Admins warned to patch NetScaler appliances

Citrix NetScaler admins are being urged to install a patch released this week to plug a critical code injection hole in the company’s Application Delivery Controller (ADC) and Gateway products.

The warning comes from the U.S. Cybersecurity and Infrastructure Security Agency (CISA), which says threat actors have been exploiting the zero-day vulnerability (CVE-2023-3519) to drop a generic webshell on an unnamed critical infrastructure organization’s non-production environment.

The webshell enabled the hackers to search and exfiltrate Active Directory (AD) data. The actors attempted to move laterally to a domain controller, but network segmentation controls for the appliance blocked movement, the CISA says.

The affected products are

• NetScaler ADC and NetScaler Gateway 13.1 before 13.1-49.13
• NetScaler ADC and NetScaler Gateway 13.0 before 13.0-91.13
• NetScaler ADC and NetScaler Gateway version 12.1, now end of life
• NetScaler ADC 13.1-FIPS before 13.1-37.159
• NetScaler ADC 12.1-FIPS before 12.1-65.36
• NetScaler ADC 12.1-NDcPP before 12.65.36

The affected appliance must be configured as a Gateway (VPN virtual server, ICA Proxy, CVPN, RDP Proxy) or authentication, authorization, and auditing (AAA) virtual server for exploitation.

The report also contains details about the techniques used by the attacker.

Through the webshell, the attacker viewed NetScaler configuration files /flash/nsconfig/keys/updated/* and /nsconfig/ns.conf.  These configuration files contain an encrypted password that can be decrypted by the key stored on the ADC appliance. They also viewed the NetScaler decryption keys (to decrypt the AD credential from the configuration file) and used the decrypted AD credential to query the AD via ldapsearch. Discovered data was then encrypted and compressed into a zip file for exfiltration.

The hacker’s other discovery activities were unsuccessful due to the critical infrastructure organization’s deployment of their NetScaler ADC appliance in a segmented environment.

The actors’ post-exploitation lateral movement attempts were also blocked by network-segmentation controls, says the report. The actors implanted a second webshell on the victim that they later removed. This was likely a PHP shell with proxying capability, the report theorizes. The actors likely used this to attempt proxying SMB traffic to the domain controller; the report notes the victim organization observed SMB connections where the actors attempted to use the previously decrypted AD credential to authenticate with the domain controller from the NetScaler ADC via a virtual machine. Firewall and account restrictions — only certain internal accounts could authenticate to the DC — blocked this activity.

In addition to installing the patch, administrators should also search for signs the vulnerability has been exploited. If compromise is detected, IT should quarantine or take offline potentially affected hosts, re-image compromised hosts and provision new account credentials.

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Howard Solomon
Howard Solomon
Currently a freelance writer. Former editor of and Computing Canada. An IT journalist since 1997, Howard has written for several of ITWC's sister publications, including Before arriving at ITWC he served as a staff reporter at the Calgary Herald and the Brampton (Ont.) Daily Times.

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