CVE-2023-4967 in NetScaler ADC
Summary
by MITRE • 10/27/2023
Denial of Service in NetScaler ADC and NetScaler Gateway when configured as a Gateway (VPN virtual server, ICA Proxy, CVPN, RDP Proxy) or AAA Virtual Server
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Analysis
by VulDB Data Team • 11/19/2023
The vulnerability identified as CVE-2023-4967 represents a critical denial of service condition affecting Citrix NetScaler ADC and NetScaler Gateway appliances when configured in specific gateway roles. This weakness manifests within the appliance's handling of certain network traffic patterns that occur during authentication and session management processes. The affected configurations include VPN virtual servers, ICA Proxy implementations, CVPN services, RDP Proxy setups, and AAA virtual servers where the appliance functions as a gateway. The vulnerability stems from inadequate input validation and improper state management within the processing pipeline that handles authentication requests and session establishment for remote access services.
The technical flaw resides in the manner in which the NetScaler appliance processes incoming authentication requests when operating in gateway mode. Specifically, the vulnerability occurs during the handling of malformed or specially crafted authentication packets that trigger an unexpected state transition within the appliance's session management subsystem. This improper handling leads to a complete service disruption where the appliance becomes unresponsive to legitimate authentication requests and fails to establish new connections for authorized users. The flaw operates at the application layer where the appliance processes authentication protocols and maintains session state information, creating a condition where normal operational traffic becomes corrupted and the system enters a non-recoverable state. This vulnerability is particularly concerning as it affects core authentication services that are fundamental to secure remote access operations.
The operational impact of CVE-2023-4967 extends beyond simple service interruption to encompass complete loss of remote access capabilities for organizations relying on Citrix NetScaler for secure connectivity. When exploited, the vulnerability can render entire remote access infrastructures unavailable, affecting thousands of users who depend on VPN services, RDP access, or application proxying capabilities. Organizations utilizing the affected appliances as central authentication points face significant business disruption since the vulnerability impacts not only the gateway functionality but also the underlying AAA services that authenticate users for various enterprise applications. The attack surface is particularly broad given that the vulnerability affects multiple service types within the NetScaler platform, making it a comprehensive threat to remote access security infrastructure. Security teams must consider the potential for cascading failures when multiple gateway services are simultaneously compromised, potentially affecting critical business operations and incident response capabilities.
Mitigation strategies for CVE-2023-4967 should prioritize immediate implementation of official patches provided by Citrix, as these updates address the root cause through improved input validation and enhanced state management protocols. Organizations should also implement network-level monitoring to detect unusual traffic patterns that may indicate exploitation attempts, particularly focusing on authentication request volumes and session establishment failures. The implementation of redundant authentication services and failover mechanisms can provide operational resilience while primary systems are patched or mitigated. Security controls should include rate limiting and connection tracking to prevent exploitation through resource exhaustion techniques, though these measures are considered temporary workarounds rather than permanent solutions. Network segmentation strategies can help limit the scope of potential exploitation by isolating vulnerable gateway services from critical internal systems, while also providing additional monitoring points for detecting anomalous behavior. Organizations should also review their incident response procedures to ensure preparedness for potential service disruptions and implement comprehensive backup authentication methods to maintain business continuity during remediation activities. The vulnerability aligns with CWE-400 which addresses improper handling of resources and CWE-20 which covers input validation issues, while the operational impact reflects ATT&CK technique T1499.004 related to network denial of service attacks and T1566.001 concerning credential access through network-based attacks.