CVE-2013-6939 in NetScaler
Summary
by MITRE
Unspecified vulnerability in Citrix NetScaler Application Delivery Controller (ADC) 9.3.x before 9.3-64.4, 10.0 before 10.0-77.5, and 10.1 before 10.1-118.7 allows attackers to cause a denial of service via unknown vectors, related to "RADIUS authentication."
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Analysis
by VulDB Data Team • 05/07/2026
The vulnerability identified as CVE-2013-6939 represents a critical security flaw within Citrix NetScaler Application Delivery Controller ADC platforms, specifically affecting versions 9.3.x prior to 9.3-64.4, 10.0 prior to 10.0-77.5, and 10.1 prior to 10.1-118.7. This issue resides within the RADIUS authentication mechanism, which serves as a fundamental component for network access control and user authentication in enterprise environments. The unspecified nature of the vulnerability vectors suggests that attackers can exploit multiple pathways within the RADIUS authentication process to disrupt service availability, potentially compromising the integrity of the entire application delivery infrastructure.
The technical implementation of this vulnerability stems from inadequate handling of RADIUS authentication requests within the NetScaler ADC platform. When the system processes authentication messages from RADIUS servers, specific conditions can trigger abnormal behavior that leads to system instability and eventual denial of service. This flaw operates at the protocol level where the ADC fails to properly validate or process incoming RADIUS packets, creating opportunities for malicious actors to craft specially crafted authentication requests that cause the system to crash or become unresponsive. The vulnerability's impact extends beyond simple service disruption as it can affect the entire network infrastructure that relies on the ADC for load balancing and application delivery services.
From an operational perspective, this vulnerability presents a significant risk to enterprise networks that depend on Citrix NetScaler ADC for their application delivery and load balancing requirements. The denial of service condition can result in complete network outages affecting critical business applications, customer-facing services, and internal enterprise systems. Attackers can exploit this weakness to systematically disrupt services by repeatedly sending malformed RADIUS authentication requests, causing the ADC to consume excessive resources or crash entirely. The implications for business continuity are severe, as organizations may experience extended downtime while the system recovers from the service disruption, potentially leading to financial losses and reputational damage.
The vulnerability aligns with CWE-119, which addresses "Improper Restriction of Operations within the Bounds of a Memory Buffer," and represents a classic example of how authentication mechanisms can become attack vectors when proper input validation and error handling are absent. From an adversarial perspective, this weakness maps to ATT&CK technique T1499.004, which involves network disruption through service exhaustion attacks. Organizations should implement immediate mitigations including applying the vendor patches released for versions 9.3-64.4, 10.0-77.5, and 10.1-118.7, implementing network segmentation to isolate vulnerable ADC instances, and deploying monitoring solutions to detect unusual authentication traffic patterns. Additionally, organizations should consider implementing rate limiting controls and configuring the ADC to properly handle authentication failures without crashing, ensuring that authentication mechanisms do not become single points of failure within the network infrastructure.