CVE-2023-4967 Overview
CVE-2023-4967 is a denial of service vulnerability affecting Citrix NetScaler Application Delivery Controller (ADC) and NetScaler Gateway. The flaw exists when these appliances are configured as a Gateway, including VPN virtual server, ICA Proxy, CVPN, or RDP Proxy roles, or as an Authentication, Authorization, and Auditing (AAA) virtual server. Unauthenticated remote attackers can exploit this issue over the network to disrupt service availability. The vulnerability maps to [CWE-119], an improper restriction of operations within the bounds of a memory buffer.
Critical Impact
An unauthenticated remote attacker can render NetScaler ADC and Gateway appliances unavailable, disrupting VPN, ICA Proxy, and authentication services for downstream users.
Affected Products
- Citrix NetScaler Application Delivery Controller (including FIPS and NDcPP variants)
- Citrix NetScaler Gateway
- NetScaler ADC and Gateway deployments configured as VPN virtual server, ICA Proxy, CVPN, RDP Proxy, or AAA virtual server
Discovery Timeline
- 2023-10-27 - CVE-2023-4967 published to the National Vulnerability Database
- 2024-11-21 - Last updated in NVD database
Technical Details for CVE-2023-4967
Vulnerability Analysis
The vulnerability resides in the request handling logic of NetScaler ADC and NetScaler Gateway when deployed in gateway or AAA virtual server configurations. The flaw is classified under [CWE-119], indicating improper restriction of operations within a memory buffer. An attacker can send specially crafted network traffic to a vulnerable virtual server to trigger the condition.
Exploitation does not require authentication, user interaction, or local access. The result is loss of availability for the affected appliance, which typically fronts remote access and authentication workflows. Because NetScaler Gateway often serves as the ingress point for enterprise VPN and ICA Proxy sessions, an outage cascades to every dependent service.
The issue affects only specific configurations. Appliances that are not configured as a Gateway (VPN virtual server, ICA Proxy, CVPN, RDP Proxy) or as an AAA virtual server are not exposed through this code path.
Root Cause
The root cause is improper buffer boundary handling within the gateway and AAA request processing components. When the appliance parses malformed or oversized input directed at a vulnerable virtual server, memory access falls outside the intended buffer bounds. The condition forces the affected process into an error state, terminating active sessions and denying further service.
Attack Vector
The attack vector is network-based. An attacker reaches the vulnerable virtual server over its standard listening port, typically TCP/443 for VPN and ICA Proxy services. No credentials are required, and exploitation complexity is low. Successful requests degrade or terminate service availability without impacting confidentiality or integrity of stored data.
No public proof-of-concept code or verified exploit examples are available for this denial of service vulnerability. Refer to the Citrix Support Article CTX579459 for vendor technical details.
Detection Methods for CVE-2023-4967
Indicators of Compromise
- Unexpected restarts, crashes, or unresponsiveness of NetScaler ADC or NetScaler Gateway management and data plane processes
- Termination of active VPN, ICA Proxy, CVPN, or RDP Proxy sessions without a corresponding administrative action
- Spikes in malformed or oversized HTTP/TLS requests targeting Gateway or AAA virtual server endpoints
Detection Strategies
- Monitor NetScaler ns.log and syslog streams for repeated process termination, core dumps, or memory-related fault messages tied to gateway or AAA handlers
- Correlate network telemetry with virtual server availability metrics to identify request patterns that immediately precede service disruption
- Alert on sudden drops in concurrent VPN or ICA Proxy session counts that do not align with maintenance windows
Monitoring Recommendations
- Forward NetScaler appliance logs and SNMP traps to a centralized SIEM for continuous availability monitoring
- Implement synthetic transactions against Gateway and AAA virtual servers to detect outages in near real time
- Track inbound traffic anomalies against published Citrix advisory indicators referenced in CTX579459
How to Mitigate CVE-2023-4967
Immediate Actions Required
- Identify every NetScaler ADC and NetScaler Gateway appliance configured as a Gateway or AAA virtual server and inventory firmware versions
- Apply the fixed firmware releases listed in the Citrix advisory to all affected appliances as soon as a maintenance window permits
- Restrict network exposure of management interfaces and limit administrative access to trusted networks during patch rollout
Patch Information
Citrix released fixed builds for NetScaler ADC and NetScaler Gateway addressing this vulnerability. Upgrade instructions, fixed version numbers, and detailed remediation guidance are published in the Citrix Support Article CTX579459. Customers running end-of-life NetScaler versions must migrate to a supported release because no fix is provided for unsupported builds.
Workarounds
- No vendor-supplied workaround replaces patching; upgrading to a fixed firmware build is required
- Where immediate patching is not feasible, restrict reachability of the Gateway and AAA virtual servers to trusted source networks through upstream firewalls
- Disable Gateway or AAA virtual server configurations that are not in active use to reduce the exposed attack surface
# Configuration example: verify NetScaler version and review virtual server exposure
show ns version
show vpn vserver
show authentication vserver
# After upgrade, confirm the appliance reports a fixed build per CTX579459
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
