CVE-2023-36054: MIT Kerberos 5 DoS Vulnerability

CVE-2023-36054 Overview

CVE-2023-36054 is an Uninitialized Memory Use vulnerability in MIT Kerberos 5 (krb5) that affects the lib/kadm5/kadm_rpc_xdr.c file. The flaw exists in the _xdr_kadm5_principal_ent_rec function, which fails to validate the relationship between n_key_data and the key_data array count, resulting in the freeing of an uninitialized pointer. A remote authenticated user can exploit this vulnerability to trigger a crash in the kadmind daemon, causing a Denial of Service condition.

Critical Impact

Remote authenticated attackers can crash the Kerberos administration daemon (kadmind), disrupting authentication services across the enterprise environment.

Affected Products

• MIT Kerberos 5 versions before 1.20.2
• MIT Kerberos 5 versions 1.21.x before 1.21.1
• Debian Linux 10.0
• NetApp Active IQ Unified Manager for VMware vSphere
• NetApp Clustered Data ONTAP 9.0
• NetApp HCI
• NetApp Management Services for Element Software
• NetApp ONTAP Tools for VMware vSphere

Discovery Timeline

• 2023-08-07 - CVE-2023-36054 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2023-36054

Vulnerability Analysis

This vulnerability stems from improper memory handling in the XDR (External Data Representation) serialization code for Kerberos administrative operations. When processing kadm5 principal entry records, the _xdr_kadm5_principal_ent_rec function does not properly validate that the n_key_data field matches the actual number of elements in the key_data array. This inconsistency leads to operations on uninitialized memory, specifically attempting to free a pointer that was never properly allocated or initialized.

The vulnerability is exploitable over the network by any authenticated user with access to the Kerberos administration protocol. While the attack requires authentication, it does not require elevated privileges, making it accessible to any valid Kerberos principal. The impact is limited to availability, as the vulnerability results in a daemon crash rather than information disclosure or code execution.

Root Cause

The root cause lies in CWE-824 (Access of Uninitialized Pointer). The _xdr_kadm5_principal_ent_rec function processes key data arrays without ensuring consistency between the declared count (n_key_data) and the actual array contents. When these values are mismatched—particularly when n_key_data is larger than the actual allocated array elements—the code attempts to free memory locations that contain uninitialized pointer values, leading to undefined behavior and a crash.

Attack Vector

The attack is conducted over the network against the kadmind service. An authenticated attacker sends a specially crafted RPC request containing a malformed principal entry record where the n_key_data count does not match the actual key_data array. When kadmind processes this request through the XDR deserialization routines, it triggers the uninitialized pointer free condition. This attack requires low complexity—only valid Kerberos credentials and network access to the kadmind service port (typically 749).

The security patch introduces validation to ensure array count consistency in kadm5 RPC operations:

 		     int v)
 {
 	unsigned int n;
+	bool_t r;
 
 	if (!xdr_krb5_principal(xdrs, &objp->principal)) {
 		return (FALSE);

Source: GitHub krb5 Commit

Detection Methods for CVE-2023-36054

Indicators of Compromise

• Unexpected kadmind service crashes or restarts in system logs
• Core dumps from the kadmind process indicating segmentation faults in XDR-related functions
• Unusual administrative RPC traffic to kadmind (port 749) from authenticated but potentially compromised accounts
• Log entries indicating failed or malformed kadm5 operations prior to service termination

Detection Strategies

• Monitor kadmind process stability and implement alerting on unexpected daemon restarts
• Analyze Kerberos administrative logs for patterns of operations that precede crashes
• Deploy network intrusion detection rules to identify malformed kadm5 RPC requests
• Implement process monitoring to detect repeated kadmind crashes that may indicate active exploitation

Monitoring Recommendations

• Enable verbose logging for the kadmind service to capture detailed operation information
• Configure system monitoring to track kadmind process state and generate alerts on abnormal terminations
• Review authentication logs to identify accounts making unusual administrative requests
• Implement centralized log collection for Kerberos infrastructure components

How to Mitigate CVE-2023-36054

Immediate Actions Required

• Upgrade MIT Kerberos 5 to version 1.20.2 or later for the 1.20.x branch
• Upgrade MIT Kerberos 5 to version 1.21.1 or later for the 1.21.x branch
• Apply vendor-specific patches for affected NetApp products as outlined in the NetApp Security Advisory
• Review the Debian LTS Announcement for Debian-specific updates

Patch Information

MIT has released patched versions addressing this vulnerability. The fix is available in krb5 versions 1.20.2 and 1.21.1. Detailed changes can be reviewed in the GitHub Version Comparison for 1.20.2 and the GitHub Version Comparison for 1.21.1. The specific fix commit is available at the GitHub krb5 Commit. Additional advisories are available at the MIT Kerberos Advisory page.

Workarounds

• Restrict network access to the kadmind service to trusted administrative networks only
• Implement firewall rules to limit connections to port 749 from authorized management hosts
• Consider implementing additional authentication controls for kadm5 administrative operations
• Monitor for service disruptions and implement automatic restart mechanisms with alerting

# Example: Restrict kadmind access via iptables
iptables -A INPUT -p tcp --dport 749 -s <trusted_admin_network> -j ACCEPT
iptables -A INPUT -p tcp --dport 749 -j DROP