CVE-2018-6186 in Netscaler VPX
Summary
by MITRE
Citrix NetScaler VPX through NS12.0 53.13.nc allows an SSRF attack via the /rapi/read_url URI by an authenticated attacker who has a webapp account. The attacker can gain access to the nsroot account, and execute remote commands with root privileges.
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Analysis
by VulDB Data Team • 02/02/2021
The vulnerability identified as CVE-2018-6186 represents a critical server-side request forgery flaw in Citrix NetScaler VPX appliances running versions through NS12.0 53.13.nc. This security weakness resides within the /rapi/read_url URI endpoint, which is designed to handle remote API requests for reading URLs. The vulnerability specifically affects authenticated users who possess web application accounts, creating a pathway for privilege escalation that ultimately allows attackers to achieve complete system compromise. The flaw demonstrates a fundamental failure in input validation and access control mechanisms within the appliance's API framework.
The technical implementation of this vulnerability stems from insufficient validation of user-supplied URLs within the read_url functionality. When an authenticated user submits a request to the /rapi/read_url endpoint, the system fails to properly sanitize or validate the input parameters, enabling an attacker to craft malicious requests that can bypass normal access controls. This vulnerability maps directly to CWE-918, which defines Server-Side Request Forgery as a condition where a web application accepts untrusted input that is used to construct HTTP requests to internal resources. The flaw allows an attacker to manipulate the target URL parameter to access internal system resources that should normally be restricted to administrative users.
The operational impact of this vulnerability is severe and far-reaching, as it enables authenticated attackers to escalate privileges from a standard web application account to the highest administrative level. Once exploited, the vulnerability provides access to the nsroot account, which represents the root administrative credentials for the NetScaler appliance. This privilege escalation capability allows attackers to execute arbitrary remote commands with full system privileges, effectively granting them complete control over the appliance and any network resources it manages. The compromise of the nsroot account can lead to data exfiltration, network disruption, and potential lateral movement within the network infrastructure that the appliance protects.
The security implications extend beyond immediate system compromise, as Citrix NetScaler appliances often serve as critical network security devices that control access to internal resources and manage SSL termination for enterprise applications. This vulnerability creates an attack vector that can be leveraged to bypass security controls, potentially allowing attackers to gain access to sensitive internal systems and data that the appliance is designed to protect. The vulnerability's exploitation requires only an authenticated web application account, making it particularly dangerous as it can be exploited by attackers who have gained access to legitimate user credentials through various means such as credential theft, phishing attacks, or weak authentication mechanisms.
Mitigation strategies for CVE-2018-6186 should include immediate application of Citrix security patches and updates that address the SSRF vulnerability in the affected NetScaler versions. Organizations should implement network segmentation and access controls to limit the exposure of the affected API endpoints, particularly by restricting access to the /rapi/read_url URI from untrusted networks. Additional defensive measures include implementing web application firewalls to monitor and filter requests to the vulnerable endpoint, conducting regular security assessments of the appliance configuration, and enforcing strict access control policies that limit the number of users with web application accounts. The vulnerability also highlights the importance of proper input validation and the principle of least privilege in API design, as outlined in the mitre ATT&CK framework under the privilege escalation and command execution techniques. Organizations should also consider implementing monitoring solutions that can detect anomalous API usage patterns that might indicate exploitation attempts, and maintain comprehensive incident response procedures that account for the potential impact of such a critical vulnerability.