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After successfully 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 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.
The chart below sums up the expected behavior of cross-protocols relay attacks depending on the mitigations in place (). All the tests and results listed in the chart were made using 's (Python).
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).
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 unsupported for this version of NTLM.
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)
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)
Below are different use-cases of ntlmrelayx.
The ntlmrelayx tool offers features making it a very valuable asset when pentesting an Active Directory domain:
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). This option can also be used when NTLMv1 is allowed (NTLMv1 doesn't support MIC).
it implements CVE-2019-1019 with the -remove-target and -machine-account arguments
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
the target can be specified with a target protocol like ldap://target but the "all" keyword can be used (all://target). If the protocol isn't specified, it defaults to smb.
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
The following mindmap sums up the overall attack paths of NTLM relay. explains how to read it here: (at 08:00).
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 that prevents this.
As of november 2020, MIC was optional, but suggest it might've become mandatory.
Reminder: if NTLMv1 is accepted, NTLM could be relayed and modified and the MIC dropped
For more details on how NTLM works, testers can read .
From UNIX-like systems, (Python) and (Python) can be used to identify and requirements for SMB, LDAP and LDAPS.
(Python), (Python) and (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 .
When combining NTLM relay with Responder for , testers need to make sure that Responder's servers are deactivated, otherwise they will interfere with ntlmrelayx ones.
The following command will try to relay the authentication over SMB and attempt a remote from the target if the relayed victim has the right privileges.
At the time of this article update (12th Feb. 2022), adding LSA dump to the existing SAM dump is pending.
The following command will try to relay the authentication and open .
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 .
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. It will also gather lots of information regarding the domain users and groups, the computers, , etc.
The following command will abuse the default value (i.e. 10) of to create a domain machine account. The tester will then be able to use it for AD operations.
In most cases, the --remove-mic option will be needed when relaying to LDAP(S) because of the .
Using LDAPS for that operation is not mandatory since Active Directory LDAP implements StartTLS. This is implemented in Impacket since April 30th 2022 ().
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 or doing some (--escalate-user) if the relayed account has sufficient privileges.
This technique is usually combined with a 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 ) to escalate a domain user privileges (--escalate-user).
The following command will 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.
A can also be operated with a relayed NTLM authentication, but only if the target domain controller is vulnerable to since the DRSUAPI always requires signing.
It can work with mitm6 (for ) 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)
Thanks to , 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.
(Python) has the ability to generate the list of possible targets for relay to SMB (hosts with SMB signing not required).
MITRE ATT&CK™ Sub-technique T1557.001
