Adversaries may steal the credentials of a specific user or service account using Credential Access techniques or capture credentials earlier in their reconnaissance process through social engineering for means of gaining Initial Access.
Accounts that an adversary may use can fall into three categories: default, local, and domain accounts. Default accounts are those that are built-into an OS such as Guest or Administrator account on Windows systems or default factory/provider set accounts on other types of systems, software, or devices. Local accounts are those configured by an organization for use by users, remote support, services, or for administration on a single system or service.  Domain accounts are those managed by Active Directory Domain Services where access and permissions are configured across systems and services that are part of that domain. Domain accounts can cover users, administrators, and services.
Compromised credentials may be used to bypass access controls placed on various resources on systems within the network and may even be used for persistent access to remote systems and externally available services, such as VPNs, Outlook Web Access and remote desktop. Compromised credentials may also grant an adversary increased privilege to specific systems or access to restricted areas of the network. Adversaries may choose not to use malware or tools in conjunction with the legitimate access those credentials provide to make it harder to detect their presence.
Default accounts are also not limited to Guest and Administrator on client machines, they also include accounts that are preset for equipment such as network devices and computer applications whether they are internal, open source, or COTS. Appliances that come preset with a username and password combination pose a serious threat to organizations that do not change it post installation, as they are easy targets for an adversary. Similarly, adversaries may also utilize publicly disclosed private keys, or stolen private keys, to legitimately connect to remote environments via Remote Services 
The overlap of account access, credentials, and permissions across a network of systems is of concern because the adversary may be able to pivot across accounts and systems to reach a high level of access (i.e., domain or enterprise administrator) to bypass access controls set within the enterprise. 
APT28 has used legitimate credentials to maintain access to a victim network and exfiltrate data. The group also used credentials stolen through a spearphishing email to login to the DCCC network.
Adversaries can instruct Duqu to spread laterally by copying itself to shares it has enumerated and for which it has obtained legitimate credentials (via keylogging or other means). The remote host is then infected by using the compromised credentials to schedule a task on remote machines that executes the malware.
Take measures to detect or prevent techniques such as Credential Dumping or installation of keyloggers to acquire credentials through Input Capture. Limit credential overlap across systems to prevent access if account credentials are obtained. Ensure that local administrator accounts have complex, unique passwords across all systems on the network. Do not put user or admin domain accounts in the local administrator groups across systems unless they are tightly controlled and use of accounts is segmented, as this is often equivalent to having a local administrator account with the same password on all systems.
Follow best practices for design and administration of an enterprise network to limit privileged account use across administrative tiers. 
Audit domain and local accounts as well as their permission levels routinely to look for situations that could allow an adversary to gain wide access by obtaining credentials of a privileged account.   These audits should also include if default accounts have been enabled, or if new local accounts are created that have not be authorized.
Applications and appliances that utilize default username and password should be changed immediately after the installation, and before deployment to a production environment.  When possible, applications that use SSH keys should be updated periodically and properly secured.
Configure robust, consistent account activity audit policies across the enterprise and with externally accessible services.  Look for suspicious account behavior across systems that share accounts, either user, admin, or service accounts. Examples: one account logged into multiple systems simultaneously; multiple accounts logged into the same machine simultaneously; accounts logged in at odd times or outside of business hours. Activity may be from interactive login sessions or process ownership from accounts being used to execute binaries on a remote system as a particular account. Correlate other security systems with login information (e.g., a user has an active login session but has not entered the building or does not have VPN access).
Perform regular audits of domain and local system accounts to detect accounts that may have been created by an adversary for persistence. Checks on these accounts could also include whether default accounts such as Guest have been activated. These audits should also include checks on any appliances and applications for default credentials or SSH keys, and if any are discovered, they should be updated immediately.
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