Cisco VPN remote code execution flaw rated 10 out of 10 for severity

Cisco issued an advisory about a critical security flaw in Cisco devices running Adaptive Security Appliance (ASA) software. Patch now because if the vulnerability is exploited, it could result in remote code execution and denial of service.

The flaw is as bad as it gets, considering CVE-2018-0101 has the most severe Common Vulnerability Score System (CVSS) score possible — 10 out of 10. Vulnerabilities rated with a 10 CVSS score mean they can be easily exploited remotely and require no authentication.

Vulnerable Cisco products

Ten is also the number of vulnerable products running Cisco ASA:

• 3000 Series Industrial Security Appliance (ISA)
• ASA 5500 Series Adaptive Security Appliances
• ASA 5500-X Series Next-Generation Firewalls
• ASA Services Module for Cisco Catalyst 6500 Series Switches and Cisco 7600 Series Routers
• ASA 1000V Cloud Firewall
• Adaptive Security Virtual Appliance (ASAv)
• Firepower 2100 Series Security Appliance
• Firepower 4110 Security Appliance
• Firepower 9300 ASA Security Module
• Firepower Threat Defense Software (FTD)

According to Cisco’s advisory:

A vulnerability in the Secure Sockets Layer (SSL) VPN functionality of the Cisco Adaptive Security Appliance (ASA) Software could allow an unauthenticated, remote attacker to cause a reload of the affected system or to remotely execute code.

WebVPN must be enabled for flaw to be exploited

There’s a catch, though, because the flaw can be exploited only if the “webvpn” feature is enabled on the device running Cisco ASA.

The vulnerability is due to an attempt to double free a region of memory when the webvpn feature is enabled on the Cisco ASA device. An attacker could exploit this vulnerability by sending multiple, crafted XML packets to a webvpn-configured interface on the affected system. An exploit could allow the attacker to execute arbitrary code and obtain full control of the system, or cause a reload of the affected device.

Cisco provided a command-line to determine if WebVPN is enabled.

According to Cisco’s documentation, WebVPN is supposed to establish a secure VPN tunnel and provide access to “web resources and web-enabled application from almost any computer on the internet” such as internal websites, MS Outlook Web Access, NT/Active Directory file shares, MAPI, and email proxies, including POP3S, IMAP45 and SMTPS.

After determining if WebVPN is enabled, users need to determine which version of ASA or FTD software is running. Cisco provided a how-to, as well as a chart listing which major ASA and FTD releases are vulnerable.

There are no workarounds, Cisco said, so patch now.

Security researcher Cedric Halbronn from the NCC Group discovered the vulnerability and reported it to Cisco. He is scheduled to talk about how he exploited it on Feb. 2 at the Recon Brussels conference.

The Cisco Product Security Incident Response Team said it is unaware of any attempts to exploit the bug. It remains to be seen if that stays true after Halbronn’s presentation at Recon Brussels 2018.