CVE-2023-39975: MIT Kerberos 5 Use-After-Free Vulnerability

CVE-2023-39975 Overview

CVE-2023-39975 is a double free vulnerability in the kdc/do_tgs_req.c file of MIT Kerberos 5 (krb5) version 1.21 before 1.21.2. The flaw occurs when an authenticated user triggers an authorization-data handling failure, causing incorrect data to be copied from one ticket to another. This memory corruption issue can be exploited over the network by authenticated attackers to potentially achieve arbitrary code execution or cause denial of service conditions on affected Key Distribution Center (KDC) servers.

Critical Impact

Authenticated attackers can exploit this double free vulnerability to compromise KDC servers, potentially leading to complete authentication infrastructure compromise, arbitrary code execution, or service disruption across the Kerberos realm.

Affected Products

• MIT Kerberos 5 version 1.21 before 1.21.2

Discovery Timeline

• 2023-08-16 - CVE CVE-2023-39975 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2023-39975

Vulnerability Analysis

This double free vulnerability (CWE-415) exists in the TGS (Ticket Granting Service) request handling code within the MIT Kerberos 5 KDC. The vulnerability is triggered during ticket validation or renewal operations when authorization-data handling fails. The root issue lies in how the code copies ticket data structures during these operations.

When processing TGS requests with the KDC_OPT_VALIDATE or KDC_OPT_RENEW options, the vulnerable code performs a shallow copy of the entire header ticket structure to the reply ticket. This creates a situation where both the original and copied tickets reference the same memory for certain fields. When authorization-data handling subsequently fails and cleanup occurs, the same memory can be freed twice, leading to heap corruption.

The vulnerability requires the attacker to be authenticated to the Kerberos realm, but once authenticated, exploitation can occur over the network without user interaction. Successful exploitation could allow an attacker to execute arbitrary code in the context of the KDC process or cause the KDC to crash, disrupting authentication services.

Root Cause

The root cause is improper memory management during ticket copying operations in the TGS request handler. The original code copied the entire ticket_reply structure from t->header_tkt, which included pointers to dynamically allocated memory. This shallow copy meant that when the enc_tkt_reply.authorization_data was set to NULL and later the original ticket's authorization data was freed, the copied ticket would still reference (and potentially attempt to free) the same memory location. The fix changes the code to perform a more selective copy, only copying the specific fields needed (server and enc_part2 fields) rather than the entire structure.

Attack Vector

The attack vector is network-based and requires the attacker to have valid authentication credentials within the Kerberos realm. The attacker must:

1. Authenticate to the Kerberos realm to obtain a valid TGT
2. Submit a specially crafted TGS request with validation or renewal options
3. Trigger an authorization-data handling failure condition
4. The double free occurs during the error handling path

     }
 
     if (t->req->kdc_options & (KDC_OPT_VALIDATE | KDC_OPT_RENEW)) {
-        /* Copy the whole header ticket except for authorization data. */
-        ticket_reply = *t->header_tkt;
+        /* Copy the header ticket server and all enc-part fields except for
+         * authorization data. */
+        ticket_reply.server = t->header_tkt->server;
         enc_tkt_reply = *t->header_tkt->enc_part2;
         enc_tkt_reply.authorization_data = NULL;
     } else {

Source: GitHub Commit Update

The fix changes the shallow copy of the entire ticket_reply structure to selective field copying, preventing the aliased memory references that lead to the double free condition.

Detection Methods for CVE-2023-39975

Indicators of Compromise

• Unexpected KDC process crashes or restarts, particularly during ticket validation or renewal operations
• Abnormal memory access patterns or heap corruption errors in KDC logs
• Increased TGS-REQ traffic with validation or renewal flags from suspicious sources
• Core dumps from krb5kdc process showing double free conditions

Detection Strategies

• Monitor KDC process stability and implement alerting on unexpected crashes or restarts
• Enable verbose logging on KDC servers to capture TGS request details and authorization-data handling failures
• Deploy memory sanitization tools in test environments to detect double free conditions
• Audit Kerberos ticket request patterns for anomalous validation or renewal activity from authenticated users

Monitoring Recommendations

• Implement centralized log collection for all KDC servers to correlate crash events
• Configure process monitoring to alert on krb5kdc process termination or excessive memory usage
• Review authentication logs for patterns of failed authorization-data handling
• Deploy SentinelOne agents on KDC hosts to detect exploitation attempts and memory corruption attacks

How to Mitigate CVE-2023-39975

Immediate Actions Required

• Upgrade MIT Kerberos 5 to version 1.21.2 or later immediately on all KDC servers
• Review KDC server logs for evidence of exploitation attempts or unexpected crashes
• Implement network segmentation to limit access to KDC servers from untrusted networks
• Ensure monitoring and alerting is in place for KDC process health

Patch Information

The vulnerability is fixed in MIT Kerberos 5 version 1.21.2. The security patch is available in commit 88a1701b423c13991a8064feeb26952d3641d840. Organizations should upgrade from any version 1.21 through 1.21.1 to version 1.21.2 or later. The fix modifies the ticket copying logic to avoid shallow copying of the entire ticket structure, instead selectively copying only the required fields.

For detailed patch information, refer to:

• GitHub Commit Update
• GitHub Version Comparison
• MIT Kerberos Advisory

Workarounds

• If immediate patching is not possible, restrict network access to KDC servers to only trusted hosts
• Implement additional monitoring on KDC servers to detect and respond to crashes quickly
• Consider deploying KDC redundancy to maintain authentication services if a server is compromised
• Audit and minimize the number of authenticated users with access to request ticket validation or renewal

# Verify krb5 version installed
krb5-config --version

# Check if vulnerable version is running
rpm -qa | grep krb5
dpkg -l | grep krb5

# Update on RHEL/CentOS
yum update krb5-server krb5-libs

# Update on Debian/Ubuntu
apt-get update && apt-get upgrade krb5-kdc krb5-admin-server