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CVE-2008-1930: WORDPRESS 2.5COOKIE INTEGRITY PROTECTION

VULNERABILITYBy Louis Nyffenegger <[email protected]>

245556888

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Table of Content

Table of ContentIntroductionAbout this exercise

LicenseSyntax of this courseThe web application

The issueIntroductionThe codeThe vulnerability

ExploitationPatchConclusion

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PentesterLab.com » CVE-2008-1930

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Introduction

This course details the exploitation of an issue in the cookies integrity mechanism ofWordpress. This issue was found in 2008 and allowed an attacker to gainadministrator access to a wordpress instance if user registration is enabled. Thisissue is a really good example of what can go wrong with cryptographic function and Ithought it will do a really good exercise.

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About this exercise

LicenseThis exercise by PentesterLab is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visithttp://creativecommons.org/licenses/by-nc-nd/3.0/.

Syntax of this course5/17


https://www.pentesterlab.com/

http://creativecommons.org/licenses/by-nc-nd/3.0/

The red boxes provide information on mistakes/issues that are likely to happen whiletesting:

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 applicationOnce the system has booted, you can then retrieve the current IP address of thesystem using the command ifconfig:

$ ifconfig eth0eth0 Link encap:Ethernet HWaddr 52:54:00:12:34:56 inet addr:10.0.2.15 Bcast:10.0.2.255 Mask:255.255.255.0 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 10.0.2.15.6/17


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 anentry to your host file with this name and the corresponding IP address. It can beeasily done by modifying:

on Windows, your C:\Windows\System32\Drivers\etc\hosts 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'tforget to update your hosts file.forget to update your hosts file.

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The issue

IntroductionThis functionnality was used to remember users after they close their browser. Acookie "AUTH_COOKIE" (named wordpress_...) is created by the application andsent back to users. Only the application is able to generate this cookie since it'sgenerated using the WordPress "secret key".

The codeThe 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:

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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;

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}

First the code retrieves the cookie AUTH_COOKIE if no cookie was provided during thefunction call. If no cookie was provided and the cookie AUTH_COOKIE is empty, thefunction 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 aguenuine 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) isgreater than the current time:

if ( $expired < time() ) return false;

The code ensures that the signature is correct:

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$key = wp_hash($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 anduse $username and $expiration to generate an unique key. You can check thisfunction'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 andthis key is unique and not predictable;

the cookie is signed based on a unique key and can't be tampered(theorically);

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The vulnerabilityThe 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, thefollowing 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 usernamesare different. The collision is interesting because it is possible to have a valid hashgenerated by the application for a user (admin1) and use it to pretend to be anotheruser admin.

The $expiration value will become even bigger for the user admin since we addedthe final 1 from admin1.

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Exploitation

As we saw above, an attacker is able to get the application to generate a valid hashfor 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 forexample (any users followed by an integer will actually work). This can be done usingthe 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 tocreate 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 validemail address and will set his own password.email address and will set his own password.

If you create a user admin1 and log in with this user. You should receive a validcookie:

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http://vulnerable/wp-login.php?action=register

HTTP/1.1 200 OK[...]Set-Cookie: wordpress_test_cookie=WP+Cookie+check; wordpress_177e685d5ab0d655bdbe4896d7cdadf4=admin1%7C1353464343%7C1ba7d82099dd6119781b54ecf8b79259[...]

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


https://addons.mozilla.org/en-US/firefox/addon/cookies-manager-plus/

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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 correctlyseparated. 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 avalid signature since $expiration and/or $username are not simply concatenate togenerate the signature.

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Conclusion

This exercise explained how this vulnerability works and how it was possible to use itto gain access to Wordpress administration pages.

To me this issue represents perfectly a common pattern in most interestingvulnerabilities: "The devil is in the detail". And that even a ridiculous small change canmake a lot the difference between secure and vulnerable code. And since Codereview is mostly a matter of “déjà vu”, you will have another thing to check for if yousearch for vulnerabilities.

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