Bastille Tracking Number 22
CVE-2017-9479

Overview

A vulnerability has been discovered that enables an attacker to launch applications on a gateway as a root user. This vulnerability can be exploited by an attacker connected to the LAN, private Wi-Fi AP, or Xfinity Home Security AP.



Affected Platforms

Cisco DPC3939, firmware version dpc3939-P20-18-v303r20421746-170221a-CMCST



Proof-of-Concept

The syseventd service provides a mechanism to launch applications on the gateway in response to certain events, such as the one-minute cron job firing. The sysevent command line application is used to communicate with the syseventd server, which is open on port 52378 to the LAN, private Wi-Fi AP, and Xfinity Home Security Wi-Fi AP.

An attacker on one of the above networks can take advantage of this, using the sysevent command line application to launch applications on the gateway. 

The following two commands, when executed from a computer connected to an Xfinity Home Security Wi-Fi AP, will cause the persistent storage configuration data to be copied to /var/IGD/. This can then be retrieved by the attacker.

./sysevent --port 52367 --ip 172.16.12.1 async </path/to/file> /bin/cp

./sysevent --port 52367 --ip 172.16.12.1 set </path/to/file> /var/IGD/<file>



Test Environment

Cisco DPC3939, firmware version dpc3939-P20-18-v303r20421746-170221a-CMCST



Mitigation

There is no apparent mechanism to allow Comcast customers to disable syseventd access.



Recommended Remediation

Update the firewall rules to disable access to syseventd from the LAN, private Wi-Fi AP, and Xfinity Home Security Wi-Fi AP.



Credits

Marc Newlin and Logan Lamb, Bastille
Chris Grayson, Web Sight.IO