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String.fromCharCode(34, 62, 60, 105, 109, 103, 32, 115, 114, 99, 61, 120, 32, 111, 110, 101, 114, 114, 111, 114, 61, 112, 114, 111, 109, 112, 116, 40, 49, 41, 59, 62)

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  2. 2. 2 4 5 5 5 6 8 8 8 12 13 16 17 Table of Content Table of Content Introduction About this exercise License Syntax of this course The web application The issue Introduction The code The vulnerability Exploitation Patch Conclusion 2/17 » CVE-2008-1930
  3. 3. 3/17 » CVE-2008-1930
  4. 4. Introduction This course details the exploitation of an issue in the cookies integrity mechanism of Wordpress. This issue was found in 2008 and allowed an attacker to gain administrator access to a wordpress instance if user registration is enabled. This issue is a really good example of what can go wrong with cryptographic function and I thought it will do a really good exercise. 4/17 » CVE-2008-1930
  5. 5. About this exercise License This exercise by PentesterLab is licensed under the Creative Commons Attribution- NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit Syntax of this course 5/17 » CVE-2008-1930
  6. 6. The red boxes provide information on mistakes/issues that are likely to happen while testing: An issue that you may encounter...An issue that you may encounter... The green boxes provide tips and information if you want to go further. You should probably check...You should probably check... The web application Once the system has booted, you can then retrieve the current IP address of the system using the command ifconfig: $ ifconfig eth0 eth0 Link encap:Ethernet HWaddr 52:54:00:12:34:56 inet addr: Bcast: Mask: inet6 addr: fe80::5054:ff:fe12:3456/64 Scope:Link UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 RX packets:88 errors:0 dropped:0 overruns:0 frame:0 TX packets:77 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:1000 RX bytes:10300 (10.0 KiB) TX bytes:10243 (10.0 KiB) Interrupt:11 Base address:0x8000 In this example the IP address is 6/17 » CVE-2008-1930
  7. 7. Throughout the training, the hostname vulnerable is used for the vulnerable machine, you can either replace it by the IP address of the machine, or you can just add an entry to your host file with this name and the corresponding IP address. It can be easily done by modifying: on Windows, your C:WindowsSystem32Driversetchosts file; on Unix/Linux and Mac OS X, your /etc/hosts file. The IP address can change if you restart the system, don'tThe IP address can change if you restart the system, don't forget to update your hosts file.forget to update your hosts file. 7/17 » CVE-2008-1930
  8. 8. The issue Introduction This functionnality was used to remember users after they close their browser. A cookie "AUTH_COOKIE" (named wordpress_...) is created by the application and sent back to users. Only the application is able to generate this cookie since it's generated using the WordPress "secret key". The code The vulnerable function is wp_validate_auth_cookie included in the file wp- includes/pluggable.php (line 470 to 499). The full code of the function is below: 8/17 » CVE-2008-1930
  9. 9. function wp_validate_auth_cookie($cookie = '') { if ( empty($cookie) ) { if ( empty($_COOKIE[AUTH_COOKIE]) ) return false; $cookie = $_COOKIE[AUTH_COOKIE]; } list($username, $expiration, $hmac) = explode('|', $cookie); $expired = $expiration; // Allow a grace period for POST and AJAX requests if ( defined('DOING_AJAX') || 'POST' == $_SERVER['REQUEST_METHOD'] ) $expired += 3600; if ( $expired < time() ) return false; $key = wp_hash($username . $expiration); $hash = hash_hmac('md5', $username . $expiration, $key); if ( $hmac != $hash ) return false; $user = get_userdatabylogin($username); if ( ! $user ) return false; return $user->ID; 9/17 » CVE-2008-1930
  10. 10. } First the code retrieves the cookie AUTH_COOKIE if no cookie was provided during the function call. If no cookie was provided and the cookie AUTH_COOKIE is empty, the function returns false and the authentication fails. Once the cookie is retrieved, it gets split into 3 values: $username: the user name; $expiration: its expiration date; $hmac: the signature of the previous values to make sure it's a guenuine cookie. The following code performs this action, | (%7C) is used as a separator: list($username, $expiration, $hmac) = explode('|', $cookie); Then the code makes sure the $expired value (based on the value $expiration) is greater than the current time: if ( $expired < time() ) return false; The code ensures that the signature is correct: 10/17 » CVE-2008-1930
  11. 11. $key = wp_hash($username . $expiration); $hash = hash_hmac('md5', $username . $expiration, $key); if ( $hmac != $hash ) return false; The function wp_hash provide the encryption, it's based on Worpress SECRET_KEY and use $username and $expiration to generate an unique key. You can check this function's behaviour in the file wp-includes/pluggable.php starting line 1071. Once the hash is validated, the current user $user is retrieved using the value $username: $user = get_userdatabylogin($username); if ( ! $user ) return false; return $user->ID; If you look at the code quickly, everything seems perfect: the cookie expired at a given time; only the application can generate the key used to sign the cookie and this key is unique and not predictable; the cookie is signed based on a unique key and can't be tampered (theorically); 11/17 » CVE-2008-1930
  12. 12. The vulnerability The issue comes from this line: $hash = hash_hmac('md5', $username . $expiration, $key); It is possible to generate a collision between two chosen values. For example, the following values will give the same hash: `$username` `$expiration` `HASH($username.$expiration)` admin1 1353464343 1ba7d82099dd6119781b54ecf8b79259 admin 11353464343 1ba7d82099dd6119781b54ecf8b79259 We see that it's possible to get a collision between two hashes even if the usernames are different. The collision is interesting because it is possible to have a valid hash generated by the application for a user (admin1) and use it to pretend to be another user admin. The $expiration value will become even bigger for the user admin since we added the final 1 from admin1. 12/17 » CVE-2008-1930
  13. 13. Exploitation As we saw above, an attacker is able to get the application to generate a valid hash for a user admin1 and reuse this signature for the user admin. To exploit this vulnerability, you need to be able to create a user named admin1 for example (any users followed by an integer will actually work). This can be done using the registration page: http://vulnerable/wp-login.php?action=register. Here the source code of Wordpress has been modified toHere the source code of Wordpress has been modified to create users with the hardcoded password `pentesterlab`. In acreate users with the hardcoded password `pentesterlab`. In a traditional Wordpress, the attacker need to provide a validtraditional Wordpress, the attacker need to provide a valid email address and will set his own address and will set his own password. If you create a user admin1 and log in with this user. You should receive a valid cookie: 13/17 » CVE-2008-1930
  14. 14. HTTP/1.1 200 OK [...] Set-Cookie: wordpress_test_cookie=WP+Cookie+check; wordpress_177e685d5ab0d655bdbe4896d7cdadf4=admin1%7C1353464343%7C1ba7 d82099dd6119781b54ecf8b79259 [...] Once you log in, you should see the traditional Worpdress page: Now that we have a valid cookie we can use this vulnerability to gain access to the admin account: Using the a valid cookie: admin1%7C1210158445%7C49718d2581bd399916b90a088a11ec84 We can generate a new valid cookie for the user admin: admin%7C11210158445%7C49718d2581bd399916b90a088a11ec84. If you're using Firefox, you can use the following extension to modify your cookies: Cookie manager +. After reloading the page, you should be able to see the "Admin version" of the website: 14/17 » CVE-2008-1930
  15. 15. 15/17 » CVE-2008-1930
  16. 16. Patch The patch for this vulnerabilty was pretty simple, to avoid the vulnerability, Worpdress' developers just had to make sure that $username and $expiration were correctly separated. To do so they introduced the following change: $hash = hash_hmac('md5', $username . '|' . $expiration, $key); With this simple |, it not possible for an attacker to tamper the cookie and still get a valid signature since $expiration and/or $username are not simply concatenate to generate the signature. 16/17 » CVE-2008-1930
  17. 17. Conclusion This exercise explained how this vulnerability works and how it was possible to use it to gain access to Wordpress administration pages. To me this issue represents perfectly a common pattern in most interesting vulnerabilities: "The devil is in the detail". And that even a ridiculous small change can make a lot the difference between secure and vulnerable code. And since Code review is mostly a matter of “déjà vu”, you will have another thing to check for if you search for vulnerabilities. 17/17 » CVE-2008-1930