CVE-2020-25710: OpenLDAP DoS Vulnerability Explained

CVE-2020-25710 Overview

A flaw was found in OpenLDAP in versions before 2.4.56 that allows an attacker to trigger a denial of service condition. This vulnerability exists in the csnNormalize23() function, where a maliciously crafted packet processed by OpenLDAP can force a failed assertion, causing the service to crash. The vulnerability is classified as CWE-617 (Reachable Assertion), meaning an attacker can trigger an assertion failure that was intended for debugging purposes but remains reachable in production code.

Critical Impact
Remote unauthenticated attackers can crash OpenLDAP directory services by sending malicious network packets, causing service disruption across dependent enterprise authentication and directory services.

Affected Products

OpenLDAP (versions before 2.4.56)
Red Hat JBoss Core Services
Red Hat JBoss Enterprise Application Platform 5.0.0
Red Hat JBoss Enterprise Web Server 2.0.0
Red Hat Enterprise Linux 5.0, 6.0, 7.0
Debian Linux 9.0
Fedora 33

Discovery Timeline

2021-05-28 - CVE-2020-25710 published to NVD
2024-11-21 - Last updated in NVD database

Technical Details for CVE-2020-25710

Vulnerability Analysis

This vulnerability resides in OpenLDAP's Change Sequence Number (CSN) normalization function, specifically within csnNormalize23(). CSN values are used in LDAP replication to track changes and ensure consistency across directory replicas. The vulnerable function fails to properly validate input data before processing, allowing an attacker to craft a malicious packet that triggers an assertion failure.

When the assertion fails, the OpenLDAP daemon (slapd) terminates unexpectedly, causing a denial of service. This is particularly impactful in enterprise environments where LDAP services are critical for authentication, authorization, and directory lookups. A successful attack could disrupt login services, email systems, and any application relying on LDAP for user management.

The vulnerability can be exploited remotely over the network without requiring any authentication or user interaction, making it highly accessible to attackers who can reach the LDAP service port (typically 389 or 636 for LDAPS).

Root Cause

The root cause is a reachable assertion (CWE-617) in the csnNormalize23() function. The function contains an assert() statement that can be triggered by specially crafted input. In production builds, assertions should either be disabled or replaced with proper error handling. In this case, the assertion remains active and reachable through normal code paths, allowing remote attackers to trigger it with malformed CSN data in LDAP requests.

Attack Vector

The attack vector is network-based, requiring no privileges or user interaction. An attacker sends a specially crafted LDAP request containing malformed CSN data to a vulnerable OpenLDAP server. When the server processes this request through the csnNormalize23() function, the assertion fails and the slapd daemon crashes. The attack can be repeated to maintain the denial of service condition, effectively keeping the LDAP service offline until mitigations are applied.

The vulnerability affects both standalone LDAP operations and replication scenarios where CSN normalization is performed. Any network path to the OpenLDAP service can potentially be used as an attack vector.

Detection Methods for CVE-2020-25710

Indicators of Compromise

Unexpected slapd service crashes or restarts in system logs
Core dump files generated by the OpenLDAP daemon with assertion failures
Repeated authentication failures across dependent services due to LDAP unavailability
Unusual network traffic patterns to LDAP ports (389/636) preceding service crashes

Detection Strategies

Monitor OpenLDAP service availability and configure alerting for unexpected restarts
Analyze system logs for assertion failure messages from slapd containing references to csnNormalize23
Deploy network intrusion detection rules to identify malformed LDAP packets
Implement application performance monitoring to track LDAP response times and service health

Monitoring Recommendations

Configure process monitoring to detect and alert on slapd crashes
Enable verbose logging for LDAP operations to capture details about requests received before crashes
Monitor network traffic to LDAP ports for anomalous connection patterns or request volumes
Set up automated service recovery with alerting to minimize downtime and track attack frequency

How to Mitigate CVE-2020-25710

Immediate Actions Required

Upgrade OpenLDAP to version 2.4.56 or later immediately
Apply vendor-specific patches from Red Hat, Debian, or Fedora as applicable to your distribution
Restrict network access to LDAP services using firewall rules to limit exposure
Implement network segmentation to isolate LDAP servers from untrusted networks

Patch Information

The vulnerability has been addressed in OpenLDAP version 2.4.56 and later. The fix is available through the OpenLDAP Git repository. Vendor-specific patches are available for major distributions:

Red Hat: See Red Hat Bug Report #1899678 for patch details
Debian: Security update available via DSA-4792 and Debian LTS announcement
NetApp: Affected product information available at NetApp Security Advisory

Workarounds

Implement strict firewall rules to limit LDAP access to trusted IP addresses only
Deploy a reverse proxy or load balancer with request validation capabilities in front of OpenLDAP
Enable connection rate limiting to reduce the impact of repeated attack attempts
Configure automatic service restart with exponential backoff to maintain availability during attacks

bash

# Firewall configuration to restrict LDAP access
# Allow LDAP traffic only from trusted networks
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