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. (Citation: Cylance Cleaver)

Typically, management services over commonly used ports are used when guessing passwords. 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)
* SNMP (161/UDP and 162/TCP/UDP)

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.(Citation: US-CERT TA18-068A 2018). Further, adversaries may abuse network device interfaces (such as `wlanAPI`) to brute force accessible wifi-router(s) via wireless authentication protocols.(Citation: Trend Micro Emotet 2020)

In default environments, LDAP and Kerberos connection attempts are less likely to trigger events over SMB, which creates Windows "logon failure" event ID 4625.
Password Guessing
source_name: mitre-attack  url: https://attack.mitre.org/techniques/T1110/001  external_id: T1110.001     source_name: Trend Micro Emotet 2020  description: Cybercrime & Digital Threat Team. (2020, February 13). Emotet Now Spreads via Wi-Fi. Retrieved February 16, 2022.  url: https://www.trendmicro.com/vinfo/us/security/news/cybercrime-and-digital-threats/emotet-now-spreads-via-wi-fi     source_name: Cylance Cleaver  description: Cylance. (2014, December). Operation Cleaver. Retrieved September 14, 2017.  url: https://web.archive.org/web/20200302085133/https://www.cylance.com/content/dam/cylance/pages/operation-cleaver/Cylance_Operation_Cleaver_Report.pdf     source_name: US-CERT TA18-068A 2018  description: US-CERT. (2018, March 27). TA18-068A Brute Force Attacks Conducted by Cyber Actors. Retrieved October 2, 2019.  url: https://www.us-cert.gov/ncas/alerts/TA18-086A     