Redundant Access

Adversaries may use more than one remote access tool with varying command and control protocols as a hedge against detection. If one type of tool is detected and blocked or removed as a response but the organization did not gain a full understanding of the adversary's tools and access, then the adversary will be able to retain access to the network. Adversaries may also attempt to gain access to Valid Accounts to use External Remote Services such as external VPNs as a way to maintain access despite interruptions to remote access tools deployed within a target network. ^[1]

Use of a Web Shell is one such way to maintain access to a network through an externally accessible Web server.

ID: T1108

Tactic: Defense Evasion, Persistence

Platform: Linux, macOS, Windows

Permissions Required: User, Administrator, SYSTEM

Data Sources: Process monitoring, Process use of network, Packet capture, Network protocol analysis, File monitoring, Authentication logs, Binary file metadata

Defense Bypassed: Network intrusion detection system, Anti-virus

Version: 1.0

Mitigations

Mitigation	Description
Network Intrusion Prevention	Network intrusion detection and prevention systems that use network signatures to identify traffic for specific adversary malware can be used to mitigate activity at the network level. Signatures are often for unique indicators within protocols and will be different across various malware families and versions. Adversaries will likely change tool signatures over time or construct protocols in such a way as to avoid detection by common defensive tools.^[10]

Mitigation

Description

Network Intrusion Prevention

Network intrusion detection and prevention systems that use network signatures to identify traffic for specific adversary malware can be used to mitigate activity at the network level. Signatures are often for unique indicators within protocols and will be different across various malware families and versions. Adversaries will likely change tool signatures over time or construct protocols in such a way as to avoid detection by common defensive tools.^[10]

Examples

Name	Description
3PARA RAT	3PARA RAT will sleep until after a date/time value loaded from a .dat file has passed. This allows the RAT to remain dormant until a set date, which could allow a means to regain access if other parts of the actors' toolset are removed from a victim.^[2]
APT3	APT3 has been known to use multiple backdoors per campaign.^[3]
Cobalt Group	Cobalt Group has used TeamViewer to preserve remote access in case control using the Cobalt Strike module was lost.^[4]
FIN5	FIN5 maintains access to victim environments by using Valid Accounts to access External Remote Services as well as establishing a backup RDP tunnel by using FLIPSIDE.^[5]
Leafminer	Leafminer used a tool called Imecab to set up a persistent remote access account on the victim machine.^[6]
OilRig	OilRig has used RGDoor via Web shell to establish redundant access. The group has also used harvested credentials to gain access to Internet-accessible resources such as Outlook Web Access, which could be used for redundant access.^[7]
Stolen Pencil	Stolen Pencil has a tool to add a Windows admin account in order to allow them to ensure continued access via RDP.^[8]
Threat Group-3390	Threat Group-3390 has deployed backup web shells and obtained OWA account credentials during intrusions that it subsequently used to attempt to regain access when evicted from a victim network.^[9]

Detection

Existing methods of detecting remote access tools are helpful. Backup remote access tools or other access points may not have established command and control channels open during an intrusion, so the volume of data transferred may not be as high as the primary channel unless access is lost.

Detection of tools based on beacon traffic, Command and Control protocol, or adversary infrastructure require prior threat intelligence on tools, IP addresses, and/or domains the adversary may use, along with the ability to detect use at the network boundary. Prior knowledge of indicators of compromise may also help detect adversary tools at the endpoint if tools are available to scan for those indicators.

If an intrusion is in progress and sufficient endpoint data or decoded command and control traffic is collected, then defenders will likely be able to detect additional tools dropped as the adversary is conducting the operation.

For alternative access using externally accessible VPNs or remote services, follow detection recommendations under Valid Accounts and External Remote Services to collect account use information.

Redundant Access

Mitigations

Examples

Detection

References