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T1556.003 Pluggable Authentication Modules

Adversaries may modify pluggable authentication modules (PAM) to access user credentials or enable otherwise unwarranted access to accounts. PAM is a modular system of configuration files, libraries, and executable files which guide authentication for many services. The most common authentication module is pam_unix.so, which retrieves, sets, and verifies account authentication information in /etc/passwd and /etc/shadow.123

Adversaries may modify components of the PAM system to create backdoors. PAM components, such as pam_unix.so, can be patched to accept arbitrary adversary supplied values as legitimate credentials.4

Malicious modifications to the PAM system may also be abused to steal credentials. Adversaries may infect PAM resources with code to harvest user credentials, since the values exchanged with PAM components may be plain-text since PAM does not store passwords.51

Item Value
ID T1556.003
Sub-techniques T1556.001, T1556.002, T1556.003, T1556.004, T1556.005, T1556.006, T1556.007, T1556.008
Tactics TA0006, TA0005, TA0003
Platforms Linux, macOS
Permissions required root
Version 2.0
Created 26 June 2020
Last Modified 17 October 2021

Procedure Examples

ID Name Description
S0377 Ebury Ebury can deactivate PAM modules to tamper with the sshd configuration.7
S0468 Skidmap Skidmap has the ability to replace the pam_unix.so file on an infected machine with its own malicious version that accepts a specific backdoor password for all users.6

Mitigations

ID Mitigation Description
M1032 Multi-factor Authentication Integrating multi-factor authentication (MFA) as part of organizational policy can greatly reduce the risk of an adversary gaining control of valid credentials that may be used for additional tactics such as initial access, lateral movement, and collecting information.
M1026 Privileged Account Management Limit access to the root account and prevent users from modifying PAM components through proper privilege separation (ex SELinux, grsecurity, AppArmor, etc.) and limiting Privilege Escalation opportunities.

Detection

ID Data Source Data Component
DS0022 File File Modification
DS0028 Logon Session Logon Session Creation

References

  1. Apple. (2011, May 11). PAM - Pluggable Authentication Modules. Retrieved June 25, 2020. ↩↩

  2. die.net. (n.d.). pam_unix(8) - Linux man page. Retrieved June 25, 2020. ↩

  3. Red Hat. (n.d.). CHAPTER 2. USING PLUGGABLE AUTHENTICATION MODULES (PAM). Retrieved June 25, 2020. ↩

  4. zephrax. (2018, August 3). linux-pam-backdoor. Retrieved June 25, 2020. ↩

  5. Fernández, J. M. (2018, June 27). Exfiltrating credentials via PAM backdoors & DNS requests. Retrieved June 26, 2020. ↩

  6. Remillano, A., Urbanec, J. (2019, September 19). Skidmap Linux Malware Uses Rootkit Capabilities to Hide Cryptocurrency-Mining Payload. Retrieved June 4, 2020. ↩

  7. Vachon, F. (2017, October 30). Windigo Still not Windigone: An Ebury Update . Retrieved February 10, 2021. ↩