CVE-2020-36221 Overview
CVE-2020-36221 is an integer underflow vulnerability discovered in OpenLDAP before version 2.4.57. The flaw exists in the Certificate Exact Assertion processing within the slapd daemon, specifically in the serialNumberAndIssuerCheck function located in schema_init.c. When exploited, this vulnerability leads to slapd crashes, resulting in a denial of service condition that can disrupt LDAP directory services.
Critical Impact
Remote attackers can crash OpenLDAP slapd servers without authentication, causing denial of service to dependent authentication and directory services across enterprise environments.
Affected Products
- OpenLDAP versions prior to 2.4.57
- Debian Linux 9.0 and 10.0
- Apple macOS and Mac OS X (multiple versions including 10.14.6 with various security updates)
Discovery Timeline
- January 26, 2021 - CVE-2020-36221 published to NVD
- November 21, 2024 - Last updated in NVD database
Technical Details for CVE-2020-36221
Vulnerability Analysis
This vulnerability is classified as CWE-191 (Integer Underflow). The flaw resides in OpenLDAP's certificate validation logic, specifically within the serialNumberAndIssuerCheck function in schema_init.c. During Certificate Exact Assertion processing, an arithmetic operation can result in an integer underflow condition. When the underflow occurs, it causes the slapd daemon to crash, effectively taking down LDAP services.
The network-accessible nature of this vulnerability means that an unauthenticated attacker can send specially crafted LDAP requests to trigger the integer underflow remotely. No user interaction is required for exploitation, and the attack complexity is low. While the vulnerability does not allow for data exfiltration or modification, the availability impact is significant as it can completely disrupt directory services.
Root Cause
The root cause is an integer underflow in the serialNumberAndIssuerCheck function within schema_init.c. The function performs arithmetic operations on certificate-related data during Certificate Exact Assertion validation without proper bounds checking. When specific input values are provided, the calculation can result in a value wrapping below zero, causing undefined behavior that leads to a crash of the slapd process.
Attack Vector
The attack is network-based and can be executed remotely without requiring authentication or user interaction. An attacker sends a maliciously crafted LDAP request containing certificate assertion data designed to trigger the integer underflow condition. When the vulnerable slapd daemon processes this request, the integer underflow occurs in the serialNumberAndIssuerCheck function, causing the daemon to crash and denying service to legitimate users.
The vulnerability affects systems using OpenLDAP for authentication, authorization, and directory services. Organizations relying on OpenLDAP for centralized user management, single sign-on, or application authentication are at risk of service disruption.
Detection Methods for CVE-2020-36221
Indicators of Compromise
- Unexpected slapd daemon crashes or restarts in system logs
- Core dumps or crash reports from slapd with stack traces pointing to schema_init.c or serialNumberAndIssuerCheck
- Unusual LDAP requests containing malformed certificate assertion data
- Repeated service interruptions in LDAP-dependent authentication systems
Detection Strategies
- Monitor slapd process stability and set alerts for unexpected crashes or restarts
- Implement LDAP query logging to identify malformed or suspicious certificate assertion requests
- Deploy network intrusion detection rules to detect anomalous LDAP traffic patterns
- Review system logs for crash signatures related to the schema_init.c module
Monitoring Recommendations
- Enable comprehensive logging for the OpenLDAP slapd daemon including access and error logs
- Set up automated monitoring for slapd process health with immediate alerting on crashes
- Monitor network traffic to LDAP ports (389, 636) for unusual request patterns
- Implement log aggregation to correlate slapd crashes with incoming LDAP requests
How to Mitigate CVE-2020-36221
Immediate Actions Required
- Upgrade OpenLDAP to version 2.4.57 or later immediately
- Apply vendor-specific security patches from Debian (DSA-4845) or Apple (HT212529, HT212530, HT212531)
- Implement network segmentation to limit exposure of LDAP services
- Configure firewall rules to restrict LDAP access to trusted networks and hosts only
Patch Information
OpenLDAP has released version 2.4.57 which addresses this vulnerability. The fixes are documented in OpenLDAP Bug #9404 and OpenLDAP Bug #9424. The specific commits addressing the issue are commit 38ac838 and commit 58c1748. The patched release is available at the OpenLDAP Release Tag 2.4.57.
Debian users should apply the security update referenced in Debian Security DSA-4845 and the Debian LTS Announcement. Apple users should refer to Apple Support Document HT212529, HT212530, and HT212531 for macOS updates.
Workarounds
- Restrict network access to LDAP services using firewall rules to allow only trusted sources
- Deploy a reverse proxy or load balancer with request filtering capabilities in front of LDAP servers
- Implement rate limiting on LDAP connections to reduce the impact of potential denial of service attempts
- Consider disabling Certificate Exact Assertion processing if not required in your environment
# Example: 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 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
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
