CVE-2023-21557 Overview
CVE-2023-21557 is a critical Denial of Service vulnerability affecting the Windows Lightweight Directory Access Protocol (LDAP) implementation across a wide range of Microsoft Windows operating systems. This vulnerability allows an unauthenticated remote attacker to cause a denial of service condition on affected systems by sending specially crafted LDAP requests over the network. The vulnerability is characterized by integer overflow (CWE-190) and resource exhaustion (CWE-400) weaknesses, which can be exploited to disrupt LDAP services and potentially expose confidential information.
Critical Impact
This vulnerability enables remote attackers to crash LDAP services on Windows systems without requiring authentication, potentially disrupting Active Directory operations and enterprise authentication services across entire network infrastructures.
Affected Products
- Microsoft Windows 10 (versions 1607, 1809, 20H2, 21H2, 22H2)
- Microsoft Windows 11 (versions 21H2, 22H2)
- Microsoft Windows 7 SP1
- Microsoft Windows 8.1
- Microsoft Windows RT 8.1
- Microsoft Windows Server 2008 SP2, 2008 R2 SP1
- Microsoft Windows Server 2012, 2012 R2
- Microsoft Windows Server 2016, 2019, 2022
Discovery Timeline
- January 10, 2023 - CVE-2023-21557 published to NVD
- November 21, 2024 - Last updated in NVD database
Technical Details for CVE-2023-21557
Vulnerability Analysis
This vulnerability resides in the Windows LDAP service implementation, a critical component used for directory services, authentication, and authorization in enterprise Windows environments. The flaw stems from improper handling of specially crafted LDAP requests, which can trigger either an integer overflow condition or resource exhaustion scenario.
When exploited, an attacker can cause the LDAP service to consume excessive resources or crash entirely, leading to service unavailability. Given that LDAP is fundamental to Active Directory operations, successful exploitation could cascade into broader authentication failures across an enterprise network.
The vulnerability can be exploited remotely over the network without requiring any user interaction or prior authentication, making it particularly dangerous in environments where LDAP services are exposed to untrusted networks.
Root Cause
The vulnerability is caused by two related weaknesses:
Integer Overflow (CWE-190): The LDAP service fails to properly validate the size of certain input parameters, allowing an attacker to trigger an integer overflow condition. When arithmetic operations on integer values wrap around due to overflow, it can lead to unexpected behavior including buffer allocations of incorrect sizes.
Resource Exhaustion (CWE-400): The LDAP service does not adequately limit resource consumption when processing malformed requests, allowing attackers to exhaust system resources and cause denial of service conditions.
Attack Vector
The attack can be launched remotely over the network against systems running vulnerable LDAP services. An unauthenticated attacker can send specially crafted LDAP requests to the target system, exploiting the integer overflow or resource exhaustion conditions. The attack does not require user interaction or any special privileges on the target system.
The exploitation mechanism involves crafting malicious LDAP packets that trigger improper arithmetic operations or excessive resource allocation within the LDAP service. This can result in service crashes, memory corruption, or system resource depletion, all leading to denial of service. Additionally, the vulnerability may allow partial disclosure of confidential information stored in memory during the exploitation process.
Detection Methods for CVE-2023-21557
Indicators of Compromise
- Unusual spikes in LDAP traffic volume, particularly from external or unexpected source IP addresses
- LDAP service crashes or unexpected restarts on domain controllers and LDAP servers
- Windows Event Log entries indicating LDAP service failures or memory-related errors (Event IDs related to lsass.exe or directory services)
- Anomalous network packets targeting TCP port 389 (LDAP) or TCP port 636 (LDAPS)
Detection Strategies
- Monitor for abnormal LDAP query patterns using network intrusion detection systems (IDS/IPS)
- Configure Windows Event forwarding to centralize security logs from domain controllers for LDAP service anomaly detection
- Deploy endpoint detection and response (EDR) solutions to monitor for suspicious activity targeting lsass.exe and related directory service processes
- Implement network traffic analysis to detect malformed or oversized LDAP packets
Monitoring Recommendations
- Enable verbose logging for LDAP services on domain controllers and configure alerts for service crashes
- Establish baseline metrics for LDAP traffic and alert on significant deviations
- Monitor system resource utilization (CPU, memory) on servers running LDAP services for signs of resource exhaustion attacks
- Implement network segmentation monitoring to detect unauthorized access attempts to LDAP ports from untrusted network segments
How to Mitigate CVE-2023-21557
Immediate Actions Required
- Apply the January 2023 security updates from Microsoft to all affected Windows systems immediately
- Restrict network access to LDAP services (ports 389 and 636) to only authorized systems and networks using firewall rules
- Enable LDAP signing and LDAP channel binding to add additional security layers to LDAP communications
- Monitor domain controllers and LDAP servers closely for signs of exploitation attempts
Patch Information
Microsoft has released security updates addressing this vulnerability as part of the January 2023 Patch Tuesday release. Organizations should apply the appropriate cumulative updates for their Windows versions. Detailed patch information and download links are available in the Microsoft Security Response Center advisory for CVE-2023-21557.
The patches address the integer overflow and resource exhaustion issues by implementing proper input validation and resource management within the LDAP service components.
Workarounds
- Implement network-level filtering to block malicious LDAP traffic patterns at perimeter firewalls and intrusion prevention systems
- Restrict LDAP service access to internal trusted networks only and require VPN for remote administrative access
- Consider deploying rate limiting on LDAP connections to mitigate resource exhaustion attack attempts
- Ensure LDAP services are not directly exposed to the internet; use secure proxies or VPN gateways for remote access requirements
# Windows Firewall rule to restrict LDAP access to specific subnets
netsh advfirewall firewall add rule name="Restrict LDAP Access" dir=in action=allow protocol=TCP localport=389 remoteip=10.0.0.0/8,172.16.0.0/12,192.168.0.0/16
netsh advfirewall firewall add rule name="Block External LDAP" dir=in action=block protocol=TCP localport=389
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