Assigned CVE: CVE-2020-8103 has been assigned and RedyOps Labs has been publicly acknowledged by the vendor.
Known to Neurosoft’s RedyOps Labs since: 18/03/2020
An Elevation of Privileges (EoP) exists in Bitdefender Antivirus Free 2020 < 126.96.36.199 . The latest version we tested is BitDefender Free Edition 188.8.131.52. The exploitation of this EoP, gives the ability to a low privileged user to gain access as NT AUTHORITY\SYSTEM . The exploitation has been tested in installation of BitDefender on Windows 10 1909 (OS Build 18363.720) 64bit .
The exploitation allows any local, low privileged user, to change the Discretionary Access Control List (DACL) of any chosen file. The access you obtain depends on which file you are going to backdoor/overwrite. In the Proof of Concept (PoC) we overwrite the file system32/wermgr.exe and we pop a cmd.exe running as NT AUTHORITY\SYSTEM . The exploitation works with the default installation of BitDefender. When the BitDefender detects a threat, it gives the ability to the user to choose what action to take. The user can choose to Quarantine the threat. After the threat has been placed in Quarantine, the user can choose to restore it. The problem arises because the BitDefeder restores the file as NT AUTHORITY\SYSTEM and without impersonating the current user. This allows the user to create a symlink and restore the file to an arbitrary location or overwrite files which are running as SYSTEM.
- Put an EICAR file (or any other file which can be detected as a threat) under an empty folder which you fully control. For example, put an EICAR file to C:\Users\Public\Music
- Scan the EICAR file (e.g C:\Users\Public\Music\eicar.txt) with the BitDefender.
- When the BitDefender detects the threat, choose to Quarantine the file.
- Give some time to the BitDefender and rename the folder C:\Users\Public\Music to C:\Users\Public\Music2 .
- Create the symlink C:\Users\Public\Music\eicar.txt (e.g with the CreateSymlink.exe from symboliclink-testing-tools of project zero) and target the file you wish to control. For example CreateSymlink.exe C:\Users\Public\Music\eicar.txt c:\windows\system32\wermgr.exe .
- Add the c:\windows\system32\wermgr.exe to the exception list of BitDefender.
- Go back to BitDefender and restore the C:\Users\Public\Music\eicar.txt . The first time it may fail. Click restore again.
- The BitDefender, running as NT AUTHORITY\SYSTEM, will follow the symlink and will overwrite the file c:\windows\system32\wermgr.exe .
- The c:\windows\system32\wermgr.exe will have a new DACL, which gives your user full access. You can overwrite the file with anything you wish.
Video PoC Step By Step
The exploit takes 2 arguments. The second argument is the file we want to overwrite. The first argument, is the file with which we want to overwrite it. As for example the execution:
Exploit.exe C:\users\attacker\Desktop\1.exe c:\windows\system32\wermgr.exe
will overwrite the file c:\windows\system32\wermgr.exe with the file C:\users\attacker\Desktop\1.exe .
The 1.exe is irrelevant to the exploitation. You can choose any file you wish and you can override any file you like. the 1.exe will just execute the cmd.exe to the current user’s session. For example you can execute
Exploit.exe C:\users\attacker\Desktop\test.txt c:\windows\system32\wermgr.exe
and you will overwrite the c:\windows\system32\wermgr.exe with the file C:\users\attacker\Desktop\test.txt
00:00 – 00:56: I present the environment. The low privilege user, the windows version, the BitDefender version and the DACL of the file c:\windows\system32\wermgr.exe . As we can see the user attacker has limited access to the file.
00:56 – 01:54: We run the exploit and the first thing we have to do is to add as an exception the file we target. In this case the file c:\windows\system32\wermgr.exe
01:54 – 02:48: After we add the exception, we go back to the exploit and we press ENTER. After the ENTER button is pressed, the exploit should print three lines which inform us about the AV. At this point, the exploit has created the EICAR file under the C:\Users\Public\Music\ folder. We go to this folder and we scan the file. We opt to move it to the Quarantine. Until this point, there is no reason for the exploit to fail. It just creates the EICAR file. If you do not see the three lines which are presented in the video and inform you about the AV, ensure you have pressed the ENTER a few times.
02:48 – 03:49: At this point the exploit checks that the EICAR file has been removed. In 30″ more or less you should see the message “I go for a coffee. Give me a sec.” . If for some reason you don’t see the message after a few seconds, press ENTER a few times inside the Exploit window. After this message, the Exploit renames the folder C:\Users\Public\Music to C:\Users\Public\Music2 and there is a sleep for 50″ . If you don’t see the second message “I am back. Oh no the bell. BRB.” after 50″ – 60″ just press ENTER again in the exploit window.
03:49 – 04:00: After the message “I am back. Oh no the bell. BRB.” the exploit creates the Symlink. It will create the symlink C:\Users\Public\Music\RESTORE_ME__* * *.txt which targets the c:\windows\system32\wermgr.exe . Then the exploit instructs us to restore the file.
04:00 – 04:20: We go back to the BitDefender and we restore the file. As we observe, we have to press the restore button two times. The BitDefender will restore the eicar file and will overwrite the c:\windows\system32\wermgr.exe . The c:\windows\system32\wermgr.exe will have a new DACL which allows us to overwrite it. The exploit will overwrite the c:\windows\system32\wermgr.exe with our binary “1.exe” . At this point the exploit should terminate in a few seconds. If you observe a delay on the exploit, press ENTER a few times.
04:20 – end: This is irrelevant with the BitDefender issue. This is just a way to trigger the execution of the file c:\windows\system32\wermgr.exe (which is now the 1.exe) as SYSTEM and gain a shell as NT AUTHORITY\SYSTEM . As we can see the c:\windows\system32\wermgr.exe has been ovewritten by the 1.exe and the new DACL gives the “attacker” full access over the file.
You can find the exploit code in our Github at https://github.com/RedyOpsResearchLabs/
RedyOps team, uses the 0-day exploits produced by Research Labs, before vendor releases any patch. They use it in special engagements and only for specific customers.
You can find RedyOps team at https://redyops.com/
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You can find Angel team at https://angelcyber.gr/
Our 0-days cannot win Illicium. Today’s information technology landscape is threatened by modern adversary security attacks, including 0-day exploits, polymorphic malwares, APTs and targeted attacks. These threats cannot be identified and mitigated using classic detection and prevention technologies; they can mimic valid user activity, do not have a signature, and do not occur in patterns. In response to attackers’ evolution, defenders now have a new kind of weapon in their arsenal: Deception.
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