Breaking In Through The Front Door

Breaking In Through The Front Door The Impact of Web Applications and Application Service Provision on Traditional Security Models Shaun Clowes – [email protected]

Overview ♦ A Brief History of Computing ♦ Web Application environments ♦ Dangers of Web Applications ♦ Exploiting PHP Applications ♦ Testing Web Applications ♦ Protecting Web Applications ♦ Questions?

A Brief History Of Computing ♦ How has the security problem space

evolved over time? ♦ What new problems are we facing? ♦ Are we eliminating old problems?

Ancient History ♦ Ancient History – 1940’s and 50’s ♦ Eniac, Univac etc. ♦ Not exactly many security issues

Commercial Computing ♦ Early 1960’s ♦ Transistors and later Integrated Circuits ♦ IBM’s 701 and later 360 ♦ Batch processing ♦ Still not exactly difficult to secure

Multi User Systems ♦ Late 1960’s ♦ IBM’s 360/67, CP/CMS and later VM/CMS ♦ Terminals ♦ Security begins to become an issue ♦ Students stealing CPU time

Centralized Computing ♦ 1970 ♦ PDP Machines ♦ The birth of Unix ♦ Word processing, true multi user operating

systems ♦ Attacks against local applications and social engineering

Client / Server ♦ Late 1970’s and early 1980’s ♦ Microcomputers interfacing with

Minicomputers and Mainframes ♦ Advent of ‘Network Services’ ♦ Security becomes a larger problem space ♦ Attacks against open services ♦ The advent of anonymity

The Internet ♦ Circa Late 1987 to now ♦ Unix supports TCP/IP, commercial

connections ♦ Security explodes, threat rises dramatically ♦ Complete anonymity ♦ Service attacks become widespread ♦ The Morris Worm - 1988

The Defence ♦ Internet not friendly ♦ Attacks: – Local Applications – Social Engineering – Externally Accessible Services

♦ Developed defence technologies

Firewalls ♦ Packet Filters – IP routers with filters

♦ Application Layer – Marcus Ranum – DEC SEAL

♦ Stateful Inspection – Gil Shwed – Firewall 1 - Checkpoint

♦ Limiting exposure of services

Intrusion Detection Systems ♦ Network Based – Network Flight Recorder, Snort etc – Detect

♦ Host Based – LIDS etc – Detect, Prevent

♦ Limit anonymity, ♦ Limit timeframe for attacks

Defence Weaknesses ♦ Local attacks still very common where

command execution is possible ♦ Administrators actively patch multi user machines ♦ Access to other machines (e.g Web Servers) limited to trusted users

Where Are We Headed? Centralized Computing

Client/Server Computing

???

Where Are We Headed? Centralized Computing

Client/Server Computing

Web/Thin Client/ASP Computing

Architecture

Architecture Benefits ♦ No client side software (web browser) – No versioning issues – No platform compatibility issues

♦ 3rd tier servers behind firewall, not directly

accessible ♦ Web Applications run on trusted machine – Can they trust their environment?

Architecture Drawbacks ♦ Processing load on web server ♦ Can be complex to develop in traditional

languages

Architecture Risks ♦ Web Applications must be secure – Can access 3rd tier servers with privileges – Subverted Web Applications not controlled (typically) by Firewall – IDS unlikely to see attacks against Web Applications ♦ Web Applications do not fit into security

model for Centralized or Client/Server computing

The Result ♦ Despite risks/drawbacks, Web Applications

being deployed in increasing numbers ♦ So what sort of attacks are we exposing ourselves to?

Attack Scenarios - 1 1. Attack SQL queries including client input 2. Gain elevated privileges to 1. Application – See Attack Scenario 2 2. Database server 3. Use access on Database server to further

elevate privileges

– –

On DB Server via local exploits On other hosts by trust relationships and service attacks

Attack Scenarios 1 cont. ♦ SQL attacks well covered elsewhere, not

focus here ♦ For more information: – Search BugTraq – David Litchfield – “Remote Web Application Dissassembly with ODBC Error Messages”

Attack Scenarios 2 1. Attack vulnerabilities in Web Application

code or environment 2. Results in:

1. Elevated Privileges in Application ♦ Exploit Application further

1. Local Code Execution ♦ Exploit local vulnerabilities (little patching) ♦ Exploit trust relationships with 3rd tier

1. End Result: – Major privileges on network

Other Attack Scenario’s ♦ Other Attacks possible against underlying

Architecture (e.g Web Server) ♦ Well covered in JD Glaser’s presentation

Driving the Change ♦ Web Applications previously developed in: – Perl –C

♦ Not designed for the Web ♦ Web Languages developed: – PHP – ASP (VBScript) – ColdFusion

Web Languages ♦ 4th Generation Languages ♦ Tags embedded in HTML pages ♦ Very feature rich ♦ Designed to be simple – Coding falls to Web Designers

Web Languages - Issues ♦ Languages make it hard to write secure

applications (especially PHP) ♦ One function can behave in many ways (for ease of use) ♦ Loosely typed, no variable declaration ♦ Blur border between user input and application variables ♦ Software written by non coders, don’t understand issues

Focus on PHP ♦ PHP = “PHP Hypertext Preprocessor” ♦ Installed on 37% of Apache servers (E-Soft

Web Survey) ♦ Lots of applications being written for it ♦ Discuss – Common mistakes with PHP – How PHP makes secure code difficult

PHP – Global Input Variables ♦ Form variables end up as global variables in

script ♦ For Example:

♦ When submitted, the variable $hello in PHP

contains the value specified in the form

PHP – Global Input Variables ♦ Attacker can pollute the global namespace ♦ For Example: – A script sets the variable $auth to true if its authenticated the remote user – The attacker provides auth as true through form input

♦ Script cannot trust any variable it hasn’t

explicitly set

PHP – File Upload ♦ SecureReality Advisory 1 (SRADV00001) ♦ Automatically handles RFC 1867 ♦ For Example:

♦ If file is less than max_file_size its saved

locally (usually in /tmp)

PHP – File Upload ♦ The script is given variables – – – –

$hello = Location of file on local system $hello_name = Remote filename $hello_size = Byte size of file $hello_type = MIME Type of file

PHP – File Upload ♦ An attacker can set max_file_size to an

arbitrary value ♦ Gain? ♦ Limited by PHP configuration directive upload_max_filesize ♦ Just submit multiple files ♦ Still limited by maximum execution time

PHP – File Upload ♦ An attacker can get the application to work on a file

local to the machine ♦ Use a form like:

♦ We’ve used our ability to affect global variables to

create the variables expected by the script

PHP – File Upload ♦ This will often lead to exposure of sensitive

file contents ♦ Hard to detect this attack in all but the most recent versions of PHP ♦ Latest versions make this easy to detect ♦ Latest documentation makes the problem clear ♦ Problem still common

PHP – Remote Files ♦ Typical file operations support Remote

Files functionality ♦ The following code opens a file:

♦ If $hello beings with http:// or ftp:// PHP

will go to network

PHP – Remote Files ♦ How does an attacker use it? ♦ Remote Files works for most file operations

in PHP ♦ Include() and require() read a file and interpret it as though it were PHP code ♦ Typically used to support “library” concept

PHP – Remote Files ♦ For example:



♦ $langdir is a configuration variable

specifying the directory for scripts for a certain language

PHP – Remote Files ♦ The attacker can set $confdir with form

input ♦ If she can prevent the script overwriting it she can submit: http://<myevilserver> ♦ If the attacker creates a file on their webserver called languages.php ♦ Code execution

PHP - Parsing ♦ Certain file types are configured in the web

server to be parsed as PHP ♦ Other types could well be returned as plain text ♦ PHP Library files are typically named .php, so they cannot have source retrieved

PHP – Parsing ♦ Thus remote user can remotely request a

library file and have it parsed ♦ Loss of dependencies ♦ Code can no longer rely on an environment

Real Life Vulnerabilities ♦ Following are several in depth examples of

security compromises through popular open source PHP Web Applications. ♦ Attacks are via Web Browser – No special software – Breaking in through the front door

♦ Attacks are normal HTTP requests

Vulnerabilities ♦ At time of writing vendors have not been

notified of the vulnerabilities to be discussed. They have thus been omitted from this version of the presentation. ♦ After the presentation the full slides will be available at http://www.securereality.com.au and http://www.blackhat.com

Vulnerabilities - Conclusion ♦ Exploiting PHP often like catching fish

from a barrel ♦ Code running on trusted host, in trusted network but still cannot trust its environment

Finding the Holes ♦ File upload issues – Change file upload fields to hidden fields – Just try

♦ Hidden fields – Often configuration variables – Modify them

♦ Normal fields – Try special characters, ‘/\:;’

Finding the Holes ♦ Getting the source: – Open source – Full or modified – IIS Holes • • • • •

Unicode exploit showcode.asp .htr hole Translate: f hole …. many more

– FTP to web server (anonymous)

Finding the Holes ♦ Getting the source: – Non parsed backup files: • .bak, .tmp, .old, .bac, .backup, .orig, .temp, .000, .~1, .php~, etc.

– File upload PHP exploitation

♦ Spotting holes in source: – Grep! – Look for ‘vulnerable’ operations with variable substitution

Finding the Holes ♦ Vulnerable operations: – PHP Functions: include(), require(), eval(), exec(), passthru(), ``, system(), popen(), fopen(), readfile(), file() – Variable functions: $hello = “echo \”hi!\””; $hello();

– Variable dereferencing: $hello = “confdir”; $$hello = “http://myhost”; echo “$confdir”;

Securing PHP ♦ Later versions of PHP are very configurable ♦ Drastic Measures: – Set open_basedir • Prevents any file open operations on files outside specified directories

– register_globals off • Protects environment from user pollution • The ultimate protection • Breaks 99.99% of existing scripts

Securing PHP ♦ Drastic Measures cont: – safe_mode on • • • • • •

Heaps of restrictions Restrict which commands can be executed Disable functions, e.g exec(), system() Restricts file access based on ownership Kills file upload Designed for use in ISP environments

Securing PHP ♦ Slightly less painful measures: – display_errors off, log_errors on • Prevent error messages giving clues as to how code works • Makes debugging hard

♦ Mostly painless measures: – allow_url_fopen off • Stops remote files functionality • No more remote includes

Securing PHP – Future? ♦ Capabilities – Defined once, enforced throughout script

♦ Taint Mode – Tainted variables cannot be used in • Include/Require Statements • File Open Operations • SQL Queries (maybe automatically filtered)

Securing Web Applications ♦ External Solutions: – Use suEXEC for CGI under Apache – SubDomain (http://www.immunix.org) under Linux

Securing Web Applications ♦ Programming Concepts – Limit calls to external programs – Pass all user input through metacharacter filter

Securing Web Applications ♦ Architecture – Adopt client/server architecture in Web Applications – Server can rely on environment – Server can limit trust in client

Web Languages ♦ The basic issue: – Web languages attempt to make rapid development as simple as possible by being feature rich. – This leads to security problems • Functions do more than expected to do • Programmers (Web Designers!) get lazy

– Security is largely a code quality issue – Coders don’t want to give up ease for security

Thankyou! ♦ Questions? ♦ SecureReality Web Site:

http://www.securereality.com.au Email: [email protected]