CVE-2021-22919 in ADC
Summary
by MITRE • 08/06/2021
A vulnerability has been discovered in Citrix ADC (formerly known as NetScaler ADC) and Citrix Gateway (formerly known as NetScaler Gateway), and Citrix SD-WAN WANOP Edition models 4000-WO, 4100-WO, 5000-WO, and 5100-WO. These vulnerabilities, if exploited, could lead to the limited available disk space on the appliances being fully consumed.
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Analysis
by VulDB Data Team • 08/08/2021
The vulnerability identified as CVE-2021-22919 represents a critical resource exhaustion flaw affecting Citrix ADC and Citrix Gateway appliances along with specific Citrix SD-WAN WANOP Edition models. This vulnerability stems from inadequate input validation and resource management within the affected Citrix products, creating a pathway for malicious actors to manipulate the system's disk space allocation. The flaw allows attackers to exploit a lack of proper bounds checking in the appliance's file handling mechanisms, leading to a condition where disk space consumption becomes unlimited or reaches critical levels. The vulnerability impacts the core operational integrity of these network infrastructure devices by potentially causing complete system failure or denial of service conditions.
Technical exploitation of CVE-2021-22919 occurs through the manipulation of file upload or processing functions within the Citrix appliances, where insufficient validation permits malicious inputs to trigger excessive disk usage patterns. The flaw typically manifests when the system receives crafted data that bypasses normal file size limitations and storage allocation checks. This allows attackers to continuously write data to disk storage until the available space is completely exhausted, effectively rendering the appliance non-functional and disrupting network services. The vulnerability is classified under CWE-400 as an Uncontrolled Resource Consumption issue, which aligns with the resource exhaustion characteristics demonstrated in this flaw. The attack vector commonly involves web-based exploitation through the appliance's management interfaces or API endpoints where file handling functions are accessible to authenticated or unauthenticated users.
The operational impact of this vulnerability extends beyond simple service disruption to encompass complete network infrastructure compromise. When disk space is consumed entirely, Citrix appliances become unable to process new requests, log events, or maintain operational state, leading to cascading failures in network connectivity and security policy enforcement. Organizations relying on these appliances for load balancing, SSL offloading, and gateway functions face significant operational risks including extended downtime, service degradation, and potential data loss from system crashes. The vulnerability particularly affects enterprises with large network footprints that depend on Citrix solutions for critical infrastructure services, as the impact can propagate across multiple network segments. According to ATT&CK framework, this vulnerability maps to T1499.004 for Network Denial of Service and T1078.002 for Valid Accounts, as exploitation typically requires some level of authentication to access the vulnerable functions, though the impact is severe enough to cause system-wide failures.
Mitigation strategies for CVE-2021-22919 require immediate patch application from Citrix, which addresses the underlying input validation issues and implements proper resource limits on file operations. Organizations should implement network segmentation to restrict access to vulnerable appliance interfaces and establish monitoring for unusual disk usage patterns that might indicate exploitation attempts. Regular disk space monitoring and automated alerting systems should be deployed to detect potential attacks before complete resource exhaustion occurs. Additionally, implementing proper access controls and privilege separation can limit the impact of potential exploitation by ensuring that only authorized personnel can access vulnerable functions. Network administrators should also consider implementing rate limiting and file size restrictions on upload operations to prevent abuse of the affected functionality. The vulnerability underscores the importance of maintaining current security patches and conducting regular vulnerability assessments of critical infrastructure components to prevent exploitation of known weaknesses in network appliances.