Saturday, October 6, 2012

Verifying NTP Reserved Mode Denial of Service Vulnerability

I recently needed to check a NTP Reserved Mode Denial of Service vulnerability CVE-2009-3563, but without causing the DoS condition on the production server.  Using Metasploit’s auxillary module auxiliary/dos/ntp/ntpd_reserved_dowas not an option so I wrote my own Ruby script to assess the remote server. This script verifies the returned UDP packet content to determine presence of vulnerability and is shared below.

#Author: Gursev Singh Kalra  
require 'socket'

if(ARGV.count != 1)
 puts "[-] Target host not provided. Usage: ntp.rb <target_server>"

target_server = ARGV[0]
target_port = 123

socket = nil
response = nil

 test_string = "\x97\x00\x00\x00\xAA\x00\x00\x00"
 socket =
 socket.send(test_string, 0, target_server, target_port)
 if select([socket], nil, nil, TIMEOUT)
  response = socket.recvfrom(10)
rescue (IOError ex)
 puts ex.to_s
 socket.close if(socket)

if(response && response[0].index("\x97\x00\x00\x00"))
 puts "[+] Vulnerable to NTP Mode 7 Request Denial Of Service"
 puts "[-] Not vulnerable to NTP Mode 7 Request Denial Of Service "

Figure 1: Image shows request capture in wireshark

Figure 2: Image shows response capture in wireshark

Figure 3: Image shows script in action

Tuesday, October 2, 2012

Bypassing CAPTCHAs by Impersonating CAPTCHA Providers

CAPTCHA service providers validate millions of CAPTCHAs each day and protect thousands of websites against the bots. A secure CAPTCHA generation and validation ecosystem forms the basis of the mutual trust model between the CAPTCHA provider and the consumer. A variety of damage can occur if any component of this ecosystem is compromised.

During Analysis of the CAPTCHA integration libraries provided by several CAPTCHA providers (including reCAPTCHA) revealed that almost all of the CAPTCHA verification API’s relied on plain text HTTP protocol to perform CAPTCHA validation. Because of this, the CAPTCHA provider’s identity was not validated, message authentication checks were not performed and the entire CAPTCHA validation was performed on an unencrypted channel. This vulnerability was also reported to reCAPTCHA team several months back. 

If you decompile the .NET Plugin, you'll be able to pull out reCAPTCHA's verification URL, which demonstrates the absense of HTTPS:

In the current scenario, two types of attacks can be launched against vulnerable CAPTCHA implementations. These attacks are based on the assumption that an attacker is able to intercept the CAPTCHA validation traffic between target website and the CAPTCHA provider.

Private Key Compromise
Most of CAPTCHA providers issue private and public keys to identify a particular consumer and to enforce an upper limit on the number of CAPTCHAs used by them. Private keys are often sent over to the CAPTCHA provider during the CAPTCHA validation process. If the public and private keys are sent using plain text HTTP, an attacker could sniff the private keys and:

Use the CAPTCHA service for without registering for the service by using the captured keys.
Exhaust the target web site’s CAPTCHA quota for the service, which depending on the CAPTCHA provider may cause a wide variety of unexpected issues.

The CAPTCHA Clipping Attack
The following image describes what I call the "CAPTCHA Clipping Attack". Notice that steps 5 and 6 in blue would be the normal operation of events. We'll go into the attack in a little more detail below.

Since the website’s application server acts as a client to CAPTCHA provider during steps 5 and 6 (in blue) and the application server often neglects to validate the CAPTCHA provider’s identity and the session integrity checks, an attacker may be able to impersonate the CAPTCHA provider and undermine the anti-automation protection (steps 5 and 6 in red). CAPTCHA validation responses are mostly Boolean (true or false, success or failure, pass or fail, 0 or 1). The response format and its contents are also publicly available as part of CAPTCHA provider’s API documentation. This allows an attacker to easily construct the finite set of possible responses, impersonate the CAPTCHA provider, and perform malicious CAPTCHA validation for the application servers. 

To exploit this vulnerability an attacker performs the following:

  1. The attacker acts as a legitimate application user and submits a large number of requests to the web application.
  2. At the same time, he/she intercepts CAPTCHA validation requests, masquerades as the CAPTCHA provider and approves all submitted requests.

Masquerading as the CAPTCHA provider and not forwarding the CAPTCHA validation requests to the actual CAPTCHA provider is the CAPTCHA Clipping Attack.

clipcaptcha is a proof of concept exploitation tool that specifically targets the vulnerabilities discussed above and allows complete bypass of CAPTCHA provider protection. clipcaptcha is built on the sslstrip codebase and has the following features:

  1. Performs signature based CAPTCHA provider detection and clipping.
  2. Can be easily extended to masquerade as any CAPTCHA provider by adding corresponding signatures to the configuration XML file.
  3. Has built in signatures of several CAPTCHA providers including reCAPTCHA, OpenCAPTCHA, Captchator etc…
  4. Logs POST requests that match any supported CAPTCHA provider to capture private and public keys. Unmatched requests are forwarded as is.
  5. clipcaptcha supports five operational modes. These are “monitor”, “stealth”, “avalanche”, “denial of service” and “random”.

clipcaptcha can be downloaded here 

This blog post is a copy of my original post here

Oct 7, 2012 Update: 
The complete whitepaper is available for download from here.

Tuesday, June 12, 2012

Oyedata for OData Security Assessments

The Open Data Protocol (OData) is an open web protocol for querying and updating data. OData enables the creation of HTTP based RESTful  data services that can be used to publish and edit resources with simple HTTP messages.  OData is intended to be used to expose and access information from a variety of sources including relational databases, file systems, content management systems, and traditional web sites. It allows a consumer to query a data source over HTTP protocol and get results back in formats like Atom, JSON or plain XML. OData can be termed as JDBC/ODBC for the internet.

The protocol is relatively new and is being adopted by many major software manufacturers such as Microsoft, IBM, and SAP but hasn’t been publically explored in terms of security. As more applications, websites, and frameworks support OData, a larger attack surface becomes available to attackers.

Oyedata is a new tool to perform black-box OData security testing and help secure OData deployments. I wrote Oyedata from a penetration testing perspective and its the major features are summarized below:
1.     Intuitive GUI based tool written in C#.
2.     Ability to create attack templates from local and remote Service Documents and Service Metadata Documents.
3.     Support for XML and JSON data formats.
4.     Ability to export attack templates in JSON and XML formats that can be fed to custom Fuzzing code.
5.     Ability to engage the OData services for manual testing.
6.     Data generator for EDMSimpleType test data generation.
7.     Ability to generate “Read URIs” for Entities, Entity Properties and Entity Property Values.
8.     Ability to generate attack templates for Creation of new Entries, updating existing Entries, Service Operation invocation, Entry deletion etc…
9.     Ability to identify Keys, Nullable and Non-Nullable Properties and indicate the same in the attack templates.
10.  Web proxy, HTTP and HTTPS support and Error logging.

Image shows Oyedata retrieving an OData Service Metadata document 

Image shows various Create, Read, Delete and Update operations of the “Categories” Feed along with the supported Service Operation nodes

The tool is now available for download from McAfee website from this URL. Please send in your suggestions and feedback.

Friday, March 2, 2012

CAPTCHA Re-Riding Attack

This attack was voted at #8 in Top Ten Web Hacking Techniques of 2012

CAPTCHA Re-Riding Attack bypasses the CAPTCHA protection built into the web applications. The attack exploits the fact that the code that verifies CAPTCHA solutions sent by the user during form submissions does not clear the CAPTCHA solution from the HTTP Session. 

Impact: A large number of successful submissions on CAPTCHA protected pages by riding on a single CAPTCHA solution. 

A typical scenario to demonstrate the vulnerability is explained below. 
1.       A user visits register page of the website.
2.       The website creates an HTTP session, assigns it a SESSIONID and returns the register page to the user along with the SESSIONID cookie. The register page also contains one image tag which directs the browser to retrieve a CAPTCHA and display it on screen.
3.       Upon parsing the image tag, the browser sends out request for the CAPTCHA.
4.       The server side code creates a new CAPTCHA with random text and CAPTCHA solution is stored in the HTTP session.
5.       CAPTCHA image is then sent to the client and is then displayed by the browser.
6.       Browser sends CAPTCHA solution along with form fields for verification.
7.       Server side code retrieves CAPTCHA solution from the HTTP Session and verifies it against the solution provided by the client.
8.       If verification is successful, client is sent to next logical step in the registration process. If not, client is redirected to the register page (step 1 above).

Figure 1: Image shows an example Register page that supports CAPTCHA

Analysis of the CAPTCHA generation and verification process reveals the following:
  1. The captcha.php is the only page responsible for updating the HTTP session with correct CAPCHA solution. The first ingredient.
  2. CAPTCHA solution inside the HTTP session is not explicitly cleared during the verification process. Yes, you guess it right. This is the second and the most important ingredient for CAPTCHA Re-Riding Attacks.
  3. When registration fails (for any reason), the web applications continue to use the same HTTP session and SESSIONID. We will not look into this further.
  4. When registration succeeds, the user is redirected to next step and the CAPTCHA generation page (/captcha.php) is not likely to be called for current SESSION again.  This allows the CAPTCHA solution to stay in the HTTP store for as long as SESSION is valid. Following are the likely scenarios to be seen when CAPTCHA verification is successful.
    1. The web application generates a new SESSIONID for the same HTTP session for known security reasons. This implementation is most likely to be seen. Combine this behavior with first and second ingredients above and you have a successful CAPTCHA Re-Riding attack.
    2. The web application continues to use the same SESSIONID for the same HTTP session.  Here we have more things to worry than just the CAPTCHA. For now, combine this behavior with first and second ingredients above and you have a successful CAPTCHA Re-Riding attack again.
    3. The web application generates a completely new HTTP session with new or same SESSIONID. For CAPTCHA Re-riding Attack, this scenario is not exploitable.

For scenarios 4.a and 4.b, the HTTP Session continues to hold the CAPTCHA solution as it is not explicitly cleared by the CAPTCHA verification code. Since /captcha.php is not going to be called again (and we will not let the call happen anyway), the same CAPTCHA solution continues to exist in HTTP session. Let us now see how 4.a & 4.b scenarios above can be exploited to make multiple successful submissions using a CAPTCHA solution.

Exploiting Scenario 4.b:
1.       Load the register page of the target website in a web browser.
2.       Solve the CAPTCHA manually, and submit the form.
3.       Record this form submission using a web proxy. This request contains a valid SESSIONID, valid form fields and a valid CAPTCHA solution.
4.       Create a custom script or use any tool like Burp intruder that can repeatedly send this request to server. With each request change the unique values (like User ID) to create multiple new accounts with a single CAPTCHA solution.

Exploiting Scenario 4.a:
1.       Load the register page of the target website in a web browser.
2.       Solve the CAPTCHA manually, and submit the form.
3.       To make things easy, trap this request in a web proxy and do not allow it to reach the web server. This request contains a valid SESSIONID, valid form fields and a valid CAPTCHA solution.
4.       Create a custom script or use any tool like Burp intruder that can repeatedly send this request to server.
5.       Submit one request.
6.       Upon successful submission, the web application will reset the current SESSIONID and send new SESSIONID back in response headers.
7.       Change the value of SESSIONID in recorded request (step 3) to the value copied from response in Step 6 above.
8.       Go to step 5.
9.     We will be able to make multiple successful submissions with single CAPTCHA solution.

Using one time tokens along with CAPTCHAs on the register pages may still be exploitable with a few additional lines of attack code. The best defense is to reset CAPTCHA solution inside the HTTP session during the CAPTCHA verification stage. It is also important to note that when a website relies on third party CAPTCHA  provider it does not maintain any session information at its end and CAPTCHA is performed by the CAPTCHA provider and these websites are not vulnerable to CAPTCHA Re-Riding Attack.

Saturday, February 25, 2012

Sqlitespy for Sqlite Database analysis

Sqlite is the ubiquitous database for mobile applications on iPad, iPhone and Android. It is also used by certain internet browsers, web application frameworks and software products for their local storage needs. While doing penetration tests, we often see sensitive information like usernames, passwords, account numbers, SSN etc… insecurely stored in these databases. Thus, every penetration test requires comprehensive analysis of the local databases being used.

While analyzing databases, a penetration tester repeatedly does the following:
  1. Opens the database in sqlite reader (sqlite3 or other readers)
  2. Views various tables and columns to understand database layout and schema.
  3. Analyzes the storage for sensitive information.
As the number and size of database increases, the analysis time increases exponentially. To escape the recurring pain, I wrote a ruby script to automate this process. The script achieves the following:
  1. Analyzes multiple databases in a single run.
  2. Queries and displays database schema.
  3. Provides an option to run search on Table and Column Names for quick analysis.
  4. Performs case-insensitive regular expression search (default). This can be controlled with command line options to one’s requirements.
  5. Displays Database, Tables and Row Number reference for every successful match.
  6. Dumps database rows on a successful match.
  7. Looks for search strings in the following:
    • Table Name
    • Column Names
    • Actual Data
Sqlitespy dependencies are listed below:
  1. Ruby
  2. Sequel Gem
  3. Sqlite3

Figure 1: Image shows sqlitespy help

Figure 2: Image shows sqlite sample run with multiple search strings and row information dump for a successful match

Figure 3: Image shows sqlitespy sample run with minimal information

Figure 4: Image shows sqlite database schema dump

Sqlitespy Code Follows:

 #Author: Gursev Singh Kalra  
 require 'rubygems'  
 require 'optparse'  
 require 'ostruct'  
 require 'sequel'  
 class CmdLineOptions  
  def self.parse(args)  
   options =  
   options.dbs = []  
   options.sstrings = []  
   options.show_schema = false  
   options.case_sensitive = false  
   options.exact = false  
   options.verbose = false  
   options.rowdump = false  
   options.metadata = false  
   opts = do |opts|  
   opts.banner = "Usage: sqlitespy.rb [options]\n\nSpecific Options:"  
   opts.on("-d", "--database DATABASE_PATH",  
    "Sqlite database to analyze.") do |db|  
    options.dbs << db  
   opts.on("-s", "--show-schema", "Show database schema") do |show|  
    options.show_schema = show;  
   opts.on("--find x,y,z", Array, "Strings to search") do |list|  
    options.sstrings = list  
   opts.on("-c", "--case-sensitive", "Perform case sensitive search. Default is case insensitive.") do |case_sensitive|  
    options.case_sensitive = case_sensitive;  
   opts.on("-e", "--exact--match", "Perform exact match for the search strings") do |v|  
    options.exact = v;  
   opts.on("-r", "--row-dump", "Dump Database Row when a match is found") do |v|  
    options.rowdump = v;  
   opts.on("-m", "--metadata", "Look for search strings only in DB metadata (table and column names)") do |v|  
    options.metadata = v;  
   opts.on("-v", "--verbose", "Verbose output") do |v|  
    options.verbose = v;  
   opts.on_tail("-h", "--help", "Show this message") do  
    puts opts  
   end# parse()  
 end# class CmdLineOptions  
 options = nil  
  options = CmdLineOptions.parse(ARGV)  
 rescue (OptionParser::InvalidOption)  
  $stderr.puts "[-] Invalid option "  
  options = CmdLineOptions.parse(ARGV+["-h"])  
 if(options.dbs.length == 0)  
  $stderr.puts "[-] No Database available. Exiting !!"  
 dbs = []  
 dbs = options.dbs.collect do |db|  
   throw Errno::ENOENT unless(File.file?(db))  
   $stderr.puts "[-] \"#{db}\" is not a sqlite database"  
 options.dbs = dbs.compact  
 if(options.dbs.length == 0)  
  $stderr.puts "[-] No Database available. Exiting."  
  puts "+"*80  
  puts "Database Schemas"  
  puts "+"*80  
  options.dbs.each do |db|  
   puts "[DATABASE] #{db}"  
   Sequel.sqlite(db) do |dbhandle|  
    dbhandle.tables.each do |table|  
     puts "\t[TABLE] #{table}"  
     puts "\t\t[COLUMNS] #{dbhandle[table.to_sym].columns.join(', ')}"  
  puts "-"*80  
 regex_strings = []  
 regex_strings = options.sstrings.collect do |search|  
  regexstr = ""  
  regex = nil  
   regexstr = "^#{search}$"  
   regexstr = "#{search}"  
   regex ="#{regexstr}")  
   regex ="#{regexstr}", Regexp::IGNORECASE)  
 options.sstrings = regex_strings  
 options.dbs.each do |database|  
   puts "+"*80  
   puts "Analyzing Database '#{database}'"  
   puts "+"*80  
  Sequel.sqlite(database) do |databasehandle|  
   databasehandle.tables.each do |table|  
     puts "-"*80  
     puts "Analyzing Table '#{table}'"  
     puts "-"*80  
    options.sstrings.each do |regex|  
      puts "[+] Table Name Match Found -> Database '#{database}' -> TABLE '#{table}'"  
    #Column Name Search  
    databasehandle[table.to_sym].columns.each do |column_name|  
     options.sstrings.each do |regex|  
       puts "[+] Column Name Match Found -> Database '#{database}' -> TABLE '#{table}' -> COLUMN '#{column_name}'"  
    #Data Search  
    if(options.sstrings.length > 0 && !options.metadata)  
     row = 0  
     databasehandle[table].each do |rowHash|  
      row = row + 1  
      rowHash.each do |key, value|  
       options.sstrings.each do |regex|  
         puts "[+] Data Match Found -> Database '#{database}' -> TABLE '#{table}', COLUMN '#{key}' -> ROW '#{row}'"  
         puts "\t[*] Row Dump\t=>\t#{rowHash.values.join('|')}" if(options.rowdump)