM @DatabaseList
SELECT @Last = MAX([RowNo]) FROM @DatabaseList
WHILE @First <= @Last
BEGIN
 SELECT @DBName = [DBName] FROM @DatabaseList WHERE [RowNo] = @First
 SET @SQLCommand = 'INSERT INTO #GuestUsersReport ([Database], [UserName], [HasDbAccess])' + CHAR(13)
     + 'SELECT ' + CHAR(39) + @DBName + CHAR(39) + ' ,[name], CASE [hasdbaccess] WHEN 0 THEN ''N'' WHEN 1 THEN ''Y'' END '
     + CHAR(13) +  'FROM [' + @DBName + ']..[sysusers] WHERE [name] LIKE ''guest'' AND [hasdbaccess] = 1'
 EXEC (@SQLCommand)
 SET @First = @First + 1
END
SELECT * FROM #GuestUsersReport WITH (NOLOCK)
SET NOCOUNT OFF
IF OBJECT_ID('tempdb..#GuestUsersReport') IS NOT NULL
 DROP TABLE #GuestUsersReport

Well after running the above script, if you find any databases with guest access enabled, then use the REVOKE CONNECT statement to revoke the access of guess access from user database:

--Specify database name in USE statement
USE [<SpecifyDatabaseName>]
GO
REVOKE CONNECT TO 

GO

This recommendation does not apply to “master”, “msdb” and “tempdb” database. For more information, see “KB #2539091 : You should not disable the guest user in the msdb database in SQL Server” and “KB #2186935 : Guidelines on revoking Guest user access in a database“.