CVE-2020-25692: OpenLDAP Server DoS Vulnerability

CVE-2020-25692 Overview

A NULL pointer dereference vulnerability was discovered in OpenLDAP server that allows unauthenticated attackers to remotely crash the slapd process. The vulnerability is triggered during a request for renaming RDNs (Relative Distinguished Names), where a specially crafted request can cause the server to dereference a NULL pointer, resulting in a Denial of Service condition.

Critical Impact

Unauthenticated remote attackers can crash OpenLDAP slapd processes, causing service disruption to directory services that may be critical for authentication and authorization in enterprise environments.

Affected Products

• OpenLDAP versions prior to 2.4.55
• Red Hat Enterprise Linux 5.0, 6.0, and 7.0
• NetApp Cloud Backup
• NetApp SolidFire Baseboard Management Controller Firmware

Discovery Timeline

• 2020-12-08 - CVE-2020-25692 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2020-25692

Vulnerability Analysis

This vulnerability is classified as CWE-476 (NULL Pointer Dereference), a memory corruption flaw that occurs when the application attempts to use a pointer that is expected to be valid but is actually NULL. In the context of OpenLDAP, this issue manifests in the RDN (Relative Distinguished Name) renaming functionality within the slapd daemon.

When the slapd server processes a modrdn (modify RDN) operation, it fails to properly validate certain input conditions before dereferencing internal data structures. An unauthenticated attacker can exploit this by sending a malformed LDAP request that triggers the code path where the NULL pointer dereference occurs. Since no authentication is required, any network-accessible OpenLDAP server running a vulnerable version is at risk.

The attack is network-accessible, requires no user interaction, and can be executed with low complexity, making it particularly dangerous for internet-facing or internally critical LDAP infrastructure.

Root Cause

The root cause of CVE-2020-25692 lies in insufficient input validation within the OpenLDAP slapd server's RDN modification handling code. When processing certain malformed modrdn requests, the server fails to verify that critical pointers have been properly initialized before dereferencing them. This oversight allows an attacker to craft a request that causes the server to attempt to read from or write to memory address zero, resulting in an immediate process crash.

Attack Vector

The attack vector for this vulnerability is network-based. An unauthenticated attacker with network access to the OpenLDAP server (typically on port 389 for LDAP or port 636 for LDAPS) can send a specially crafted modrdn request. The request exploits the lack of NULL pointer validation in the RDN renaming logic.

The attack flow involves:

1. Establishing a connection to the target OpenLDAP server
2. Sending a malformed modrdn request with specific parameters that trigger the vulnerable code path
3. The slapd process crashes immediately upon processing the request
4. Directory services become unavailable until the service is restarted

This vulnerability is particularly impactful because LDAP servers often serve as centralized authentication backends for enterprise applications, meaning a successful exploit could disrupt authentication services across an organization.

Detection Methods for CVE-2020-25692

Indicators of Compromise

• Unexpected slapd process crashes or restarts in system logs
• Multiple modrdn operation failures logged in OpenLDAP access logs
• Segmentation fault (SIGSEGV) signals in slapd crash dumps
• Increased connection attempts to LDAP ports (389/636) followed by service unavailability

Detection Strategies

• Monitor slapd process stability and configure alerting for unexpected service restarts
• Implement network-level monitoring for anomalous LDAP traffic patterns, particularly modrdn operations
• Review OpenLDAP access logs for unusual modrdn requests from untrusted sources
• Deploy intrusion detection signatures that identify malformed LDAP modrdn requests

Monitoring Recommendations

• Enable detailed logging in OpenLDAP to capture modrdn operation details
• Configure process monitoring to automatically restart slapd and alert administrators on crashes
• Implement rate limiting on LDAP connections to mitigate repeated exploitation attempts
• Use SIEM solutions to correlate LDAP service disruptions with network activity

How to Mitigate CVE-2020-25692

Immediate Actions Required

• Upgrade OpenLDAP to version 2.4.55 or later immediately
• Apply vendor-specific patches for Red Hat Enterprise Linux and NetApp products
• Restrict network access to OpenLDAP servers to trusted networks and clients only
• Implement firewall rules to limit exposure of LDAP ports (389/636) to the internet

Patch Information

The vulnerability was fixed in OpenLDAP version 2.4.55. Organizations should upgrade to this version or later to remediate the vulnerability. For Red Hat Enterprise Linux systems, refer to the Red Hat Bug Report #1894567 for specific patch information and availability. NetApp customers should consult the NetApp Security Advisory ntap-20210108-0006 for guidance on affected products and firmware updates.

Workarounds

• Implement network segmentation to isolate LDAP servers from untrusted networks
• Use firewall rules to restrict access to LDAP services to known, trusted IP addresses
• Deploy a reverse proxy or load balancer with request filtering capabilities in front of LDAP servers
• Enable TLS and require client certificate authentication to limit unauthenticated access

# Restrict LDAP access using iptables
iptables -A INPUT -p tcp --dport 389 -s 10.0.0.0/8 -j ACCEPT
iptables -A INPUT -p tcp --dport 389 -j DROP
iptables -A INPUT -p tcp --dport 636 -s 10.0.0.0/8 -j ACCEPT
iptables -A INPUT -p tcp --dport 636 -j DROP