Product: Open-Xchange AppSuite / HTMLCleaner
Vendor: Open-Xchange GmbH / HTMLCleaner team

Internal reference: 27708 (Open-Xchange Bug ID), 86 (HTMLcleaner ticket)
Vulnerability type: Race condition within a thread (CWE-366)
Vulnerable version: 7.2.2
Vulnerable component: backend
Fixed version: 7.2.2-rev13
Solution status: Fixed by Vendor (Open-Xchange), Fixed by third party (HTMLCleaner)
Vendor notification: 2013-07-22
Solution date: 2013-08-06
Public disclosure: 2013-08-16
CVE reference: CVE-2013-5035
CVSSv2: 7.6 (AV:N/AC:L/Au:S/C:P/I:P/A:N/E:F/RL:U/RC:C/CDP:H/TD:H/CR:ND/IR:ND/AR:ND)

Vulnerability Details:
If multiple requests to save E-Mail as “draft”, or send E-Mail, occur within a very narrow window of time, it is possible that E-Mail content get swapped between requests. The root cause for this is a HTML sanitising library that turned out not to be thread-safe despite it claims to be. Further research showed, that the issue has been introduced with OX 7.2.2 by updating to the latest version of this library (2.2 to 2.5). OX Versions 7.2.1 and earlier are not vulnerable.

Risk:
Not properly handling concurrent access within the sanitising library leads to a potential privacy issue. Content can become modified unintentionally or available to unprivileged users and recipients, when mixing with other users content while processing mail. An attacker could potentially trigger a lot of these requests to provoke content switches in order to randomly access mail content and personal data. Apart from using the library for this specific use-case, results for general HTML sanitising under high load may not be accurate and could contain corrupted content.

Steps to reproduce:
1. Have a multiple clients constantly saving draft mail or sending mail (quicker than 100ms per mail)

Proof of concept:
The issue has been reproduced using Unit tests, load tests and has been confirmed by the HTMLcleaner development team. At typical load, we experienced a probability of less than 0.5% that mail content of the same client either gets duplicated or mixed with mail content of another client. Higher system load and concurrent usage (per OX node) leads to a higher probability that this issue can arise.
We have reported this issue back to the maintainers of the library, adding a test case and proof-of-concept code. The issue has been fixed with HTMLCleaner 2.6, see http://sourceforge.net/p/htmlcleaner/bugs/86/ for more details.

Solution:
The solution is to create one instance of the sanitizer per request rather than passing multiple requests to the same instance. This avoids potential multithreading issues using this library, however it does not solve the root cause and other consumers of this library should either downgrade, upgrade or implement a similar workaround.
Users should update to the latest available patch releases 7.2.2-rev13

Martin Braun
Open-Xchange GmbH