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Account Manipulation: SSH Authorized Keys

ID Name
T1098.001 Additional Cloud Credentials
T1098.002 Additional Email Delegate Permissions
T1098.003 Additional Cloud Roles
T1098.004 SSH Authorized Keys
T1098.005 Device Registration

Adversaries may modify the SSH authorized_keys file to maintain persistence on a victim host. Linux distributions and macOS commonly use key-based authentication to secure the authentication process of SSH sessions for remote management. The authorized_keys file in SSH specifies the SSH keys that can be used for logging into the user account for which the file is configured. This file is usually found in the user's home directory under <user-home>/.ssh/authorized_keys.[1] Users may edit the system’s SSH config file to modify the directives PubkeyAuthentication and RSAAuthentication to the value "yes" to ensure public key and RSA authentication are enabled. The SSH config file is usually located under /etc/ssh/sshd_config.

Adversaries may modify SSH authorized_keys files directly with scripts or shell commands to add their own adversary-supplied public keys. In cloud environments, adversaries may be able to modify the SSH authorized_keys file of a particular virtual machine via the command line interface or rest API. For example, by using the Google Cloud CLI’s "add-metadata" command an adversary may add SSH keys to a user account.[2][3] Similarly, in Azure, an adversary may update the authorized_keys file of a virtual machine via a PATCH request to the API.[4] This ensures that an adversary possessing the corresponding private key may log in as an existing user via SSH.[5][6]

Where authorized_keys files are modified via cloud APIs or command line interfaces, an adversary may achieve privilege escalation on the target virtual machine if they add a key to a higher-privileged user.

ID: T1098.004
Sub-technique of:  T1098
ⓘ
Tactic: Persistence
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Platforms: IaaS, Linux, macOS
Contributors: Dror Alon, Palo Alto Networks; Or Kliger, Palo Alto Networks; Tony Lambert, Red Canary
Version: 1.1
Created: 24 June 2020
Last Modified: 20 April 2022

Procedure Examples

ID Name Description
S0482 Bundlore

Bundlore creates a new key pair with ssh-keygen and drops the newly created user key in authorized_keys to enable remote login.[7]
S0468 Skidmap

Skidmap has the ability to add the public key of its handlers to the authorized_keys file to maintain persistence on an infected host.[8]
G0139 TeamTNT

TeamTNT has added RSA keys in authorized_keys.[9]
S0658 XCSSET

XCSSET will create an ssh key if necessary with the ssh-keygen -t rsa -f $HOME/.ssh/id_rsa -P command. XCSSET will upload a private key file to the server to remotely access the host without a password.[10]

Mitigations

ID Mitigation Description
M1042 Disable or Remove Feature or Program

Disable SSH if it is not necessary on a host or restrict SSH access for specific users/groups using /etc/ssh/sshd_config.
M1022 Restrict File and Directory Permissions

Restrict access to the authorized_keys file.
M1018 User Account Management

In cloud environments, ensure that only users who explicitly require the permissions to update instance metadata or configurations can do so.

Detection

ID Data Source Data Component
DS0017 Command Command Execution
DS0022 File File Modification
DS0009 Process Process Creation

Use file integrity monitoring to detect changes made to the authorized_keys file for each user on a system. Monitor for suspicious processes modifying the authorized_keys file. In cloud environments, monitor instances for modification of metadata and configurations.

Monitor for changes to and suspicious processes modifiying /etc/ssh/sshd_config.

References

  1. ssh.com. (n.d.). Authorized_keys File in SSH. Retrieved June 24, 2020.
  2. Google Cloud. (2022, March 31). gcloud compute instances add-metadata. Retrieved April 1, 2022.
  3. Chris Moberly. (2020, February 12). Tutorial on privilege escalation and post exploitation tactics in Google Cloud Platform environments. Retrieved April 1, 2022.
  4. Microsoft. (n.d.). Virtual Machines - Update. Retrieved April 1, 2022.
  5. Blachman, Y. (2020, April 22). Growing Abuse of SSH Keys: Commodity Malware Campaigns Now Equipped with SSH Capabilities. Retrieved June 24, 2020.
  1. Cybereason Nocturnus. (2019, June 13). New Pervasive Worm Exploiting Linux Exim Server Vulnerability. Retrieved June 24, 2020.
  2. Sushko, O. (2019, April 17). macOS Bundlore: Mac Virus Bypassing macOS Security Features. Retrieved June 30, 2020.
  3. Remillano, A., Urbanec, J. (2019, September 19). Skidmap Linux Malware Uses Rootkit Capabilities to Hide Cryptocurrency-Mining Payload. Retrieved June 4, 2020.
  4. Kol, Roi. Morag, A. (2020, August 25). Deep Analysis of TeamTNT Techniques Using Container Images to Attack. Retrieved September 22, 2021.
  5. Mac Threat Response, Mobile Research Team. (2020, August 13). The XCSSET Malware: Inserts Malicious Code Into Xcode Projects, Performs UXSS Backdoor Planting in Safari, and Leverages Two Zero-day Exploits. Retrieved October 5, 2021.