CVE-2021-27212: OpenLDAP Denial of Service Vulnerability

CVE-2021-27212 Overview

CVE-2021-27212 is a denial of service vulnerability affecting OpenLDAP through version 2.4.57 and 2.5.x through 2.5.1alpha. The vulnerability exists in the issuerAndThisUpdateCheck function within schema_init.c, where an assertion failure can be triggered via a crafted packet containing a short timestamp. When exploited, this causes the slapd daemon to exit unexpectedly, resulting in a complete service disruption for directory services dependent on OpenLDAP.

This vulnerability is particularly concerning for organizations relying on OpenLDAP for authentication and directory services, as successful exploitation can disrupt critical infrastructure components without requiring any authentication.

Critical Impact

Remote attackers can crash the OpenLDAP slapd daemon without authentication, causing denial of service to all dependent authentication and directory services.

Affected Products

• OpenLDAP versions through 2.4.57
• OpenLDAP 2.5.0 alpha
• OpenLDAP 2.5.1 alpha
• Debian Linux 9.0
• Debian Linux 10.0

Discovery Timeline

• 2021-02-14 - CVE-2021-27212 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-27212

Vulnerability Analysis

This vulnerability is classified as CWE-617 (Reachable Assertion), where the OpenLDAP server contains an assertion that can be reached by an attacker through specially crafted network input. The flaw resides in the time validation logic within the checkTime function called by issuerAndThisUpdateCheck in schema_init.c.

The network attack vector allows unauthenticated remote exploitation with low complexity. While the vulnerability does not compromise confidentiality or integrity, it has a complete impact on availability, as the assertion failure causes the slapd daemon to terminate immediately.

Root Cause

The root cause lies in improper handling of timestamp validation in the issuerAndThisUpdateCheck function. When processing certificate-related data, specifically in CRL (Certificate Revocation List) handling, the code fails to properly validate the length of timestamp values before processing. A malformed packet containing a short timestamp triggers an assertion failure in the checkTime function, which is called during the validation process.

The OpenLDAP developers addressed this through commits that added proper boundary checking for timestamp values before the assertion logic is reached.

Attack Vector

An attacker can exploit this vulnerability by sending a specially crafted LDAP packet to a vulnerable OpenLDAP server. The attack does not require authentication and can be performed remotely over the network.

The exploitation flow involves:

1. The attacker crafts an LDAP request containing malformed timestamp data with insufficient length
2. The packet is sent to the target OpenLDAP slapd service
3. When processing the request, the issuerAndThisUpdateCheck function is invoked
4. The checkTime function receives the malformed short timestamp
5. An assertion check fails due to the unexpected input length
6. The slapd daemon exits, causing a denial of service condition

The vulnerability is related to how OpenLDAP handles time validation during schema initialization and CRL processing. For detailed technical analysis, refer to the OpenLDAP Bug Report #9454.

Detection Methods for CVE-2021-27212

Indicators of Compromise

• Unexpected slapd daemon terminations or crashes in system logs
• Core dump files generated by slapd with assertion failure indicators
• LDAP service unavailability incidents without corresponding resource exhaustion
• Log entries referencing assertion failures in schema_init.c or checkTime functions

Detection Strategies

• Monitor slapd process stability and implement automatic alerting on unexpected daemon exits
• Analyze core dumps for assertion failure patterns in issuerAndThisUpdateCheck or checkTime functions
• Review network traffic for malformed LDAP packets with unusually short timestamp fields
• Implement LDAP-aware intrusion detection rules to identify anomalous certificate-related requests

Monitoring Recommendations

• Configure centralized logging for all OpenLDAP slapd instances with real-time alerting on crashes
• Deploy network-level monitoring to detect patterns of repeated LDAP connection attempts following service crashes
• Implement health check probes for LDAP services to detect availability impacts immediately
• Enable OpenLDAP debug logging temporarily during incident investigation to capture detailed request information

How to Mitigate CVE-2021-27212

Immediate Actions Required

• Upgrade OpenLDAP to patched versions immediately (versions after 2.4.57 with security patches applied, or 2.5.x releases after 2.5.1alpha)
• Apply operating system vendor patches for affected Debian Linux systems
• Implement network-level access controls to restrict LDAP service access to trusted sources
• Configure service restart policies to minimize downtime in case of exploitation attempts

Patch Information

OpenLDAP has released security patches to address this vulnerability. The fixes are available in the following commits:

• OpenLDAP Commit 3539fc3 - Initial fix for the assertion failure
• OpenLDAP Commit 9badb73 - Additional hardening

For Debian users, security advisories DSA-4860 and the Debian LTS Announcement provide packaged updates. NetApp users should refer to NetApp Security Advisory NTAP-20210319-0005.

Workarounds

• Restrict network access to the LDAP service using firewall rules to limit exposure to trusted clients only
• Deploy a reverse proxy or load balancer with input validation capabilities in front of OpenLDAP
• Implement rate limiting on LDAP connections to slow potential exploitation attempts
• Configure automatic service restart with monitoring to minimize denial of service impact

# Configuration example - Restrict LDAP access via iptables
# Allow LDAP connections only from trusted network segments
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