[Full-disclosure] WordPress User Photo Component Remote File Upload Vulnerability

[ADVtools Advisories <advisories@xxxxxxxxxxxx>]

WordPress User Photo Component Remote File Upload Vulnerability

Product Name: User Photo
Vendor: http://wordpress.org/extend/plugins/user-photo/
Versions Affected: 0.9.4 and probably previous versions
Severity: High

Credit: Sebastien Andrivet, Flora Bottaccio, ADVtools.com
Website: http://www.advtools.com
Contact: advisories@xxxxxxxxxxxx
Date: 2011-02-17


I. Product Description

User Photo is a WordPress component that allows a user to associate a
photo with her account and for this photo to be displayed in  posts
and comments.


II. Vulnerability description

When a photo is uploaded, it is only partially validated and it is
possible to upload a backdoor on the server hosting WordPress. This
backdoor can be called (executed) even if the photo has not been yet
approved.


III. Analysis

1. Image type validation

When a file is uploaded, its type is validated. Only the following
types are accepted:

        $userphoto_validtypes = array(
                "image/jpeg" => true,
                "image/pjpeg" => true,
                "image/gif" => true,
                "image/png" => true,
                "image/x-png" => true
        );

The type is validated by the following code:

        if(@!$userphoto_validtypes[$_FILES['userphoto_image_file']['type']])
                $error = sprintf(__("The uploaded file type “%s” is 
not
allowed.", 'user-photo'), $_FILES['userphoto_image_file']['type']);

This code verifies the MIME type of the uploaded file. A navigator
infers the MIME type from the file itself or from its extension but it
is possible to intercept the HTTP request and change it (using a proxy
such as WebScarab). This way, any file can be uploaded as if it were
an image. The HTTP header to change is Content-type:

Content-type: image/gif

2. Image resizing

When a photo (an image) is uploaded, its size is checked. If it is too
big, it is resized. To avoid this resizing, the uploaded file has to
look like a small image. The verification of the size of the image is
done with code such as:

        $imageinfo = getimagesize($tmppath);

In the case of GIF, this PHP function simply looks at the beginning of
the GIF header and extracts the size of the image. A GIF header starts
with:

Offset   Length   Contents
  0      3 bytes  "GIF"
  3      3 bytes  "87a" or "89a"
  6      2 bytes  <Logical screen width in little-endian byte order>
  8      2 bytes  <Logical screen height in little-endian byte order>

getimagesize ignores the remaining of the binary data. It is thus easy
to create a file that looks like a small GIF image but that is in fact
something else.

3. PHP file

A PHP file can contain binary data. This data are reflected on the
output steam without interpretation. Only data between <?php and ?>
are interpreted as PHP code (see
http://www.php.net/manual/en/language.basic-syntax.phpmode.php). Using
this characteristic and the previous point, it is thus possible to
construct a file that looks like a small GIF image but that is in fact
a PHP file. For example (in hexadecimal):

        47 49 46 38 39 61 14 00 14 00 3C 3F 70 68 70 20 70 68 70 69 6E 66 6F
28 29 3B 20 3F 3E

This file is recognized as a GIF image with a width and a height of 20
pixels and also as a PHP file containing a call to phpinfo(). Using
the same technic, it is possible to upload a backdoor.

4. Uploading

Once uploaded, the PHP file is always located at the same place:

        wp-content/uploads/userphoto/alice.php

where alice is the login name (nickname) of the user uploading the file.

Important: This file is present even if it has not yet been approved
by the moderator.

5. Limitation

Since the PHP file begins with a fake GIF header, this header will be
output for every response. In practice, this is not really a problem:
it can be simply ignored (in the case of a backdoor outputting HTML)
or manually removed (in the case of the downloading of file). In some
cases (for example when images are dynamically returned), a backdoor
has to be slightly modified to avoid outputting two GIF headers.

6. Special case

In some installations, PHP files are interpreted as Unicode (16 bits).
Since the beginning of the GIF header is 16-bit aligned, it is not an
issue. The PHP code has to be written in Unicode.

7. Other concerns

This component contains also a XSS vulnerability located in the same
lines of code.


IV. Versions affected

Version 0.9.4 (latest version as of January 2011).

Other versions were not tested.


V. Impact

The exact impact depends of the configuration of the web server and of
the operating system:

- In the worst case, if Apache is running as root or as an
Administrator, the server is compromised (owned).

- If the Apache server is running as a dedicated low privilege user,
the backdoor will have limited access. Most of the time, the backdoor
will have read access but no write access except in very specific
places. To compromise the server, another vulnerability is necessary
(escalation).


VI. Proof of concept / Exploit

See part III.


VII. Mitigation, Workaround and fix

The hardening of the web server will limit the impact.


VIII. Disclosure time-line

2010.07 First researches, vulnerability discovered
2011.01.24 Proof of concept
2011.01.27 Vendor notified
2011.02.17 Public disclosure


IX. References

Graphics Interchange Format
http://en.wikipedia.org/wiki/Graphics_Interchange_Format

GIF specification
http://www.w3.org/Graphics/GIF/spec-gif89a.txt

PHP Escaping from HTML
http://www.php.net/manual/en/language.basic-syntax.phpmode.php

Passing Malicious PHP Through getimagesize()
http://ha.ckers.org/blog/20070604/passing-malicious-php-through-getimagesize/

PHP Exploit Code in a GIF
http://isc2.sans.org/diary.html?storyid=2997

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