High
CVE-2023-6553
CVE-2023-6553
Description
The Backup Migration plugin for WordPress is vulnerable to Remote Code Execution in all versions up to, and including, 1.3.7 via the /includes/backup-heart.php file. This is due to an attacker being able to control the values passed to an include, and subsequently leverage that to achieve remote code execution. This makes it possible for unauthenticated attackers to easily execute code on the server.
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Technical Analysis
The Backup Migration Wordpress plugin describes itself as an all in one solution if you need to migrate your WordPress site to another host or just restore the site from a backup. The plugin is quite popular with over 80,000 active installs. Versions 1.3.7 and earlier suffer from an unauthenticated remote code execution vulnerability. The vulnerability is exploitable through the Content-Dir header which is sent to the /wp-content/plugins/backup-backup/includes/backup-heart.php endpoint.
Inside /includes/backup-heart.php we can see all the headers of the request get loaded into the $fields variable:
31 // Get fields from header 32 if (isFunctionEnabled('getallheaders')) { 33 $fields = getallheaders(); 34 }
Later on in the file we see the value of the user controlled content-dir header is loaded into the BMI_ROOT_DIR variable.
62 define('BMI_ROOT_DIR', $fields['content-dir']);
Then BMI_INCLUDES gets defined as BMI_ROOT_DIR . 'includes'
64 define('BMI_INCLUDES', BMI_ROOT_DIR . 'includes');
And finally the user controlled variable which is now BMI_INCLUDES gets fed into a require_once statement.
118 require_once BMI_INCLUDES . '/bypasser.php';
Now you might be thinking – a user controlled variable being fed into a require statement? Sounds like a Local File Inclusion vulnerability. You would be correct if it weren’t for a very neat technique called PHP Filter Chaining.
PHP Filter Chaining
php://filter is a kind of meta-wrapper designed to permit the application of filters to a stream at the time of opening. The php://filter target has one required parameter which is resource and it specifies the stream that you would like to filter. For the purposes of PHP filter chaining we can set the resource to resource=php://temp as we’re not actually interested in filtering any specific stream we just want to abuse the functionality of PHP filters.
There are thousands of languages throughout the world and our basic ASCII encoding table is far too small to represent all the characters used to express all of these different languages. So many encoding tables were created to convert or translate characters from one language to another. These conversion tables are also accessible through php://convert.iconv.*.* wrappers.
The key to PHP filter chaining is that in some cases byte signatures are prepended by switching encoding. These signatures do not get removed from the string when you switch encoding a second time, they stack. In the case of Unicode (UTF-16), it is required to give to your system the order of the bytes to use (Byte Order Mark BOM), by digging a bit in the RFC 2781 referring to it:
The Unicode Standard and ISO 10646 define the character “ZERO WIDTH
NON-BREAKING SPACE” (0xFEFF), which is also known informally as “BYTE
ORDER MARK” (abbreviated “BOM”).This usage, suggested by Unicode
and ISO 10646 Annex F (informative), is to prepend a 0xFEFF character
to a stream of Unicode characters as a “signature”; a receiver of such
a serialized stream may then use the initial character both as a hint
that the stream consists of Unicode characters and as a way to recognize
the serialization order.
The technique was first introduced by a Computer Security enthusiast by the name of Gynvael Coldwind and since the introduction of the technique people have figured out how to chain encoding switches to prepend every letter in the base64 character set. We’re able to base64 decode the string using the php filter convert.base64-decode so the base64 character set is all we need to know how to convert. Here is an excerpt from the conversion table:
"A" => "convert.iconv.8859_3.UTF16|convert.iconv.863.SHIFT_JISX0213", "a" => "convert.iconv.CP1046.UTF32|convert.iconv.L6.UCS-2|convert.iconv.UTF-16LE.T.61-8BIT|convert.iconv.865.UCS-4LE", "B" => "convert.iconv.CP861.UTF-16|convert.iconv.L4.GB13000", "b" => "convert.iconv.JS.UNICODE|convert.iconv.L4.UCS2|convert.iconv.UCS-2.OSF00030010|convert.iconv.CSIBM1008.UTF32BE", "C" => "convert.iconv.UTF8.CSISO2022KR", "c" => "convert.iconv.L4.UTF32|convert.iconv.CP1250.UCS-2",
If wish to prepend the character “A” to your string you simply need to chain these two filter conversion together: "convert.iconv.8859_3.UTF16|convert.iconv.863.SHIFT_JISX0213.
Turning the LFI in RCE
Now we can use the conversion table to prepend a PHP payload to the start of the of the string that gets interpreted by the require_once statement on line 118 of
/includes/backup-heart.php. The Content-Dir header becomes very long and is subject to length HTTP request header length restrictions, however we are still able to send relatively small payloads such as <?phpecho>/tmp/pwned;?>. The following POST request will send the aforementioned payload:
POST /wp-content/plugins/backup-backup/includes/backup-heart.php HTTP/1.1 Host: 127.0.0.1 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 14_0) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/16.5 Safari/605.1.15 Content-Dir: 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Content-Type: application/x-www-form-urlencoded Content-Length: 0
References
https://www.wordfence.com/blog/2023/12/critical-unauthenticated-remote-code-execution-found-in-backup-migration-plugin/
https://www.synacktiv.com/en/publications/php-filters-chain-what-is-it-and-how-to-use-it
https://wordpress.org/plugins/backup-backup/
CVSS V3 Severity and Metrics
General Information
Vendors
- backupbliss
Products
- backup migration
Metasploit Modules
References
Exploit
Additional sources will be added here as they become relevant.
Notes: We will only add the top 3 POCs for a given CVE. POCs added here must have at least 2 GitHub stars.
Miscellaneous
