CVE-2023-2953: OpenLDAP Null Pointer Dereference Flaw

CVE-2023-2953 Overview

A null pointer dereference vulnerability was discovered in OpenLDAP, specifically within the ber_memalloc_x() function. This security flaw allows remote attackers to cause a denial of service condition by triggering the null pointer dereference, potentially crashing the LDAP service and disrupting directory services operations across affected systems.

Critical Impact

Remote attackers can exploit this vulnerability to cause service disruption through denial of service attacks against OpenLDAP servers, affecting authentication and directory services that depend on LDAP infrastructure.

Affected Products

• OpenLDAP 2.4
• Red Hat Enterprise Linux 8.0 and 9.0
• Apple macOS (multiple versions)
• NetApp Active IQ Unified Manager for VMware vSphere
• NetApp Clustered Data ONTAP
• NetApp ONTAP Tools for VMware vSphere
• NetApp H300S, H500S, H700S, H410S, and H410C firmware

Discovery Timeline

• May 30, 2023 - CVE-2023-2953 published to NVD
• January 10, 2025 - Last updated in NVD database

Technical Details for CVE-2023-2953

Vulnerability Analysis

This vulnerability is classified as CWE-476 (NULL Pointer Dereference). The flaw exists in the ber_memalloc_x() function within OpenLDAP's Basic Encoding Rules (BER) library. When specific conditions are met during memory allocation operations, the function fails to properly validate pointers before dereferencing them, leading to a null pointer dereference condition.

The BER library is a fundamental component of OpenLDAP that handles encoding and decoding of LDAP protocol messages. The ber_memalloc_x() function is responsible for memory allocation during BER operations. When the function encounters certain malformed or specially crafted LDAP requests, it can attempt to dereference a null pointer, causing the OpenLDAP daemon to crash.

The vulnerability is network-exploitable without authentication or user interaction, making it particularly dangerous for internet-facing LDAP servers. Successful exploitation results in denial of service, as the affected OpenLDAP service becomes unavailable until restarted.

Root Cause

The root cause of this vulnerability lies in insufficient null pointer validation within the ber_memalloc_x() function. During memory allocation operations in the BER library, the function does not adequately check whether a pointer is null before attempting to dereference it. This oversight allows an attacker to craft LDAP requests that trigger code paths where the null pointer condition occurs, ultimately causing the service to crash.

Attack Vector

The attack vector for CVE-2023-2953 is network-based. An attacker can exploit this vulnerability remotely by sending specially crafted LDAP requests to a vulnerable OpenLDAP server. The attack does not require any privileges or authentication, and no user interaction is needed.

The exploitation flow involves:

1. Attacker identifies a vulnerable OpenLDAP server accessible over the network
2. Attacker crafts malicious LDAP requests designed to trigger the null pointer dereference in ber_memalloc_x()
3. The vulnerable server processes the request and encounters the null pointer condition
4. The OpenLDAP daemon crashes, causing denial of service
5. Services dependent on LDAP authentication or directory lookups become unavailable

For detailed technical information about this vulnerability, refer to the OpenLDAP Bug Report #9904.

Detection Methods for CVE-2023-2953

Indicators of Compromise

• Unexpected OpenLDAP service crashes or restarts without apparent cause
• Segmentation fault errors in OpenLDAP logs related to memory operations
• Repeated crash/restart cycles of the slapd daemon
• Unusual LDAP request patterns from external IP addresses preceding service failures

Detection Strategies

• Monitor system logs for segmentation faults or null pointer dereference errors in OpenLDAP processes
• Implement network-level monitoring for anomalous LDAP traffic patterns that may indicate exploitation attempts
• Deploy intrusion detection signatures to identify malformed LDAP requests targeting the BER library
• Use application performance monitoring to detect abnormal OpenLDAP service behavior or unexpected restarts

Monitoring Recommendations

• Enable comprehensive logging for OpenLDAP services to capture detailed error information
• Set up automated alerting for OpenLDAP service crashes or unexpected terminations
• Monitor LDAP port traffic (389/TCP and 636/TCP for LDAPS) for unusual request volumes or patterns
• Implement centralized log aggregation to correlate LDAP service events across multiple servers

How to Mitigate CVE-2023-2953

Immediate Actions Required

• Identify all systems running affected versions of OpenLDAP, including standalone installations and embedded components
• Apply vendor-provided security patches for OpenLDAP and all affected downstream products
• Restrict network access to LDAP services to only trusted networks and clients where possible
• Implement rate limiting on LDAP connections to mitigate potential exploitation attempts

Patch Information

Security updates addressing CVE-2023-2953 are available from multiple vendors. Consult the following resources for platform-specific patches:

• OpenLDAP: Refer to OpenLDAP Bug Report #9904 for fix information
• Red Hat: See the Red Hat CVE-2023-2953 Advisory for RHEL patches
• Apple: Apply updates from HT213843, HT213844, and HT213845
• NetApp: Review the NetApp Security Advisory NTAP-20230703-0005 for affected product updates

Workarounds

• Implement firewall rules to restrict LDAP access to trusted IP addresses and networks only
• Deploy a reverse proxy or load balancer with request filtering capabilities in front of LDAP servers
• Enable connection limits and timeouts to reduce the impact of potential denial of service attempts
• Consider temporarily disabling external LDAP access if patches cannot be immediately applied

# Example: Restrict LDAP access using iptables
# Allow LDAP traffic only from trusted network
iptables -A INPUT -p tcp --dport 389 -s 10.0.0.0/8 -j ACCEPT
iptables -A INPUT -p tcp --dport 636 -s 10.0.0.0/8 -j ACCEPT
iptables -A INPUT -p tcp --dport 389 -j DROP
iptables -A INPUT -p tcp --dport 636 -j DROP