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// HackTricks · Network Services

88tcp/udp - Pentesting Kerberos

88tcp/udp - Pentesting Kerberos

Basic Information

Kerberos is a network authentication protocol: it establishes identities and session keys, while each application service makes its own authorization decision about the authenticated principal.[5]

In environments like Active Directory, Kerberos is instrumental in establishing the identity of users by validating their secret passwords. This process ensures that each user’s identity is confirmed before they interact with network resources. However, Kerberos does not extend its functionality to evaluate or enforce the permissions a user has over specific resources or services. Instead, it provides a secure way of authenticating users, which is a critical first step in the security process.

After Kerberos authentication, the target service evaluates the user’s rights and permissions using its local policy and any authorization data carried in the ticket. This separates authentication from application-specific access control.[5]

Default Port: 88/tcp/udp

PORT   STATE SERVICE
88/tcp open  kerberos-sec

To learn how to abuse Kerberos you should read the post about Active Directory.

Kerberos-only environments: client prep and troubleshooting

When NTLM is disabled on domain services (SMB/WinRM/etc.), you must authenticate with Kerberos. Common pitfalls and a working workflow:[4]

  • Time synchronization is mandatory. If your host clock is skewed by more than a few minutes you will see KRB_AP_ERR_SKEW and all Kerberos auth will fail. Sync against the DC:
# quick one-shot sync (requires sudo)
sudo ntpdate <dc.fqdn> || sudo chronyd -q 'server <dc.fqdn> iburst'
  • Generate a valid krb5.conf for the target realm/domain. netexec (CME fork) can output one for you while testing SMB:[1]
# Generate krb5.conf and install it
netexec smb <dc.fqdn> -u <user> -p '<pass>' -k --generate-krb5-file krb5.conf
sudo cp krb5.conf /etc/krb5.conf
  • Obtain a TGT and verify the ccache:[3]
kinit <user>
klist
  • Use Kerberos with SMB tooling (no passwords sent, uses your ccache):
# netexec / CME
netexec smb <dc.fqdn> -k            # lists shares, runs modules using Kerberos
# impacket examples also support -k / --no-pass to use the ccache
smbclient --kerberos //<dc.fqdn>/IPC$
  • GSSAPI SSH single sign-on (OpenSSH to Windows OpenSSH server):[2]
# Ensure krb5.conf is correct and you have a TGT (kinit)
# Use the FQDN that matches the host SPN. Wrong names cause: "Server not found in Kerberos database"
ssh -o GSSAPIAuthentication=yes <user>@<host.fqdn>

Tips:

  • Ensure your /etc/hosts resolves the exact FQDN you will SSH/SMB to, and that it comes before any bare domain entries if you are overriding DNS. SPN mismatches break GSSAPI.
  • If NTLM is disabled on SMB you may see STATUS_NOT_SUPPORTED with NTLM attempts; add -k to force Kerberos.

More

Shodan

  • port:88 kerberos

MS14-068

MS14-068 allowed a domain user to forge authorization data in a Kerberos ticket and have a vulnerable Domain Controller accept elevated group membership, potentially yielding Domain Admin privileges.[6]

The pykek exploit is archived in the SecWiki Windows kernel exploits collection.[7]

HackTricks Automatic Commands

Protocol_Name: Kerberos    #Protocol Abbreviation if there is one.
Port_Number:  88   #Comma separated if there is more than one.
Protocol_Description: AD Domain Authentication         #Protocol Abbreviation Spelled out

Entry_1:
  Name: Notes
  Description: Notes for Kerberos
  Note: |
    Kerberos operates on a principle where it authenticates users without directly managing their access to resources. This is an important distinction because it underlines the protocol's role in security frameworks.
    In environments like **Active Directory**, Kerberos is instrumental in establishing the identity of users by validating their secret passwords. This process ensures that each user's identity is confirmed before they interact with network resources. However, Kerberos does not extend its functionality to evaluate or enforce the permissions a user has over specific resources or services. Instead, it provides a secure way of authenticating users, which is a critical first step in the security process.

    https://book.hacktricks.wiki/en/network-services-pentesting/pentesting-kerberos-88/index.html

Entry_2:
  Name: Pre-Creds
  Description: Brute Force to get Usernames
  Command: nmap -p 88 --script=krb5-enum-users --script-args krb5-enum-users.realm="{Domain_Name}",userdb={Big_Userlist} {IP}

Entry_3:
  Name: With Usernames
  Description: Brute Force with Usernames and Passwords
  Note: consider git clone https://github.com/ropnop/kerbrute.git ./kerbrute -h

Entry_4:
  Name: With Creds
  Description: Attempt to get a list of user service principal names
  Command: GetUserSPNs.py -request -dc-ip {IP} active.htb/svc_tgs

References