Bastille Tracking Number 28
CVE-2017-9485

Overview

A vulnerability has been discovered that enables an attacker to write arbitrary data to a session cookie stored at a known location on a target gateway.



Affected Platforms

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



Proof-of-Concept

Using the syseventd vulnerability (Bastille Tracking Number 22), an attacker can put a target gateway into UI dev mode by creating the file /var/ui_dev_mode on disk.

When this file is present on the gateway, the web UI bypasses the normal authentication logic, and accepts any password, as long as a valid username is entered.

As a result of bypassing the normal authentication logic, when an invalid username is entered, session data is written to disk, but never deleted.

An attacker can therefore enter arbitrary data into the username field, which gets written to disk at the following path:

/var/tmp/sess_[PHP Session ID]

The PHP Session ID is included in the login attempt HTTP response, giving the attacker control of the contents of a file at a known location on the gateway.



Test Environment

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



Mitigation

There is no apparent mechanism to allow Comcast customers to control or change this behavior.



Recommended Remediation

Modify the web UI to delete the session data when a user enters an invalid username.



Credits

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