Adversaries with no prior knowledge of legitimate credentials within the system or environment may guess passwords to attempt access to accounts. Without knowledge of the password for an account, an adversary may opt to systematically guess the password using a repetitive or iterative mechanism. An adversary may guess login credentials without prior knowledge of system or environment passwords during an operation by using a list of common passwords. Password guessing may or may not take into account the target's policies on password complexity or use policies that may lock accounts out after a number of failed attempts.
Guessing passwords can be a risky option because it could cause numerous authentication failures and account lockouts, depending on the organization's login failure policies. 
Typically, management services over commonly used ports are used when guessing passwords. Commonly targeted services include the following:
In addition to management services, adversaries may "target single sign-on (SSO) and cloud-based applications utilizing federated authentication protocols," as well as externally facing email applications, such as Office 365.. Further, adversaries may abuse network device interfaces (such as
wlanAPI) to brute force accessible wifi-router(s) via wireless authentication protocols.
In default environments, LDAP and Kerberos connection attempts are less likely to trigger events over SMB, which creates Windows "logon failure" event ID 4625.
APT28 has used a brute-force/password-spray tooling that operated in two modes: in brute-force mode it typically sent over 300 authentication attempts per hour per targeted account over the course of several hours or days. APT28 has also used a Kubernetes cluster to conduct distributed, large-scale password guessing attacks.
|M1036||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.
Use multi-factor authentication. Where possible, also enable multi-factor authentication on externally facing services.
Refer to NIST guidelines when creating password policies. 
Upgrade management services to the latest supported and compatible version. Specifically, any version providing increased password complexity or policy enforcement preventing default or weak passwords.
|ID||Data Source||Data Component||Detects|
|DS0015||Application Log||Application Log Content||
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.
|DS0002||User Account||User Account Authentication||
Monitor for many failed authentication attempts across various accounts that may result from password spraying attempts. It is difficult to detect when hashes are cracked, since this is generally done outside the scope of the target network. (ex: Windows EID 4625 or 5379)