CVE-2020-27780 in Linux-PAM
Summary
by MITRE • 12/18/2020
A flaw was found in Linux-Pam in versions prior to 1.5.1 in the way it handle empty passwords for non-existing users. When the user doesn't exist PAM try to authenticate with root and in the case of an empty password it successfully authenticate.
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Analysis
by VulDB Data Team • 06/11/2026
This vulnerability in Linux-PAM affects versions prior to 1.5.1 and represents a critical authentication bypass flaw that stems from improper handling of empty passwords for non-existent user accounts. The vulnerability occurs when a user attempts to authenticate with an empty password against a non-existent account, causing the PAM module to fall back to root authentication and successfully grant access. This behavior violates fundamental security principles by allowing unauthorized access through a predictable authentication path that should not be available to non-existent users. The flaw creates a significant backdoor that could be exploited by attackers to gain system access without proper credentials or account creation.
The technical implementation of this vulnerability resides in the PAM authentication flow where the system fails to properly validate authentication attempts for non-existent users. When a login attempt is made with an empty password against a username that does not exist in the system, the PAM module incorrectly handles this edge case by attempting to authenticate against the root account instead of rejecting the attempt outright. This misconfiguration allows an attacker to bypass normal authentication procedures and gain access to the system with root privileges. The vulnerability is classified as a weakness in authentication mechanisms and aligns with CWE-287 which addresses improper authentication scenarios. From an operational perspective, this flaw represents a severe risk to system integrity as it provides a direct path to root access without requiring valid credentials or account existence.
The operational impact of this vulnerability extends beyond simple unauthorized access to encompass broader system compromise and potential lateral movement within network environments. Attackers can exploit this flaw to establish persistent access points, escalate privileges, and potentially gain control over entire systems or networks. The vulnerability creates an authentication bypass that can be leveraged in various attack scenarios including privilege escalation, unauthorized system access, and potential data exfiltration. Security professionals should consider this vulnerability in their threat modeling exercises as it provides a straightforward method for attackers to bypass traditional authentication controls. The flaw also impacts compliance requirements and security posture assessments, as it represents a failure in basic authentication security controls that should be in place to prevent such scenarios.
Mitigation strategies for this vulnerability require immediate patching of Linux-PAM to version 1.5.1 or later where the issue has been addressed through proper authentication handling. System administrators should implement comprehensive monitoring to detect unusual authentication patterns, particularly attempts to log in with empty passwords against non-existent accounts. Additional security controls including account lockout mechanisms, authentication policy enforcement, and regular security audits should be implemented to reduce the attack surface. Organizations should also consider implementing multi-factor authentication and privileged access management solutions to provide additional layers of protection. The remediation process should include thorough testing of authentication systems to ensure that the patch has been correctly applied and that no regression issues have been introduced. Security teams should conduct vulnerability assessments to identify systems running affected versions of Linux-PAM and prioritize remediation based on risk exposure and system criticality. This vulnerability demonstrates the importance of proper input validation and authentication flow management in security-critical software components, aligning with ATT&CK technique T1078 for valid accounts and T1566 for credential harvesting.