CVE-2025-6543

Citrix NetScaler ADC and NetScaler Gateway appliances contain a memory buffer overflow (CWE-119) that can lead to unintended control flow and denial of service. The flaw is only reachable when the appliance is configured as a Gateway (VPN virtual server, ICA Proxy, CVPN, or RDP Proxy) or as an AAA virtual server, and it is network-exploitable without authentication or user interaction, though attack complexity is rated high. A successful attacker could achieve unintended control flow — with the CVSS 4.0 vector rating impact high across confidentiality, integrity, and availability — or crash the appliance, disrupting VPN access and application delivery. Any organization running NetScaler ADC or NetScaler Gateway in an affected Gateway/AAA configuration is exposed, a population that public scan data places in the tens of thousands of internet-exposed devices. The vulnerability was added to CISA's KEV catalog on 2025-06-30, confirming exploitation in the wild, with EPSS at 10.1% and no public proof-of-concept known.

What to do: Apply the patched NetScaler release specified in Citrix's security bulletin for CVE-2025-6543 immediately, prioritizing appliances in Gateway or AAA configurations, per CISA KEV and BOD 22-01 requirements. Audit which virtual servers (VPN, ICA Proxy, CVPN, RDP Proxy, AAA) are in use and whether they are internet-exposed, and check appliances for signs of compromise before and after upgrading.