CVE-2023-4966: Citrix NetScaler ADC and NetScaler Gateway Buffer Overflow Vulnerability

What it is

Sensitive information disclosure in NetScaler ADC and NetScaler Gateway when configured as a Gateway (VPN virtual server, ICA Proxy, CVPN, RDP Proxy) or AAA  virtual server.

Exploitation evidence

First exploit evidence seen: 2023-10-18 (earliest date across the sources below).

Exploited in the wild

• CISA KEV: listed since 2023-10-18. Source

Public exploit code

Metasploit

• auxiliary/scanner/http/citrix_bleed_cve_2023_4966: Citrix ADC (NetScaler) Bleed Scanner (disclosure 2023-10-25)

GitHub proof-of-concept repositories

14 public repositories from 14 accounts, first seen 2023-10-24. Most starred: Chocapikk/CVE-2023-4966 (78 stars), view repository.

Verify before running: public PoCs can be malicious. Read the code and only run it in an isolated lab. A PoC means exploit code is public, not that this CVE is exploited in the wild.

Sources: CISA KEV · Rapid7 Metasploit Framework module metadata · nomi-sec/PoC-in-GitHub.

How to fix it

CISA required action

Apply mitigations and kill all active and persistent sessions per vendor instructions [https://www.netscaler.com/blog/news/cve-2023-4966-critical-security-update-now-available-for-netscaler-adc-and-netscaler-gateway/] OR discontinue use of the product if mitigations are unavailable. (source: CISA KEV)

Fixed versions

• citrix netscaler application delivery controller: upgrade to 12.1-55.300 or later (affected from 12.1)
• citrix netscaler application delivery controller: upgrade to 13.0-92.19 or later (affected from 13.0)
• citrix netscaler application delivery controller: upgrade to 13.1-37.164 or later (affected from 13.1)
• citrix netscaler application delivery controller: upgrade to 13.1-49.15 or later (affected from 13.1)
• citrix netscaler application delivery controller: upgrade to 14.1-8.50 or later (affected from 14.1)
• citrix netscaler gateway: upgrade to 13.0-92.19 or later (affected from 13.0)
• citrix netscaler gateway: upgrade to 13.1-49.15 or later (affected from 13.1)
• citrix netscaler gateway: upgrade to 14.1-8.50 or later (affected from 14.1)

Derived from the NVD CPE configuration (versionEndExcluding / versionEndIncluding). Always confirm with the vendor advisory.

Vendor advisories and patches

• https://www.netscaler.com/blog/news/cve-2023-4966-critical-security-update-now-available-for-netscaler-adc-and-netscaler-gateway/ (CISA KEV notes)
• https://support.citrix.com/article/CTX579459/netscaler-adc-and-netscaler-gateway-security-bulletin-for-cve20234966-and-cve20234967 (CISA KEV notes)
• https://support.citrix.com/article/CTX579459 (Vendor Advisory)

MITRE ATT&CK mitigations for this chain

• M1052 User Account Control (for T1574) : Turn off UAC's privilege elevation for standard users <code>[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Policies\System]</code> to automatically deny elevation requests, add: <code>"ConsentPromptBehaviorUser"=dword:00000000</code>. Consider enabling installer detection for all users by adding: <code>"
• M1040 Behavior Prevention on Endpoint (for T1574) : Some endpoint security solutions can be configured to block some types of behaviors related to process injection/memory tampering based on common sequences of indicators (ex: execution of specific API functions).
• M1044 Restrict Library Loading (for T1574) : Disallow loading of remote DLLs. This is included by default in Windows Server 2012+ and is available by patch for XP+ and Server 2003+. Enable Safe DLL Search Mode to force search for system DLLs in directories with greater restrictions (e.g. <code>%SYSTEMROOT%</code>)to be used before local directory DLLs (e.g. a us
• M1047 Audit (for T1574) : Use auditing tools capable of detecting hijacking opportunities on systems within an enterprise and correct them. Toolkits like the PowerSploit framework contain PowerUp modules that can be used to explore systems for hijacking weaknesses. Use the program sxstrace.exe that is included with Windows along with manual in
• M1013 Application Developer Guidance (for T1574) : When possible, include hash values in manifest files to help prevent side-loading of malicious libraries.
• M1018 User Account Management (for T1574, T1134.001) : Limit privileges of user accounts and groups so that only authorized administrators can interact with service changes and service binary target path locations. Deny execution from user directories such as file download directories and temp directories where able. Ensure that proper permissions and directory access con
• M1051 Update Software (for T1574) : Update software regularly to include patches that fix DLL side-loading vulnerabilities.
• M1038 Execution Prevention (for T1574) : Adversaries may use new payloads to execute this technique. Identify and block potentially malicious software executed through hijacking by using application control solutions also capable of blocking libraries loaded by legitimate software.
• M1022 Restrict File and Directory Permissions (for T1574) : Install software in write-protected locations. Set directory access controls to prevent file writes to the search paths for applications, both in the folders where applications are run from and the standard library folders.
• M1024 Restrict Registry Permissions (for T1574) : Ensure proper permissions are set for Registry hives to prevent users from modifying keys for system components that may lead to privilege escalation.

Detection Generic guidance

• Execution: Watch for the service process spawning shells or script interpreters (cmd.exe, powershell, sh, bash, python) via EDR process-tree rules.
• Privilege Escalation: Alert on processes gaining SYSTEM/root unexpectedly, and on exploitation crashes of privileged services.
• Collection: Watch for bulk reads of databases, file shares or mailboxes by the affected service.

Response checklist Generic guidance

1. Inventory: find every instance of the affected product and version (include internet-facing and forgotten ones).
2. Patch or mitigate per the vendor advisory; if you cannot patch, remove internet exposure or apply the vendor workaround.
3. Assume compromise if the system was exposed before patching: hunt for the indicators in the advisory and review logs back to the disclosure date.
4. Rotate credentials and secrets stored on or used by the affected system if compromise cannot be ruled out.