Menu
OpenVPN servers can be vulnerable to Shellshock Bash vulnerability

OpenVPN servers can be vulnerable to Shellshock Bash vulnerability

Some OpenVPN configuration options open remote attack vectors for Bash vulnerabilities, VPN provider warns

Virtual private network servers based on OpenVPN might be vulnerable to remote code execution attacks through Shellshock and other recent flaws that affect the Bash Unix shell.

The OpenVPN attack vector was described in a post on Hacker News Tuesday by Fredrik Strömberg, co-founder of a commercial VPN service called Mullvad.

"OpenVPN has a number of configuration options that can call custom commands during different stages of the tunnel session," Strömberg said. "Many of these commands are called with environmental variables set, some of which can be controlled by the client."

Shellshock and several other flaws found in the Bash Unix shell over the past week stem from errors in how the command-line interpreter parses strings passed to it as environment variables. These strings can be crafted to trick Bash into evaluating parts of them as separate commands.

Various applications call Bash in different circumstances and could be used by attackers to pass malicious strings to the shell. This is the case of CGI scripts running on Web servers, the CUPS printing system for Unix-like operating systems, the Secure Shell (SSH) and others.

The security community is still investigating the full scope of the Shellshock flaws and which applications open up remote attack vectors for them. Security researcher Rob Fuller has put together a list of proof-of-concept exploits published so far.

One OpenVPN configuration option that allows for Shellshock exploitation is called auth-user-pass-verify. According to the software's official documentation this directive provides a plug-in-style interface for extending an OpenVPN server's authentication capabilities.

The option executes an administrator-defined script via the command-line interpreter in order to validate user names and passwords supplied by connecting clients. This opens up the possibility of clients supplying maliciously crafted user names and passwords that exploit the Shellshock vulnerability when passed to Bash as strings.

Amagicom, the Swedish company that owns Mullvad, informed the OpenVPN developers and some VPN service providers about the auth-user-pass-verify issue last week, but waited before going public to allow them to take the appropriate actions. This Shellshock attack vector is one of the more serious ones, because it does not require authentication.

However, it does appear that the developers of OpenVPN knew about the general security risks associated with auth-user-pass-verify even before the recent Bash flaws were discovered.

"Care must be taken by any user-defined scripts to avoid creating a security vulnerability in the way that these strings are handled," the official OpenVPN documentation warns for this configuration option. "Never use these strings in such a way that they might be escaped or evaluated by a shell interpreter."

In other words, the script author needs to make sure that the user name and password strings received from clients do not contain any dangerous characters or sequence of characters before passing them to the shell interpreter. However, instead of relying on the ability of script writers to filter out possible exploits, it's probably best to deploy the latest Bash patch in this case.


Follow Us

Join the newsletter!

Or

Sign up to gain exclusive access to email subscriptions, event invitations, competitions, giveaways, and much more.

Membership is free, and your security and privacy remain protected. View our privacy policy before signing up.

Error: Please check your email address.

Tags patchesAccess control and authenticationExploits / vulnerabilitiesAmagicom

Brand Post

Featured

Slideshows

The Kiwi channel gathers for the 2020 Reseller News Women in ICT Awards

The Kiwi channel gathers for the 2020 Reseller News Women in ICT Awards

Hundreds of leaders from the New Zealand IT industry gathered at the Hilton in Auckland on 17 November to celebrate the finest female talent in the Kiwi channel and recognise the winners of the Reseller News Women in ICT Awards (WIICTA) 2020.

The Kiwi channel gathers for the 2020 Reseller News Women in ICT Awards
Leading female front runners honoured at the 2020 Reseller News Women in ICT Awards

Leading female front runners honoured at the 2020 Reseller News Women in ICT Awards

The leading female front runners of the New Zealand ICT industry joined together for the annual Reseller News Women in ICT Awards event at the Hilton in Auckland, during which hundreds of guests celebrated 13 outstanding individuals who won awards, chosen from more than 50 finalists representing over 30 organisations.

Leading female front runners honoured at the 2020 Reseller News Women in ICT Awards
Channel gathers to celebrate the Reseller News Innovation Awards 2020 winners

Channel gathers to celebrate the Reseller News Innovation Awards 2020 winners

More than 500 channel leaders gathered in Auckland on 21 October at the ​Reseller News Innovation Awards ​2020 to celebrate the achievements of the New Zealand technology industry's top partners, start-ups, vendors, distributors and individuals.

Channel gathers to celebrate the Reseller News Innovation Awards 2020 winners
Show Comments