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.
|APT3||APT3 has been known to brute force password hashes to be able to leverage plain text credentials. |
|APT33||APT33 has used password spraying to gain access to target systems. |
|Chaos||Chaos conducts brute force attacks against SSH services to gain initial access. |
|China Chopper||China Chopper's server component can perform brute force password guessing against authentication portals. |
|Dragonfly 2.0||Dragonfly 2.0 dropped and executed tools used for password cracking, including Hydra.   |
|Emotet||Emotet has been observed using a hard coded list of passwords to brute force user accounts.     |
|Lazarus Group||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.  |
|Leafminer||Leafminer used a tool called BruteForcer to perform a brute force attack. |
|Linux Rabbit||Linux Rabbit brute forces SSH passwords in order to attempt to gain access and install its malware onto the server. |
|Net Crawler||Net Crawler uses a list of known credentials gathered through credential dumping to guess passwords to accounts as it spreads throughout a network. |
|OilRig||OilRig has used brute force techniques to obtain credentials. |
|PoshC2||PoshC2 has modules for brute forcing local administrator and AD user accounts. |
|SpeakUp||SpeakUp can perform brute forcing using a pre-defined list of usernames and passwords in an attempt to log in to administrative panels. |
Turla may attempt to connect to systems within a victim's network using
|Xbash||Xbash can obtain a list of weak passwords from the C2 server to use for brute forcing as well as attempt to brute force services with open ports.  |
|Account Use Policies||Set account lockout policies after a certain number of failed login attempts to prevent passwords from being guessed. Too strict a policy may create a denial of service condition and render environments un-usable, with all accounts used in the brute force being locked-out.|
|Multi-factor Authentication||Use multi-factor authentication. Where possible, also enable multi-factor authentication on externally facing services.|
|Password Policies||Refer to NIST guidelines when creating password policies. |
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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