Execution Flow

Acquire known credentials: The adversary must obtain known credentials in order to access the target system, application, or service.

Techniques
An adversary purchases breached username/password combinations or leaked hashed passwords from the dark web.
An adversary leverages a key logger or phishing attack to steal user credentials as they are provided.
An adversary conducts a sniffing attack to steal credentials as they are transmitted.
An adversary gains access to a database and exfiltrates password hashes.
An adversary examines outward-facing configuration and properties files to discover hardcoded credentials.

Determine target's password policy: Determine the password policies of the target system/application to determine if the known credentials fit within the specified criteria.

Techniques
Determine minimum and maximum allowed password lengths.
Determine format of allowed passwords (whether they are required or allowed to contain numbers, special characters, etc., or whether they are allowed to contain words from the dictionary).
Determine account lockout policy (a strict account lockout policy will prevent brute force attacks if multiple passwords are known for a single user account).

Attempt authentication: Try each username/password combination until the target grants access.

Techniques
Manually or automatically enter each username/password combination through the target's interface.

Impersonate: An adversary can use successful experiments or authentications to impersonate an authorized user or system or to laterally move within a system or application
Spoofing: Malicious data can be injected into the target system or into a victim user's system by an adversary. The adversary can also pose as a legitimate user to perform social engineering attacks.
Data Exfiltration: The adversary can obtain sensitive data contained within the system or application.

Credential Stuffing