Unusual Linux Usernameedit

A machine learning job detected activity for a username that is not normally active, which can indicate unauthorized changes, activity by unauthorized users, lateral movement, or compromised credentials. In many organizations, new usernames are not often created apart from specific types of system activities, such as creating new accounts for new employees. These user accounts quickly become active and routine. Events from rarely used usernames can point to suspicious activity. Additionally, automated Linux fleets tend to see activity from rarely used usernames only when personnel log in to make authorized or unauthorized changes, or threat actors have acquired credentials and log in for malicious purposes. Unusual usernames can also indicate pivoting, where compromised credentials are used to try and move laterally from one host to another.

Rule type: machine_learning

Machine learning job: linux_anomalous_user_name_ecs

Machine learning anomaly threshold: 50

Severity: low

Risk score: 21

Runs every: 15 minutes

Searches indices from: now-45m (Date Math format, see also Additional look-back time)

Maximum signals per execution: 100



  • Elastic
  • Linux
  • ML

Version: 1

Added (Elastic Stack release): 7.7.0

Potential false positivesedit

Uncommon user activity can be due to an engineer logging in to a server instance in order to perform manual troubleshooting or reconfiguration.

Investigation guideedit

Signals from this rule indicate activity for a Linux user name that is rare and unusual. Here are some possible avenues of investigation:

  • Consider the user as identified by the username field. Is this program part of an expected workflow for the user who ran this program on this host? Could this be related to troubleshooting or debugging activity by a developer or site reliability engineer?
  • Examine the history of user activity. If this user manifested only very recently, it might be a service account for a new software package. If it has a consistent schedule - for example if it runs monthly or quarterly - it might be part of a monthly or quarterly business process.
  • Examine the process arguments, title and working directory. These may provide indications as to the source of the program or the nature of the tasks that the user is performing.