Threat actors have been observed exploiting a critical pre-authentication command injection vulnerability in Citrix NetScaler ADC and NetScaler Gateway to drop web shells and attempt theft of configuration data.

LevelBlue’s Threat Hunt Operations & Research (THOR) team, which analyzed the exploitation activity across multiple customer environments, said it identified malicious NetScaler authentication events containing attacker-controlled usernames designed to weaponize CVE-2026–88771.

CVE-2026–88771 (CVSS score: 9.5) is an improper input validation vulnerability that could allow an unauthenticated attacker to execute arbitrary commands.