CVE-2017-17549 in Netscaler Application Delivery Controller
Summary
by MITRE
Citrix NetScaler Application Delivery Controller (ADC) and NetScaler Gateway 10.5 before build 67.13, 11.0 before build 71.22, 11.1 before build 56.19, and 12.0 before build 53.22 allow remote attackers to obtain sensitive information from the backend client TLS handshake by leveraging use of TLS with Client Certificates and a Diffie-Hellman Ephemeral (DHE) key exchange.
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Analysis
by VulDB Data Team • 01/27/2021
The vulnerability identified as CVE-2017-17549 affects Citrix NetScaler ADC and NetScaler Gateway appliances across multiple versions, representing a significant information disclosure risk that stems from improper handling of TLS client certificate authentication combined with specific key exchange mechanisms. This flaw exists in versions prior to specific build numbers across the 10.5, 11.0, 11.1, and 12.0 release lines, creating a persistent security gap that remote attackers can exploit to access sensitive backend information.
The technical root cause of this vulnerability lies in the improper implementation of TLS protocol handling when client certificates are used in conjunction with Diffie-Hellman Ephemeral key exchange. When the NetScaler appliance processes TLS connections with client certificate authentication and DHE key exchange, it fails to properly protect the sensitive information exchanged during the TLS handshake process. This vulnerability specifically manifests when the system is configured to use TLS with client certificates and the DHE key exchange mechanism, creating an information disclosure channel that allows attackers to obtain backend client TLS handshake information.
The operational impact of this vulnerability is substantial as it enables remote attackers to perform information disclosure attacks against affected Citrix appliances. Attackers can leverage this weakness to extract sensitive data from the backend client TLS handshake process, potentially obtaining information that could be used for further exploitation or to compromise the security posture of the affected network infrastructure. The vulnerability particularly affects environments where client certificate authentication is implemented, making it a critical concern for organizations relying on strong authentication mechanisms.
This vulnerability maps to CWE-200, which describes "Information Exposure," and aligns with ATT&CK technique T1046 for network service scanning and T1071.1 for application layer protocols. The flaw represents a classic example of improper information protection where the system fails to adequately secure sensitive data during cryptographic operations. Organizations using Citrix NetScaler appliances in environments where client certificate authentication is required and DHE key exchange is implemented are particularly at risk. The vulnerability essentially creates a side-channel information leak that undermines the security of the entire TLS communication framework.
Mitigation strategies for this vulnerability include applying the official Citrix security patches and updates released for the affected versions, which address the improper handling of TLS handshake information. Organizations should also consider implementing network segmentation to limit access to affected appliances, reviewing and updating TLS configurations to avoid using DHE key exchange when possible, and monitoring network traffic for suspicious activities related to TLS handshake information disclosure. Additionally, organizations should conduct comprehensive vulnerability assessments to identify all instances of affected Citrix appliances within their environment and ensure proper patch management procedures are in place to prevent similar issues from occurring in the future.