[FD] usd20210005: Privileged File Write in Check Point Identity Agent < R81.018.0000

[Responsible Disclosure via Fulldisclosure <fulldisclosure@xxxxxxxxxxxx>]

### Advisory: Privileged File Write



Description

===========

The Check Point Identity Agent allows low privileged users to write files to 
protected locations of the file system.





Details

=======

Advisory ID: usd-2021-0005

Product: Check Point Identity Agent

Affected Version: < R81.018.0000

Vulnerability Type: Symlink Vulnerability

Security Risk: High

Vendor URL: https://www.checkpoint.com

Vendor Status: Fixed

Advisory URL: https://herolab.usd.de/security-advisories/usd-2021-0005/

First Published: 2021-03-31





Introduction

============

The Check Point Identity Agent allows low privileged users to write files to 
protected locations of the file system.



Privileged file write vulnerabilities allow low privileged users to create or 
overwrite files in arbitrary locations of the file system.

The impact of these attacks largely depends on the content that is written to 
the files. If the content is user controlled, privilege escalations are usually 
possible. Otherwise, the vulnerability can be used to perform Denial of Service 
attacks.





Proof of Concept

================

The Check Point Identity Agent allows users to collect information for the 
technical support. This information is collected to a Windows Cabinet file and 
stored within a user defined location. During the write operation that creates 
the Cabinet file, the service uses the permissions of the SYSTEM account, which 
allows low privileged users to create the Cabinet file in arbitrary locations 
of the file system. By using a symbolic link, the file name is also fully user 
controlled and the write operation can also be redirected to already existing 
files.



In the following screenshot, a low privileged user account sets the log folder 
of the Check Point Identity Agent to a user controlled path on the file system. 
This is possible within the tray menu of the agent.



Screenshot URL: 
https://herolab.usd.de/wp-content/uploads/sites/4/01-set-log-folder.png



The Cabinet file that we will create soon has a naming scheme of 
CP_Identity_Agent_Logs_25-01-2021_17.07.11.cab. As the name contains the 
current time and the export takes a while, an reliable attack requires multiple 
symbolic links. For our demonstration, we use the following simple script to 
create these:



```

C:\Users\tony\Desktop> type link.bat

C:\ProgramData\CreateSymlink.exe -p 
C:\Linker\CP_Identity_Agent_Logs_31-01-2021_12.13.00.cab C:\Windows\win.ini

C:\ProgramData\CreateSymlink.exe -p 
C:\Linker\CP_Identity_Agent_Logs_31-01-2021_12.13.01.cab C:\Windows\win.ini

C:\ProgramData\CreateSymlink.exe -p 
C:\Linker\CP_Identity_Agent_Logs_31-01-2021_12.13.02.cab C:\Windows\win.ini

C:\ProgramData\CreateSymlink.exe -p 
C:\Linker\CP_Identity_Agent_Logs_31-01-2021_12.13.03.cab C:\Windows\win.ini

C:\ProgramData\CreateSymlink.exe -p 
C:\Linker\CP_Identity_Agent_Logs_31-01-2021_12.13.04.cab C:\Windows\win.ini

[...]

```



Within the script, we make obviously use of the symboliclink-testing-tools 
(https://github.com/googleprojectzero/symboliclink-testing-tools) by James 
Forshaw (https://twitter.com/tiraniddo). After invoking the script, all 
possible file system locations for the to be generated Cabinet file point now 
to C:\Windows\win.ini, which is not writable for low privileged user accounts. 
Now we can request technical support information within the agents tray menu:



Screenshot URL: 
https://herolab.usd.de/wp-content/uploads/sites/4/02-request-technical-support-info.png



When the export finished, the C:\Windows\win.ini file should be overwritten 
with the contents of the Cabinet file. The following listing shows the original 
and the new content of the file:



```

C:\> type Windows\win.ini

; for 16-bit app support

[fonts]

[extensions]

[mci extensions]

[files]

[Mail]

MAPI=1



C:\> type C:\Windows\win.ini

MSCF [...]

```





Fix

=======

Whenever high privileged services write data into user controlled parts of the 
file system, they should make sure to impersonate the corresponding user during 
the operation. Additionally, protection mechanisms can be implemented to avoid 
following symlinks during write operations.





Timeline

=========

2021-02-12 Vulnerability discovered by Tobias Neitzel

2021-02-13 Initial contact with vendor

2021-03-02 Vendor provides patch for retest

2021-03-18 Patch published by vendor in release R81.018.0000

2021-03-31 Security Advisory released





Credits

================

This security vulnerability was discovered by Tobias Neitzel 
(https://twitter.com/qtc_de) of usd AG (https://www.usd.de/).





Disclaimer

==========

The information provided in this security advisory is provided "as is" and 
without warranty of any kind. Details of this security advisory may be updated 
in order to provide as accurate information as possible.






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