Brute Force

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Brute Force
Technique
ID T1110
Tactic Credential Access
Platform Windows Server 2003, Windows Server 2008, Windows Server 2012, Windows XP, Windows 7, Windows 8, Windows Server 2003 R2, Windows Server 2008 R2, Windows Server 2012 R2, Windows Vista, Windows 8.1, Linux, Windows 10, MacOS, OS X
Permissions Required User
Data Sources Authentication logs
Contributors John Strand

Adversaries may use brute force techniques to attempt access to accounts when passwords are unknown or when password hashes are obtained.

Credential Dumping to obtain password hashes 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. Cracking hashes is usually done on adversary-controlled systems outside of the target network.1

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.2

A related technique called password spraying uses one password, 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.3

Examples

  • 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.4
  • Turla may attempt to connect to systems within a victim's network using net use commands and a predefined list or collection of passwords.5
  • Net Crawler uses a list of known credentials gathered through credential dumping to guess passwords to accounts as it spreads throughout a network.2

Mitigation

Set account lockout policies after a certain number of failed login attempts to prevent passwords from being guessed. Use multifactor authentication. Follow best practices for mitigating access to Valid Accounts

Detection

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.