CVE-2019-6485 in Netscaler Gateway
Summary
by MITRE
Citrix NetScaler Gateway 12.1 before build 50.31, 12.0 before build 60.9, 11.1 before build 60.14, 11.0 before build 72.17, and 10.5 before build 69.5 and Application Delivery Controller (ADC) 12.1 before build 50.31, 12.0 before build 60.9, 11.1 before build 60.14, 11.0 before build 72.17, and 10.5 before build 69.5 allow remote attackers to obtain sensitive plaintext information because of a TLS Padding Oracle Vulnerability when CBC-based cipher suites are enabled.
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Analysis
by VulDB Data Team • 07/19/2023
The vulnerability identified as CVE-2019-6485 represents a critical TLS padding oracle flaw affecting Citrix NetScaler Gateway and Application Delivery Controller appliances across multiple versions. This weakness stems from improper handling of TLS decryption operations when CBC-based cipher suites are active, creating a scenario where remote attackers can exploit the vulnerability to recover plaintext information through carefully crafted padding oracle attacks. The vulnerability impacts Citrix NetScaler appliances running versions 12.1 before build 50.31, 12.0 before build 60.9, 11.1 before build 60.14, 11.0 before build 72.17, and 10.5 before build 69.5, making it a widespread concern across the Citrix NetScaler product line.
The technical implementation of this vulnerability occurs within the TLS protocol handling mechanism of the Citrix appliances, specifically when they process encrypted data using CBC (Cipher Block Chaining) cipher suites. The padding oracle vulnerability arises because the system provides different error responses when decrypting data with incorrect padding versus other decryption failures, allowing attackers to iteratively determine the correct padding values and ultimately reconstruct the plaintext content. This weakness directly maps to CWE-129 and CWE-310 in the Common Weakness Enumeration catalog, representing both improper input validation and cryptographic issues. The vulnerability classifies under the ATT&CK technique T1071.001 for application layer protocol and T1566 for credential access through the potential for plaintext credential recovery.
The operational impact of this vulnerability extends far beyond simple information disclosure, as it enables attackers to obtain sensitive plaintext information including authentication credentials, session tokens, and potentially confidential business data transmitted through the affected appliances. Network administrators must consider that this vulnerability could be exploited to establish persistent access to corporate networks, particularly in environments where the Citrix appliances serve as primary entry points for remote access. The attack vector requires only network connectivity to the vulnerable appliance, making it particularly dangerous as it can be exploited from external networks without requiring physical access or prior authentication. Organizations utilizing these appliances face significant risk of data breaches and unauthorized access to their internal systems.
Mitigation strategies for CVE-2019-6485 require immediate action to address the underlying TLS implementation flaw. Organizations should prioritize upgrading their Citrix NetScaler appliances to the latest available builds that contain the necessary patches, specifically targeting the versions mentioned in the advisory. Network administrators should disable CBC-based cipher suites on affected appliances and instead implement modern TLS configurations using GCM or other authenticated encryption modes. Additionally, implementing network segmentation and monitoring for unusual TLS traffic patterns can help detect exploitation attempts. The vulnerability demonstrates the critical importance of maintaining current security patches and following secure configuration practices for enterprise security appliances, as outlined in NIST SP 800-53 and ISO/IEC 27001 standards for information security management.