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[Global AppSec Amsterdam, September 23-27]
Brute force attack
Last revision (mm/dd/yy): 08/9/2013
Related Security Activities
How to Test for Brute Force Vulnerabilities
Last revision (mm/dd/yy): 08/9/2013
A brute force attack can manifest itself in many different ways, but primarily consists in an attacker configuring predetermined values, making requests to a server using those values, and then analyzing the response. For the sake of efficiency, an attacker may use a dictionary attack (with or without mutations) or a traditional brute-force attack (with given classes of characters e.g.: alphanumerical, special, case (in)sensitive). Considering a given method, number of tries, efficiency of the system which conducts the attack, and estimated efficiency of the system which is attacked the attacker is able to calculate approximately how long it will take to submit all chosen predetermined values.
Brute-force attacks are often used for attacking authentication and discovering hidden content/pages within a web application. These attacks are usually sent via GET and POST requests to the server. In regards to authentication, brute force attacks are often mounted when an account lockout policy in not in place.
A web application can be attacked via brute force by taking a word list of known pages, for instance from a popular content management system, and simply requesting each known page then analyzing the HTTP response code to determine if the page exists on the target server.
In the first scenerio, where the goal of brute-forcing is to know the password in its decrypted form, it may appear that John the Ripper is a very helpful tool. The TOP10 tools for password cracking with different methods, including brute-force, may be found on http://sectools.org/crackers.html.
For testing web services there are tools like:
- dirb (http://sourceforge.net/projects/dirb/) - WebRoot (http://www.cirt.dk/tools/webroot/WebRoot.txt)
dirb belongs to more advanced tools. With its help we are able to:
- set cookies - add any HTTP header - use PROXY - mutate objects which were found - test http(s) connections - seek catalogues and/or files using defined dictionaries and templates - and much much more
The simplest test to perform is:
rezos@dojo ~/d/owasp_tools/dirb $ ./dirb http://testsite.test/ ----------------- DIRB v1.9 By The Dark Raver ----------------- START_TIME: Mon Jul 9 23:13:16 2007 URL_BASE: http://testsite.test/ WORDLIST_FILES: wordlists/common.txt SERVER_BANNER: lighttpd/1.4.15 NOT_EXISTANT_CODE: 404 [NOT FOUND] (Location: '' - Size: 345) ----------------- Generating Wordlist... Generated Words: 839 ---- Scanning URL: http://testsite.test/ ---- FOUND: http://testsite.test/phpmyadmin/ (***) DIRECTORY (*)
In the output the attacker is informed that phpmyadmin/ catalogue was found. The attacker who knows that is now able to perform the attack on this application. In dirb's templates there is, among others, a dictionary containing information about invalid httpd configuration. This dictionary will detect weaknesses of this kind.
One of the main problems with tools like dirb is recognition if the given response from the server is expected and reliable. With more advanced server configuration (e.g. with mod_rewrite) automatic tools are unable to determine if server response informs about an error or that the file, which the attacker is after, was found.
The application WebRoot.pl, written by CIRT.DK, has embedded mechanisms for parsing server responses, and based on the phrase specified by the attacker, measures if the server response is expected.
./WebRoot.pl -noupdate -host testsite.test -port 80 -verbose -match "test" -url "/private/<BRUTE>" -incremental lowercase -minimum 1 -maximum 1
oo00oo00oo00oo00oo00oo00oo00oo00oo00oo00oo00oo00 o Webserver Bruteforcing 1.8 o 0 ************* !!! WARNING !!! ************ 0 0 ******* FOR PENETRATION USE ONLY ********* 0 0 ****************************************** 0 o (c)2007 by Dennis Rand - CIRT.DK o oo00oo00oo00oo00oo00oo00oo00oo00oo00oo00oo00oo00
[X] Checking for updates - NO CHECK [X] Checking for False Positive Scan - OK [X] Using Incremental - OK [X] Starting Scan - OK GET /private/b HTTP/1.1 GET /private/z HTTP/1.1
[X] Scan complete - OK [X] Total attempts - 26 [X] Sucessfull attempts - 1 oo00oo00oo00oo00oo00oo00oo00oo00oo00oo00oo00oo00
WebRoot.pl found one file "/private/b" on testsite.test, which contains phrase "test".
Another example is to examine ranges of the variable's values:
./WebRoot.pl -noupdate -host testsite.test -port 80 -verbose -diff "Error" -url "/index.php?id=<BRUTE>" -incremental integer -minimum 1 -maximum 1
Detect your web servers being scanned by brute force tools such as WFuzz, OWASP DirBuster and vulnerability scanners such as Nessus, Nikto, Acunetix ..etc. This helps you quickly identify probable probing by bad guys who's wanna dig possible security holes.
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