CVE-2023-3326: FreeBSD pam_krb5 Auth Bypass Vulnerability

CVE-2023-3326 Overview

CVE-2023-3326 is an authentication bypass vulnerability in FreeBSD's pam_krb5 module that allows attackers to authenticate as any user on vulnerable systems. The pam_krb5 module authenticates users by running kinit with the provided password and obtaining a ticket-granting ticket (TGT) from the Kerberos Key Distribution Center (KDC) over the network. However, when a keytab is not provisioned on the system, pam_krb5 has no mechanism to validate the KDC's response, essentially trusting any TGT received over the network as valid.

In non-default FreeBSD installations that use pam_krb5 for authentication without a provisioned keytab, an attacker who can control both the password input and intercept/modify KDC responses can forge a valid TGT. This allows the attacker to authenticate as any user on the system, completely bypassing the authentication mechanism.

Critical Impact

Complete authentication bypass enabling attackers to impersonate any user on FreeBSD systems using pam_krb5 without a provisioned keytab, potentially leading to full system compromise.

Affected Products

• FreeBSD 12.4 (all patch levels through p2)
• FreeBSD 13.1 (all patch levels through p7)
• FreeBSD 13.2

Discovery Timeline

• June 22, 2023 - CVE-2023-3326 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2023-3326

Vulnerability Analysis

This authentication bypass vulnerability stems from a fundamental design issue in how pam_krb5 validates Kerberos authentication responses. The PAM module performs authentication by obtaining a TGT from the KDC, but without a local keytab file, it cannot cryptographically verify that the response actually came from a legitimate KDC. This creates a trust-but-verify scenario where the verification step is missing entirely.

The vulnerability requires a specific configuration: a FreeBSD system using pam_krb5 for authentication that does not have a keytab provisioned. While this is not the default FreeBSD configuration, organizations that have deployed Kerberos authentication without properly configuring keytabs are at risk.

Root Cause

The root cause is improper authentication (CWE-287) combined with incorrect implementation of authentication algorithm (CWE-303). The pam_krb5 module fails to properly validate the authenticity of KDC responses when no keytab is available. A keytab contains shared secret keys that allow the system to verify that TGT responses genuinely originated from the trusted KDC. Without this verification mechanism, the module blindly trusts network responses that claim to be from the KDC.

Attack Vector

The attack requires network-level access to intercept or manipulate traffic between the target FreeBSD system and the Kerberos KDC. An attacker positioned to perform a man-in-the-middle attack, or who has compromised the network path, can:

1. Intercept the authentication request from the target system to the KDC
2. Respond with a crafted TGT that validates the attacker-controlled password
3. The pam_krb5 module accepts this forged TGT without verification
4. The attacker successfully authenticates as any user on the system

The attack is particularly dangerous in environments where DNS spoofing or ARP poisoning is possible, as attackers can redirect KDC traffic to their own malicious server. Since no cryptographic verification occurs without a keytab, the attack leaves minimal forensic evidence in standard authentication logs.

Detection Methods for CVE-2023-3326

Indicators of Compromise

• Unexpected successful authentications from users who should not have valid credentials
• Authentication events occurring when legitimate users were not actively logging in
• Network traffic anomalies between the FreeBSD system and Kerberos KDC infrastructure
• Multiple successful authentications for privileged accounts from unusual source IPs

Detection Strategies

• Audit PAM configuration files to identify systems using pam_krb5 without provisioned keytabs
• Monitor for authentication attempts that bypass normal Kerberos validation workflows
• Implement network monitoring to detect potential man-in-the-middle attacks against Kerberos traffic
• Review authentication logs for successful logins that lack corresponding valid TGT acquisition from the legitimate KDC

Monitoring Recommendations

• Enable verbose logging for pam_krb5 authentication events to capture detailed authentication flow
• Deploy network intrusion detection rules to identify anomalous Kerberos protocol traffic
• Implement correlation rules to detect authentication success without corresponding legitimate KDC communication
• Monitor for configuration changes to PAM modules and keytab files

How to Mitigate CVE-2023-3326

Immediate Actions Required

• Verify that all FreeBSD systems using pam_krb5 have properly provisioned keytab files
• Audit PAM configuration to identify vulnerable authentication setups
• Apply the latest FreeBSD security patches addressing this vulnerability
• Consider temporarily disabling pam_krb5 authentication on systems without keytabs until patching is complete

Patch Information

FreeBSD has released security advisories addressing this vulnerability. Administrators should apply the patches referenced in FreeBSD Security Advisory SA-23:04 and FreeBSD Security Advisory SA-23:09. Additionally, NetApp has published guidance in their Security Advisory NTAP-20230714-0005 for affected NetApp products running FreeBSD.

Affected versions include FreeBSD 12.4 (through p2), FreeBSD 13.1 (through p7), and FreeBSD 13.2. Organizations should upgrade to patched versions as soon as possible.

Workarounds

• Provision a valid keytab file on all systems using pam_krb5 for authentication to enable proper KDC response validation
• Use alternative authentication mechanisms such as pam_krb5afs or other PAM modules that enforce keytab validation
• Implement network segmentation to protect Kerberos traffic from man-in-the-middle attacks
• Deploy mutual TLS or IPsec between FreeBSD systems and the Kerberos KDC to protect authentication traffic integrity

# Verify keytab presence and configuration
# Check if keytab file exists
ls -la /etc/krb5.keytab

# List keys in the keytab to verify proper provisioning
klist -k /etc/krb5.keytab

# Verify PAM configuration for pam_krb5
grep pam_krb5 /etc/pam.d/*