### 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.