Assigned CVE: CVE-2020-5835 has been assigned and RedyOps Labs has been publicly acknowledged by the vendor.
Known to Neurosoft’s RedyOps Labs since: 16/03/2020
Vendor’s Advisory: https://support.broadcom.com/security-advisory/security-advisory-detail.html?notificationId=SYMSA1762
An Elevation of Privilege (EoP) exists in SEPM 14.2 RU2 MP1. The latest version we tested is SEPM Version 14 (14.2 RU2 MP1) build 5569 (14.2.5569.2100). The exploitation of this EoP , gives the ability to a low privileged user to execute any file as SYSTEM . The exploitation can take place the moment where a remote installation of the SEP is happening. The attacker escalates privileges, not in the machine which has the SEPM installed, but in the machine which we are going to remotely push (install) the SEP in. The attacker controls the file vpremote.dat which is used in order to provide the command line for the execution of the setup. By altering the command line, the attacker can execute any chosen file. Moreover, the installation uses the C:\TEMP folder, which the user fully controls and thus further attacks with symlinks seem to be possible.
Whenever Symantec Endpoint Protection Manager (SEPM) performs a client installation with remote push (Symantec Endpoint Protection Manager->Clients->Install a Client->Next->Next->Remote Push), the following actions are happening:
- By providing the credentials of a user who has Administrative rights on the remote machine (local Administrator, Domain Admin, etc), the SEPM connects to the remote system in order to install the client.
- In the remote system, the folder c:\TEMP\Clt-Inst\ is being created and a variety of actions are being performed as SYSTEM.
Any user with low privileges has full access in the new folders which are being created under the C:\ drive, by default. Any single user can create any folder under the C:\ drive . The inherited permissions allow him and everyone else, to edit/add/remove any file or sub folders, thus any user has full access on subfolders/files under the c:\TEMP folder.
As long as many file operations are being performed as SYSTEM, it seems possible for a user with low privileges to perform a variety of attacks with symlinks, such as Arbitrary file Creations, Arbitrary Deletes and more. These attacks are well documented, so I am not going to use any of those attacks for this blog post.
In order to present the issue, I am going to escalate my privileges from a user with low privileges and gain access as NT AUTHORITY/SYSTEM. This will be accomplished by modifying the vpremote.dat which is being created to the victim machine, in which we try to install the SEP with “remote push”. I remind that this machine is not necessarily the one which has the SEPM installed and usually is a new machine in which we try install the SEP client for first time.
During the installation process, the file vpremote.dat contains the command line which is going to be executed, probably by the vpremote.exe. This command line, instructs the execution of the setup.exe .By modifying the vpremote.dat, we can instruct the execution of our executable.
In order for this to succeed, we create a vpremote.dat with the following contents:
1.exe & setup.exe
and we add our executable 1.exe , in the same folder. The 1.exe is irrelevant to the vulnerability. It’s only the file you want to execute as SYSTEM.
No special exploit is need.
In the following paragraph a step by step explanation of the Video PoC, is provided.
Video PoC Step By Step
00:00-00:49: The Symantec Endpoint Manager Administrator, wants to install the SEP client on a the machine 192.168.2.9 . In this time frame, we watch the non malicious SEP Manager Administrator, who opts to install the client to the machine 192.168.2.9.
00:50-end: This is the view of the machine 192.168.2.9. The SEPM Administrator, from the previous machine, is trying to install the SEP Client on this machine (192.168.2.9 ).
An attacker with low privilege access, has compromised the machine 192.168.2.9 . The attacker at this point has no access to the SEP Manager, or to the Administrator’s password for this machine (192.168.2.9) . He only has low privilege access as user “attacker” on this machine (192.168.2.9).
01:15-01:19: As we can see, the folder C:\Temp\Clt-Inst\ is created and some files are added to this folder. One of the files is the vpremote.dat .
01:24-01:25: We replace the vpremote.dat with our vpremote.dat, which contains the payload “1.exe & setup.exe” and we add to the folder our malicious binary 1.exe .
The file 1.exe is irrelevant. You can use any binary you like. In case you will opt to use your own binary, you may not see the window because it is being executed from the session of the system user. You can observe the process creation from the procmon or you can validate that your 1.exe has been executed, from the Task Manager.
02:06-end: The 1.exe has been executed and the cmd.exe has opened as SYSTEM. My application 1.exe, opens the cmd.exe in the attacker’s session, and this is why it is visible. If you choose your own binary (e.g the notepad.exe) it might not be visible, but you can see it running in the task manager.
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