Exploitable PHP functions

Answer 1

I guess you won't be able to really find all possible exploits by parsing your source files.

• also if there are really great lists provided in here, you can miss a function which can be exploitet

• there still could be "hidden" evil code like this

$myEvilRegex = base64_decode('Ly4qL2U=');

preg_replace($myEvilRegex, $_POST['code']);

• you could now say, i simply extend my script to also match this

• but then you will have that mayn "possibly evil code" which additionally is out of it's context

• so to be (pseudo-)secure, you should really write good code and read all existing code yourself

Answer 2

To build this list I used 2 sources. A Study In Scarlet and RATS. I have also added some of my own to the mix and people on this thread have helped out.

Edit: After posting this list I contacted the founder of RIPS and as of now this tools searches PHP code for the use of every function in this list.

Most of these function calls are classified as Sinks. When a tainted variable (like $_REQUEST) is passed to a sink function, then you have a vulnerability. Programs like RATS and RIPS use grep like functionality to identify all sinks in an application. This means that programmers should take extra care when using these functions, but if they where all banned then you wouldn't be able to get much done.

"With great power comes great responsibility."

--Stan Lee

Command Execution

exec           - Returns last line of commands output
passthru       - Passes commands output directly to the browser
system         - Passes commands output directly to the browser and returns last line
shell_exec     - Returns commands output
`` (backticks) - Same as shell_exec()
popen          - Opens read or write pipe to process of a command
proc_open      - Similar to popen() but greater degree of control
pcntl_exec     - Executes a program

PHP Code Execution

Apart from eval there are other ways to execute PHP code: include/require can be used for remote code execution in the form of Local File Include and Remote File Include vulnerabilities.

eval()
assert()  - identical to eval()
preg_replace('/.*/e',...) - /e does an eval() on the match
create_function()
include()
include_once()
require()
require_once()
$_GET['func_name']($_GET['argument']);
$func = new ReflectionFunction($_GET['func_name']); $func->invoke(); or $func->invokeArgs(array());

List of functions which accept callbacks

These functions accept a string parameter which could be used to call a function of the attacker's choice. Depending on the function the attacker may or may not have the ability to pass a parameter. In that case an Information Disclosure function like phpinfo() could be used.

Function                     => Position of callback arguments
'ob_start'                   =>  0,
'array_diff_uassoc'          => -1,
'array_diff_ukey'            => -1,
'array_filter'               =>  1,
'array_intersect_uassoc'     => -1,
'array_intersect_ukey'       => -1,
'array_map'                  =>  0,
'array_reduce'               =>  1,
'array_udiff_assoc'          => -1,
'array_udiff_uassoc'         => array(-1, -2),
'array_udiff'                => -1,
'array_uintersect_assoc'     => -1,
'array_uintersect_uassoc'    => array(-1, -2),
'array_uintersect'           => -1,
'array_walk_recursive'       =>  1,
'array_walk'                 =>  1,
'assert_options'             =>  1,
'uasort'                     =>  1,
'uksort'                     =>  1,
'usort'                      =>  1,
'preg_replace_callback'      =>  1,
'spl_autoload_register'      =>  0,
'iterator_apply'             =>  1,
'call_user_func'             =>  0,
'call_user_func_array'       =>  0,
'register_shutdown_function' =>  0,
'register_tick_function'     =>  0,
'set_error_handler'          =>  0,
'set_exception_handler'      =>  0,
'session_set_save_handler'   => array(0, 1, 2, 3, 4, 5),
'sqlite_create_aggregate'    => array(2, 3),
'sqlite_create_function'     =>  2,

Information Disclosure

Most of these function calls are not sinks. But rather it maybe a vulnerability if any of the data returned is viewable to an attacker. If an attacker can see phpinfo() it is definitely a vulnerability.

phpinfo
posix_mkfifo
posix_getlogin
posix_ttyname
getenv
get_current_user
proc_get_status
get_cfg_var
disk_free_space
disk_total_space
diskfreespace
getcwd
getlastmo
getmygid
getmyinode
getmypid
getmyuid

Other

extract - Opens the door for register_globals attacks (see study in scarlet).
parse_str -  works like extract if only one argument is given.  
putenv
ini_set
mail - has CRLF injection in the 3rd parameter, opens the door for spam. 
header - on old systems CRLF injection could be used for xss or other purposes, now it is still a problem if they do a header("location: ..."); and they do not die();. The script keeps executing after a call to header(), and will still print output normally. This is nasty if you are trying to protect an administrative area. 
proc_nice
proc_terminate
proc_close
pfsockopen
fsockopen
apache_child_terminate
posix_kill
posix_mkfifo
posix_setpgid
posix_setsid
posix_setuid

Filesystem Functions

According to RATS all filesystem functions in php are nasty. Some of these don't seem very useful to the attacker. Others are more useful than you might think. For instance if allow_url_fopen=On then a url can be used as a file path, so a call to copy($_GET['s'], $_GET['d']); can be used to upload a PHP script anywhere on the system. Also if a site is vulnerable to a request send via GET everyone of those file system functions can be abused to channel and attack to another host through your server.

// open filesystem handler
fopen
tmpfile
bzopen
gzopen
SplFileObject->__construct
// write to filesystem (partially in combination with reading)
chgrp
chmod
chown
copy
file_put_contents
lchgrp
lchown
link
mkdir
move_uploaded_file
rename
rmdir
symlink
tempnam
touch
unlink
imagepng   - 2nd parameter is a path.
imagewbmp  - 2nd parameter is a path. 
image2wbmp - 2nd parameter is a path. 
imagejpeg  - 2nd parameter is a path.
imagexbm   - 2nd parameter is a path.
imagegif   - 2nd parameter is a path.
imagegd    - 2nd parameter is a path.
imagegd2   - 2nd parameter is a path.
iptcembed
ftp_get
ftp_nb_get
// read from filesystem
file_exists
file_get_contents
file
fileatime
filectime
filegroup
fileinode
filemtime
fileowner
fileperms
filesize
filetype
glob
is_dir
is_executable
is_file
is_link
is_readable
is_uploaded_file
is_writable
is_writeable
linkinfo
lstat
parse_ini_file
pathinfo
readfile
readlink
realpath
stat
gzfile
readgzfile
getimagesize
imagecreatefromgif
imagecreatefromjpeg
imagecreatefrompng
imagecreatefromwbmp
imagecreatefromxbm
imagecreatefromxpm
ftp_put
ftp_nb_put
exif_read_data
read_exif_data
exif_thumbnail
exif_imagetype
hash_file
hash_hmac_file
hash_update_file
md5_file
sha1_file
highlight_file
show_source
php_strip_whitespace
get_meta_tags

Answer 3

Most of attacks in the code use multiple access sources, or multiple steps to execute themselves. I would search not only for a code, or method having malicious code, but all methods, function executing or calling it. The best security would also include encoding and validating form data as it comes in and out.

Watch also out from defining system variables, they can afterwards be called from any function or method in the code.

Answer 4

There was some discussion of this on security.stackexchange.com recently

functions that can be used for arbitrary code execution

Well that reduces the scope a little - but since 'print' can be used to inject javascript (and therefore steal sessions etc) its still somewhat arbitrary.

isn't to list functions that should be blacklisted or otherwise disallowed. Rather, I'd like to have a grep-able list

That's a sensible approach.

Do consider writing your own parser though - very soon you're going to find a grep based approach getting out of control (awk would be a bit better). Pretty soon you're also going to start wishing you'd implemented a whitelist too!

In addition to the obvious ones, I'd recommend flagging up anything which does an include with an argument of anything other than a string literal. Watch out for __autoload() too.

Answer 5

You'd have to scan for include($tmp) and require(HTTP_REFERER) and *_once as well. If an exploit script can write to a temporary file, it could just include that later. Basically a two-step eval.

And it's even possible to hide remote code with workarounds like:

 include("data:text/plain;base64,$_GET[code]");

Also, if your webserver has already been compromised you will not always see unencoded evil. Often the exploit shell is gzip-encoded. Think of include("zlib:script2.png.gz"); No eval here, still same effect.

Answer 6

My VPS is set to disable the following functions:

root@vps [~]# grep disable_functions /usr/local/lib/php.ini
disable_functions = dl, exec, shell_exec, system, passthru, popen, pclose, proc_open, proc_nice, proc_terminate, proc_get_status, proc_close, pfsockopen, leak, apache_child_terminate, posix_kill, posix_mkfifo, posix_setpgid, posix_setsid, posix_setuid

PHP has enough potentially destructible functions that your list might be too big to grep for. For example, PHP has chmod and chown, which could be used to simply deactivate a website.

EDIT: Perhaps you may want to build a bash script that searches for a file for an array of functions grouped by danger (functions that are bad, functions that are worse, functions that should never be used), and then calculate the relativity of danger that the file imposes into a percentage. Then output this to a tree of the directory with the percentages tagged next to each file, if greater than a threshold of say, 30% danger.