CVE-2021-27212
📋 TL;DR
This vulnerability in OpenLDAP allows remote attackers to cause a denial of service by sending a specially crafted packet with a short timestamp, triggering an assertion failure that crashes the slapd daemon. It affects OpenLDAP servers running vulnerable versions, potentially disrupting directory services. The attack requires network access to the LDAP service port.
💻 Affected Systems
- OpenLDAP
📦 What is this software?
Openldap by Openldap
Openldap by Openldap
Openldap by Openldap
⚠️ Risk & Real-World Impact
Worst Case
Complete service outage of LDAP directory services, disrupting authentication, authorization, and directory lookups for all dependent applications and users.
Likely Case
Intermittent slapd daemon crashes requiring manual restart, causing temporary authentication failures and service disruptions.
If Mitigated
Minimal impact with proper network segmentation and monitoring allowing quick detection and restart of affected services.
🎯 Exploit Status
The vulnerability is triggered by sending malformed LDAP packets with short timestamps. Proof of concept details are available in the bug report and commit references.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: OpenLDAP 2.4.58 and 2.5.2 or later
Vendor Advisory: https://bugs.openldap.org/show_bug.cgi?id=9454
Restart Required: Yes
Instructions:
1. Download and install OpenLDAP 2.4.58+ or 2.5.2+ from openldap.org. 2. Stop slapd service. 3. Install updated packages. 4. Restart slapd service. 5. Verify service is running and responding.
🔧 Temporary Workarounds
Network Access Control
linuxRestrict network access to LDAP ports (389/636) to only trusted sources using firewall rules.
iptables -A INPUT -p tcp --dport 389 -s trusted_network -j ACCEPT
iptables -A INPUT -p tcp --dport 389 -j DROP
Service Monitoring and Auto-restart
linuxConfigure monitoring to detect slapd crashes and automatically restart the service.
systemctl enable slapd
systemctl edit slapd (add Restart=always)
🧯 If You Can't Patch
- Implement strict network segmentation to limit LDAP access to only required systems
- Deploy intrusion detection systems to monitor for malformed LDAP packets and alert on crash events
🔍 How to Verify
Check if Vulnerable:
Check OpenLDAP version: slapd -VV 2>&1 | grep -i 'openldap' and compare against affected versions (2.4.0-2.4.57, 2.5.0-2.5.1alpha)Check Version:
slapd -VV 2>&1 | grep -i 'openldap' || ldapsearch -x -H ldap://localhost -b '' -s base 'objectclass=*' vendorVersionVerify Fix Applied:
Verify version is 2.4.58+ or 2.5.2+: slapd -VV 2>&1 | grep -i 'openldap' and check service is running: systemctl status slapd📡 Detection & Monitoring
Log Indicators:
- slapd daemon crash/restart messages in syslog
- Assertion failed messages related to issuerAndThisUpdateCheck or schema_init.c
- Unexpected slapd process termination
Network Indicators:
- Malformed LDAP packets with short timestamp fields
- Multiple connection attempts to LDAP port followed by service unavailability
SIEM Query:
source="syslog" AND (process="slapd" AND (event="crash" OR event="terminated" OR message="assertion fail*"))🔗 References
- https://bugs.openldap.org/show_bug.cgi?id=9454
- https://git.openldap.org/openldap/openldap/-/commit/3539fc33212b528c56b716584f2c2994af7c30b0
- https://git.openldap.org/openldap/openldap/-/commit/9badb73425a67768c09bcaed1a9c26c684af6c30
- https://lists.apache.org/thread.html/r58af02e294bd07f487e2c64ffc0a29b837db5600e33b6e698b9d696b%40%3Cissues.bookkeeper.apache.org%3E
- https://lists.apache.org/thread.html/rf4c02775860db415b4955778a131c2795223f61cb8c6a450893651e4%40%3Cissues.bookkeeper.apache.org%3E
- https://lists.debian.org/debian-lts-announce/2021/02/msg00035.html
- https://security.netapp.com/advisory/ntap-20210319-0005/
- https://www.debian.org/security/2021/dsa-4860
- https://bugs.openldap.org/show_bug.cgi?id=9454
- https://git.openldap.org/openldap/openldap/-/commit/3539fc33212b528c56b716584f2c2994af7c30b0
- https://git.openldap.org/openldap/openldap/-/commit/9badb73425a67768c09bcaed1a9c26c684af6c30
- https://lists.apache.org/thread.html/r58af02e294bd07f487e2c64ffc0a29b837db5600e33b6e698b9d696b%40%3Cissues.bookkeeper.apache.org%3E
- https://lists.apache.org/thread.html/rf4c02775860db415b4955778a131c2795223f61cb8c6a450893651e4%40%3Cissues.bookkeeper.apache.org%3E
- https://lists.debian.org/debian-lts-announce/2021/02/msg00035.html
- https://security.netapp.com/advisory/ntap-20210319-0005/
- https://www.debian.org/security/2021/dsa-4860
