CVE-2025-6543: Citrix NetScaler ADC Memory Overflow DoS Flaw

CVE-2025-6543 Overview

Memory overflow vulnerability leading to unintended control flow and Denial of Service in NetScaler ADC and NetScaler Gateway when configured as Gateway (VPN virtual server, ICA Proxy, CVPN, RDP Proxy) OR AAA virtual server.

Critical Impact
Unintended control flow and potential Denial of Service

Affected Products

Citrix NetScaler Application Delivery Controller (ADC)
Citrix NetScaler Gateway

Discovery Timeline

2025-06-25 - CVE CVE-2025-6543 published to NVD
2025-10-24 - Last updated in NVD database

Technical Details for CVE-2025-6543

Vulnerability Analysis

The vulnerability hinges on a memory overflow condition that can be triggered when the Citrix NetScaler ADC and Gateway are configured incorrectly. This can lead to unintended code execution paths, culminating in a potential Denial of Service (DoS).

Root Cause

The root cause is a lack of proper bounds checking within the memory handling functions of the application, leading to buffer overflow conditions.

Attack Vector

The primary attack vector is through network access, leveraging misconfigured Gateway or AAA virtual servers that expose the vulnerability.

// Example exploitation code (sanitized)
void vulnerable_function(char *input) {
    char buffer[256];
    strcpy(buffer, input); // No bounds checking
}

Detection Methods for CVE-2025-6543

Indicators of Compromise

Unusual application crashes
Logs indicating segmentation faults
Increase in memory usage

Detection Strategies

Utilize network-based intrusion detection systems (IDS) to monitor for inconsistent network patterns or unexpected traffic to virtual servers. Implement application layer monitoring to capture anomalies in server responses and error logs.

Monitoring Recommendations

Proactively assess application logs for signs of abnormal termination. Employ memory usage monitoring tools to detect unusual spikes indicating possible overflow exploitation.

How to Mitigate CVE-2025-6543

Immediate Actions Required

Disable unnecessary virtual server configurations
Implement strict memory policy boundaries
Monitor network traffic for anomalies

Patch Information

Citrix has released a security patch that addresses this vulnerability. All users are advised to update to the latest version as per Citrix Vendor Advisory.

Workarounds

If immediate patching is not possible, restrict access to virtual servers and use network-level defenses to limit exposure.

bash

# Configuration example
iptables -A INPUT -p tcp --dport 443 -j DROP
service iptables save