CRLF injection in OpenVPN Access Server - CVE-2017-5868

Detailed vulnerability description

The vulnerability allows a remote attacker to inject arbitrary session cookies and gain control over victim's VPN session.

The vulnerability exists due to insufficient sanitization of user-supplied data (e.g. %0A character), passed via URL. A remote attacker can create a specially crafted link, trick the victim into following it and spoof HTTP headers, used by OpenVPN Access Server to authenticate users.

Successful exploitation of the vulnerability may allow an attacker to inject arbitrary cookie values, including session identifier, and gain full access to victim's session.

Exploitation example for injecting victim's session identifier, controlled by the attacker:

https://[host]/__session_start__/%0aSet-Cookie:
openvpn_sess_******=******cf23; Path=/; Secure; HttpOnly

How to mitigate CVE-2017-5868

Sources

• http://seclists.org/oss-sec/2017/q2/332