CVE-2020-27780: Linux-PAM Auth Bypass Vulnerability

CVE-2020-27780 Overview

A critical authentication bypass vulnerability was discovered in Linux-PAM (Pluggable Authentication Modules) in versions prior to 1.5.1. The flaw exists in how Linux-PAM handles empty passwords for non-existing users. When authentication is attempted for a user that doesn't exist, PAM incorrectly falls back to authenticating with root credentials, and in cases where an empty password is provided, it successfully authenticates the user. This improper authentication logic (CWE-287) could allow unauthorized access to systems protected by PAM-based authentication.

Critical Impact

Attackers can bypass authentication by supplying an empty password for non-existing usernames, potentially gaining root-level access to vulnerable systems.

Affected Products

• Linux-PAM versions prior to 1.5.1
• Systems using PAM-based authentication with vulnerable configurations
• Linux distributions shipping affected Linux-PAM versions

Discovery Timeline

• 2020-12-18 - CVE-2020-27780 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2020-27780

Vulnerability Analysis

This vulnerability represents a severe authentication bypass flaw in the Linux-PAM authentication framework. The core issue lies in PAM's handling of authentication requests for users that do not exist in the system. Under normal circumstances, authentication should fail immediately when a non-existent username is provided. However, due to this flaw, PAM incorrectly redirects the authentication attempt to the root account.

The vulnerability is particularly dangerous because when an empty password is submitted during this flawed authentication flow, PAM accepts it as valid authentication, effectively granting access without proper credential verification. This behavior completely undermines the fundamental security guarantee that authentication modules are supposed to provide.

Root Cause

The root cause stems from improper error handling in the PAM authentication logic. When PAM encounters a non-existent user, instead of returning an authentication failure, it attempts to authenticate against the root account. Combined with insufficient validation of empty password strings, this creates a path where attackers can authenticate without valid credentials. The flaw represents a fundamental failure in the authentication decision logic that should reject any authentication attempt where the supplied credentials cannot be properly verified against the intended user account.

Attack Vector

The attack vector is network-based and requires no privileges or user interaction to exploit. An attacker can target any service that uses Linux-PAM for authentication, such as SSH, sudo, or web applications. The attack is straightforward: the attacker supplies a non-existent username combined with an empty password. Due to the flawed authentication logic, PAM redirects the authentication to root and accepts the empty password, granting the attacker unauthorized access.

The exploitation process involves:

1. Identifying a target system using vulnerable Linux-PAM versions for authentication
2. Attempting to authenticate with a username that does not exist on the system
3. Providing an empty password when prompted
4. Gaining authentication success due to the flawed fallback to root authentication

For more technical details on the authentication flow, see the Red Hat Bug Report #1901094.

Detection Methods for CVE-2020-27780

Indicators of Compromise

• Successful authentication events logged for usernames that don't exist in /etc/passwd or directory services
• Unusual root-level activity following authentication attempts with non-standard usernames
• Authentication logs showing empty password submissions with successful outcomes
• Spike in failed authentication attempts followed by successful authentications for non-existent users

Detection Strategies

• Monitor authentication logs (/var/log/auth.log, /var/log/secure) for successful logins with usernames not present in the user database
• Implement alerting on authentication success events that don't correlate with known user accounts
• Deploy intrusion detection rules to flag empty password authentication attempts
• Audit PAM configuration files for vulnerable module versions and configurations

Monitoring Recommendations

• Enable verbose PAM logging to capture detailed authentication flow information
• Configure SIEM rules to correlate authentication events with user account existence checks
• Implement real-time monitoring for root account access following unusual authentication patterns
• Review and baseline normal authentication patterns to detect anomalous access attempts

How to Mitigate CVE-2020-27780

Immediate Actions Required

• Upgrade Linux-PAM to version 1.5.1 or later immediately
• Audit authentication logs for signs of exploitation
• Review PAM configurations and ensure proper authentication module ordering
• Consider implementing additional authentication layers such as multi-factor authentication

Patch Information

The vulnerability is addressed in Linux-PAM version 1.5.1 and later. Organizations should update their Linux-PAM packages through their distribution's package management system. Check your distribution's security advisories for specific package update instructions.

For Red Hat-based systems, consult the Red Hat Bug Report #1901094 for vendor-specific guidance.

Workarounds

• Configure PAM to explicitly deny authentication for non-existent users before other modules process the request
• Implement network-level access controls to limit exposure of PAM-authenticated services
• Enable account lockout policies to limit authentication attempts
• Deploy host-based intrusion prevention to block suspicious authentication patterns

# Verify Linux-PAM version installed
rpm -qa | grep pam
# or for Debian-based systems
dpkg -l | grep libpam

# Check PAM configuration for authentication modules
cat /etc/pam.d/system-auth
cat /etc/pam.d/common-auth

# Monitor authentication attempts in real-time
tail -f /var/log/auth.log