CVE-2023-4966 in Citrix NetScaler: Navigating the Storm

Threat Overview – CVE-2023-4966

A significant security concern has emerged for organizations utilizing Citrix NetScaler Gateways and ADC appliances. In October 2023, Citrix identified a critical vulnerability, designated CVE-2023-4966, posing a severe threat to these devices. This sensitive information disclosure vulnerability, scoring a high 9.4 on the CVSS scale, has the potential to allow attackers to hijack legitimate user sessions on the affected appliances, leading to a range of security risks.

Campaign Overview

CVE-2023-4966 impacts several versions of NetScaler ADC and NetScaler Gateway appliances. Mandiant researchers noted exploitation of this vulnerability as early as August 2023, indicating its active use in the wild. The threat is exacerbated by its ability to bypass multi-factor authentication, usernames, or passwords, granting attackers unauthorized access and control over the compromised device.

Technical Details

The vulnerability is present in specific endpoints of the affected appliances. Attackers can exploit it by sending crafted HTTP GET requests with an overly long HTTP Host header to specific URLs. This action causes the appliance to return contents of its system memory, potentially including valid session cookies. These cookies can then be used to authenticate to the appliance, allowing unauthorized access and control.

Two primary vulnerable endpoints have been identified:

  • hxxps:///oauth/idp/.well-known/openid-configuration
  • hxxps:///oauth/rp/.well-known/openid-configuration

Given its critical nature, the vulnerability necessitates immediate attention from organizations using the affected Citrix products.

Taking Action Against CVE-2023-4966

In response to CVE-2023-4966, Citrix has released urgent updates for NetScaler ADC and NetScaler Gateway. Organizations are strongly advised to update their appliances to the latest versions immediately to mitigate the risks associated with this vulnerability.

In addition to applying these updates, vigilance is required in monitoring for any signs of unauthorized access or unusual system activity. Early detection of these signs can be crucial in preventing or limiting the impact of a breach.

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