Update: NetScaler ADC and Gateway Zero-Day Vulnerability

THE THREAT

August 10th Update: “What You Should Do About It” section updated to include additional details on searching for post-exploitation webshell deployment.

eSentire is aware of reports of the widespread exploitation of the critical NetScaler ADC (formerly Citrix ADC) and NetScaler Gateway (formerly Citrix Gateway) vulnerability CVE-2023-3519 (CVSS: 9.8). It is a Remote Code Execution (RCE) vulnerability that may be exploited by a remote and unauthenticated threat actor to achieve code execution. The vulnerability was initially disclosed on July 18th, 2023, and the earliest signs of exploitation have been traced back to June 2023.

Based on both public reporting and eSentire observations, exploitation of CVE-2023-3519 has transitioned from targeted attacks to widespread opportunistic exploitation; all vulnerable Citrix devices mentioned are at high risk of compromise.

It is critical that organizations, using the impacted Citrix devices, immediately apply relevant security patches and review all unpatched devices following disclosure of vulnerability for signs of compromise.

What we’re doing about it

• eSentire MDR for Network has a rule in place to identify exploitation attempts
• eSentire Managed Vulnerability Service (MVS) has plugins in place to identify this vulnerability
• New detections have been deployed as of August 10th, to identify exploitation attempts via eSentire MDR for Log
• Based on details shared by Citrix and CISA, the eSentire Tactical Threat Response (TTR) team created new eSentire MDR for LOG detections
• eSentire MDR for Network has rules in place to identify the China Chopper webshell
• eSentire released an initial advisory on this topic on July 19th

What you should do about it

• After performing a business impact review, apply the relevant security patches released by Citrix
  • Alternative mitigations are not currently available
  • Organizations that require assistance relating to upgrades are recommended to contact Citrix Technical Support
• Ensure logging is enabled on all NetScaler ADC and NetScaler Gateway devices
• Review potentially impacted devices for signs of compromise, as outlined by CISA and ShadowServer
  • ShadowServer has provided detection and hunting recommendations for known post compromise activity; it is critical that potentially impacted organizations query for webshell artifacts

Additional information

As of August 1st, researchers from the non-profit security organization Shadowserver Foundation have identified 581 compromised Citrix servers that were impacted by webshells; the total number of impacted devices is likely significantly higher.

On July 20th, CISA disclosed some details on real-world attacks. In observed incidents, threat actors were identified exploiting CVE-2023-3519 to deliver webshells to victim devices. Shadowserver has confirmed that attacks resulted in the deployment of the ChinaChopper webshell, a tool known to be used by Chinese affiliated threat actor groups in attacks related to both espionage and ransomware deployment. The end goal of recent attacks has not been identified at this time.

Impacted Citrix Products:

• 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 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

While patching is critical, if a webshell was deployed prior to the security patch being implemented, threat actors will achieve persistence despite the security patch. As such, it is highly recommended to review any potentially impacted devices for signs of compromise, as outlined in the CISA advisory Threat Actors Exploiting Citrix CVE-2023-3519 to Implant Webshells and the ShadowServer report Technical Summary of Observed Citrix CVE-2023-3519 Incidents.

It should be noted that for successful exploitation of the vulnerability, a vulnerable device must be configured as a Gateway or AAA virtual server. Citrix servers are not configured as Gateways or AAA virtual servers by default. NetScaler Gateway and NetScaler ADC version 12.1 are out of support and will not receive any additional updates. Organizations using version 12.1 are strongly recommended to update to a supported version as soon as possible.

References:

[1] https://nvd.nist.gov/vuln/detail/CVE-2023-3519
[2] https://support.citrix.com/article/CTX561482/citrix-adc-and-citrix-gateway-security-bulletin-for-cve20233519-cve20233466-cve20233467
[3] https://www.esentire.com/security-advisories/netscaler-adc-and-gateway-zero-day-vulnerability
[4] https://www.citrix.com/support/open-a-support-case
[5] https://www.cisa.gov/news-events/cybersecurity-advisories/aa23-201a
[6] https://www.shadowserver.org/news/technical-summary-of-observed-citrix-cve-2023-3519-incidents/
[7] https://twitter.com/Shadowserver/status/1686778896962797576
[8] https://www.bleepingcomputer.com/news/security/over-640-citrix-servers-backdoored-with-web-shells-in-ongoing-attacks/
[9] https://www.esentire.com/security-advisories/ransomware-hackers-attack-a-top-safety-testing-org-using-tactics-and-techniques-borrowed-from-chinese-espionage-groups