=============================================
- Release date: November 11th, 2009
- Discovered by: Dawid Golunski
- Severity: Moderately High
=============================================
I. VULNERABILITY
-------------------------
WordPress <= 2.8.5 Unrestricted File Upload Arbitrary PHP Code Execution
II. BACKGROUND
-------------------------
WordPress is a state-of-the-art publishing platform with a focus on
aesthetics, web standards,
and usability. WordPress is both free and priceless at the same time.
More simply, WordPress is
what you use when you want to work with your blogging software, not
fight it.
III. DESCRIPTION
-------------------------
Wordpress allows authorised users to add an attachment to a blog post.
It does not sanitize provided file properly before moving it to an
uploads directory.
The part of the code responsible for uploading files looks as follows:
wp-admin/includes/file.php:
---[cut]---
line 217:
function wp_handle_upload( &$file, $overrides = false, $time = null ) {
---[cut]---
// All tests are on by default. Most can be turned off by
$override[{test_name}] = false;
$test_form = true;
$test_size = true;
// If you override this, you must provide $ext and $type!!!!
$test_type = true;
$mimes = false;
---[cut]---
// A properly uploaded file will pass this test. There should be no
reason to override this one.
if (! @ is_uploaded_file( $file['tmp_name'] ) )
return $upload_error_handler( $file, __( 'Specified file
failed upload test.' ));
// A correct MIME type will pass this test. Override $mimes or use the
upload_mimes filter.
if ( $test_type ) {
$wp_filetype = wp_check_filetype( $file['name'], $mimes );
extract( $wp_filetype );
if ( ( !$type || !$ext ) && !
current_user_can( 'unfiltered_upload' ) )
return $upload_error_handler( $file,
__( 'File type does not meet
security guidelines. Try
another.' ));
if ( !$ext )
$ext = ltrim(strrchr($file['name'], '.'), '.');
if ( !$type )
$type = $file['type'];
} else {
$type = '';
}
// A writable uploads dir will pass this test. Again, there's no point
overriding this one.
if ( ! ( ( $uploads = wp_upload_dir($time) ) && false ===
$uploads['error'] ) )
return $upload_error_handler( $file, $uploads['error'] );
$filename = wp_unique_filename( $uploads['path'], $file['name'],
$unique_filename_callback );
// Move the file to the uploads dir
$new_file = $uploads['path'] . "/$filename";
if ( false === @ move_uploaded_file( $file['tmp_name'], $new_file ) ) {
return $upload_error_handler( $file,
sprintf( __('The uploaded file could not be moved to %s.' ),
$uploads['path'] ) );
}
---[cut ]---
From the above code we can see that provided filename gets checked
with:
$wp_filetype = wp_check_filetype( $file['name'], $mimes );
Here is how the wp_check_filetype() function looks like:
wp-includes/functions.php:
---[cut]---
line 2228:
function wp_check_filetype( $filename, $mimes = null ) {
// Accepted MIME types are set here as PCRE unless provided.
$mimes = ( is_array( $mimes ) ) ? $mimes :
apply_filters( 'upload_mimes', array(
'jpg|jpeg|jpe' => 'image/jpeg',
'gif' => 'image/gif',
'png' => 'image/png',
'bmp' => 'image/bmp',
'tif|tiff' => 'image/tiff',
'ico' => 'image/x-icon',
'asf|asx|wax|wmv|wmx' => 'video/asf',
'avi' => 'video/avi',
---[cut, more mime types]---
line 2279:
$type = false;
$ext = false;
foreach ( $mimes as $ext_preg => $mime_match ) {
$ext_preg = '!\.(' . $ext_preg . ')$!i';
if ( preg_match( $ext_preg, $filename,
$ext_matches ) ) {
$type = $mime_match;
$ext = $ext_matches[1];
break;
}
}
return compact( 'ext', 'type' );
}
We can see that type of the file gets set to a predefined MIME type
that matches supplied
extension, and that the extension is obtained from a regexp that
matches a mime ext. string after
the LAST dot.
If extension is not on the list $type and $ext will be set to FALSE
and wordpress will
produce an error ("File type does not meet security guidelines. Try
another").
Let's look at the other check that is performed on the filename before
a file gets uploaded,
that is a call to the following function:
$filename = wp_unique_filename( $uploads['path'], $file['name'],
$unique_filename_callback );
wp-includes/functions.php:
line 2096:
function wp_unique_filename( $dir, $filename,
$unique_filename_callback = null ) {
// sanitize the file name before we begin processing
$filename = sanitize_file_name($filename);
---[cut, code that only matters if uploaded file already
exists]---
line 2126:
return $filename;
}
To have a complete view on file sanitization performed by wordpress we
need to look into the
sanitize_file_name() function:
wp-includes/formatting.php:
line 601:
function sanitize_file_name( $filename ) {
$filename_raw = $filename;
$special_chars = array("?", "[", "]", "/", "\\", "=", "<",
">", ":", ";", ",", "'", "\"",
"&", "$", "#", "*", "(", ")", "|", "~", "`",
"!", "{", "}", chr(0));
$special_chars = apply_filters('sanitize_file_name_chars',
$special_chars, $filename_raw);
$filename = str_replace($special_chars, '', $filename);
$filename = preg_replace('/[\s-]+/', '-', $filename);
$filename = trim($filename, '.-_');
return apply_filters('sanitize_file_name', $filename,
$filename_raw);
}
This function removes special characters shown above, replaces spaces
and consecutive dashes with
a single dash, trims period, dash and underscore from beginning and
end of the filename.
The sanitization process appears quite extensive however it does not
take into account files that
have multiple extensions.
It is possible to upload a file containing an arbitrary PHP script
with an extension of '.php.jpg'
and execute it by requesting the uploaded file directly.
The execution of the PHP code despite the .php.jpg extension is
possible because Apache
allows for multiple extensions. Here is a quote from Apache docs
regarding this matter:
"
Files can have more than one extension, and the order of the
extensions is normally irrelevant.
For example, if the file welcome.html.fr maps onto content type text/
html and language French then
the file welcome.fr.html will map onto exactly the same information.
If more than one extension is
given that maps onto the same type of meta-information, then the one
to the right will be used,
except for languages and content encodings. For example, if .gif maps
to the MIME-type image/gif
and .html maps to the MIME-type text/html, then the file
welcome.gif.html will be associated with
the MIME-type text/html.
Care should be taken when a file with multiple extensions gets
associated with both a MIME-type
and a handler. This will usually result in the request being handled
by the module associated with
the handler. For example, if the .imap extension is mapped to the
handler imap-file
(from mod_imagemap) and the .html extension is mapped to the MIME-type
text/html, then the file
world.imap.html will be associated with both the imap-file handler and
text/html MIME-type.
When it is processed, the imap-file handler will be used, and so it
will be treated as a
mod_imagemap imagemap file.
"
IV. PROOF OF CONCEPT
-------------------------
Browser is enough to replicate this issue. Simply log in to your
wordpress blog as a low privileged
user or admin. Create a new post and use the media file upload feature
to upload a file:
test-image.php.jpg
containing the following code:
<?php
phpinfo();
?>
After the upload you should receive a positive response saying:
test-vuln.php.jpg
image/jpeg
2009-11-11
and it should be possible to request the uploaded file via a link:
http://link-to-our-wp-unsecured-blog.com/wp-content/uploads/2009/11/test-vuln.php.jpg
thus executing the PHP code it contains.
In the above code example, a php info page will be shown.
V. BUSINESS IMPACT
-------------------------
An attacker that has already obtained login details (for example by
stealing user's cookies with
an XSS attack) to the blog as one of the existing users could exploit
this vulnerability to get
access to the system in the Apache user's context.
From there he could use local bugs to further escalate the
privileges. Apache account would be
enough in most cases to view the source codes and gain access to the
databases.
Some wordpress users of the 2.8.5 release have reported that some php
files have been added to
their wordpress directory. It could be possible that they have been
hit by this bug. Therefore it
is important to take some countermeasures as soon as possible.
VI. SYSTEMS AFFECTED
-------------------------
Most likely all of the wordpress releases contain this bug. Including
the current hardened stable
release 2.8.5 and the beta version.
VII. SOLUTION
-------------------------
Vendor has been informed about the bug. Currently wordpress developers
and contributors are in
the process of bug hunting and fixing reported bugs in beta versions
before the new stable release,
so hopefully it should not take long for them to take this problem
into account.
You can apply the temporary solutions for this problem which I provide
below before an official
patch is made.
You can create a .htaccess file in the uploads dir (wordpress/wp-
content/uploads) with
the following content:
deny from all
<Files ~ "^\w+\.(gif|jpe?g|png|avi)$">
order deny,allow
allow from all
</Files>
Adjust allowed file extensions in the brackets if necessary.
This will prevent Apache from serving files with double extensions
inside the uploads directory.
Alternatively you can try to patch the source code yourself by editing
the
wp-admin/includes/file.php file and the wp_handle_upload() function it
contains. An example patch
could be to add the following three lines of code at the line 260:
// Fix Unrestricted File Upload Arbitrary PHP Code Execution bug,
return if more than 1 extension provided
if ( count(explode('.', $file['name'])) > 2 );
return $upload_error_handler( $file, __( 'File type does not
meet security guidelines. Try another.' ));
VIII. REFERENCES
-------------------------
http://www.wordpress.org
http://httpd.apache.org/docs/2.2/mod/mod_mime.html#multipleext
IX. CREDITS
-------------------------
This vulnerability has been discovered by Dawid Golunski
golunski (at) onet (dot) eu
Greetings go to: robxt, sajanek, xsoti, bart, falcon (for the old
time's sake :) and complexmind
X. REVISION HISTORY
-------------------------
November 11th, 2009: Initial release
XI. LEGAL NOTICES
-------------------------
The information contained within this advisory is supplied "as-is"
with no warranties or guarantees of fitness of
use or otherwise. I accept no responsibility for any damage caused by
the use or misuse of this information.
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