Adversaries may use brute force techniques to attempt access to accounts when passwords are unknown or when password hashes are obtained.
Credential Dumping is used to obtain password hashes, this may only get an adversary so far when Pass the Hash is not an option. Techniques to systematically guess the passwords used to compute hashes are available, or the adversary may use a pre-computed rainbow table to crack hashes. Cracking hashes is usually done on adversary-controlled systems outside of the target network. 
Adversaries may attempt to brute force logins without knowledge of passwords or hashes during an operation either with zero knowledge or by attempting a list of known or possible passwords. This is a riskier option because it could cause numerous authentication failures and account lockouts, depending on the organization's login failure policies. 
A related technique called password spraying uses one password (e.g. 'Password01'), or a small list of passwords, that matches the complexity policy of the domain and may be a commonly used password. Logins are attempted with that password and many different accounts on a network to avoid account lockouts that would normally occur when brute forcing a single account with many passwords. 
Typically, management services over commonly used ports are used when password spraying. Commonly targeted services include the following:
- SSH (22/TCP)
- Telnet (23/TCP)
- FTP (21/TCP)
- NetBIOS / SMB / Samba (139/TCP & 445/TCP)
- LDAP (389/TCP)
- Kerberos (88/TCP)
- RDP / Terminal Services (3389/TCP)
- HTTP/HTTP Management Services (80/TCP & 443/TCP)
- MSSQL (1433/TCP)
- Oracle (1521/TCP)
- MySQL (3306/TCP)
- VNC (5900/TCP)
In default environments, LDAP and Kerberos connection attempts are less likely to trigger events over SMB, which creates Windows "logon failure" event ID 4625.
Lazarus Group malware attempts to connect to Windows shares for lateral movement by using a generated list of usernames, which center around permutations of the username Administrator, and weak passwords.
Set account lockout policies after a certain number of failed login attempts to prevent passwords from being guessed. Too strict a policy can create a denial of service condition and render environments un-usable, with all accounts being locked-out permanently. Use multifactor authentication. Follow best practices for mitigating access to Valid Accounts
Refer to NIST guidelines when creating passwords.
Where possible, also enable multi factor authentication on external facing services.
It is difficult to detect when hashes are cracked, since this is generally done outside the scope of the target network.
Monitor authentication logs for system and application login failures of Valid Accounts. If authentication failures are high, then there may be a brute force attempt to gain access to a system using legitimate credentials.
Also monitor for many failed authentication attempts across various accounts that may result from password spraying attempts.
For password spraying consider the following:
- Domain Controllers: "Audit Logon" (Success & Failure) for event ID 4625.
- Domain Controllers: "Audit Kerberos Authentication Service" (Success & Failure) for event ID 4771.
- All systems: "Audit Logon" (Success & Failure) for event ID 4648.
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