CVE-2026-19490: Critical Vulnerability Affecting Citrix NetScaler ADC and NetScaler Gateway

Citrix NetScaler ADC and NetScaler Gateway contain an authentication-bypass vulnerability (CWE-288, 'using an alternate path or channel') that an unauthenticated remote threat actor can exploit. The flaw is triggerable when the appliance is configured as an AAA virtual server or as a Gateway, including SSL VPN, ICA Proxy, CVPN, or RDP Proxy deployments, allowing the attacker to bypass authentication without valid credentials. A successful bypass could give an attacker access to VPN-protected or AAA-gated resources as an authenticated user; no CVSS score has been published yet. Organizations running affected NetScaler appliances in these configurations are exposed, and affected version ranges are not specified in the available data, so defenders should consult Citrix advisory AL26-019. The flaw was added to CISA's KEV on 2026-09-09, indicating exploitation in the wild; ransomware use is unknown, no public PoC is known, and EPSS assigns a 3.4% probability of exploitation within 30 days (88th percentile).

Do: Prioritize applying vendor fixes or mitigations per Citrix advisory AL26-019 in line with CISA BOD 26-04, focusing first on internet-facing appliances configured as AAA virtual servers or Gateways (SSL VPN, ICA Proxy, CVPN, RDP Proxy). Until patched, restrict internet exposure and review VPN/AAA authentication logs for signs of unauthenticated access, following CISA's Forensics Triage Requirements if compromise is suspected.