Hackers Exploit Two Critical Citrix NetScaler Zero-Days to Deploy Web Shells and Enable Lateral Movement
Threat actors are actively exploiting two critical zero-day vulnerabilities in Citrix NetScaler appliances to gain unauthenticated remote code execution, deploy stealthy web shells, and move laterally across compromised networks.
The vulnerabilities, identified as CVE-2026-88771 and CVE-2026-88772, both carry a CVSS v4.0 score of 9.5. CVE-2026-88771 stems from improper input validation and affects default deployments of NetScaler ADC and NetScaler Gateway. CVE-2026-88772 results from a memory overflow that can lead to RCE or denial of service when DTLS is enabled, a setting active by default on many VPN vServers.
Attackers have moved beyond proof-of-concept exploits. They install password-protected PHP web shells and modify web server configurations so that files with seemingly harmless extensions such as .css or image files are processed as PHP scripts. This technique hinders manual detection and allows malicious payloads to blend with legitimate resources.
Additional persistence techniques include setting the setuid bit on /bin/sh to grant root privileges to commands executed through the web shell. Observed artifacts include the file /var/netscaler/logon/LogonPoint/custom/.ctxs.receiver, suspicious Alias or AliasMatch directives in httpd.conf, and Python processes launched with nohup alongside files such as /tmp/.uxdport and /tmp/.uxdlock.
The campaign has been linked to the WHIPSHOT PHP web shell and the SLAPSHOT TCP tunneling tool written in Python. Citrix has issued patches for NetScaler ADC 13.1 before 13.1-64.23 and 14.1 before 14.1-73.37, along with corresponding NetScaler Gateway builds and FIPS/NDcPP variants. CISA has added both CVEs to its Known Exploited Vulnerabilities catalog and set a September 30, 2026 deadline for federal agencies.
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