Citrix Patches Critical Zero Days Under Active Exploitation

Citrix has published updates for eight new vulnerabilities, including two critical zero-day CVEs that had been under active exploitation.

In a bulletin on September 27 the vendor confirmed eight new flaws in Citrix NetScaler ADC (formerly Citrix ADC) and Citrix NetScaler Gateway (formerly Citrix Gateway). They have CVSS scores ranging from 7 to 9.5.

The two most urgent are:

• CVE-2026-88771: a remote code execution (RCE) flaw due to improper input validation, enabling an unauthenticated attacker to execute arbitrary commands. It affects all NetScaler ADC and NetScaler Gateway deployments with default configuration
• CVE-2026-88772: a memory overflow vulnerability leading to RCE or denial of service. It affects any deployment with DTLS configuration enabled (which it is by default on VPN vServers)

“Exploitation of CVE-2026-88771 and CVE-2026-88772 on unmitigated NetScaler deployments has been observed,” Citrix said in a blog post. “Citrix strongly urges affected customers to install the relevant updated versions as soon as possible.”

Read more on Citrix vulnerabilities: Citrix Urges Immediate Patching for Critical NetScaler Vulnerabilities

Also noteworthy is CVE-2026-88773, a critical HTTP request smuggling flaw which is present when the HTTP configuration is enabled on NetScaler ADC or NetScaler Gateway. It has a CVSS score of 9.3.

Reports had been circulating before the Citrix bulletin of exploitation of the zero-day bugs.

The Australian Signals Directorate’s Australian Cyber Security Centre (ACSC) issued a critical alert on September 28 urging organizations to patch.

Reports online also suggested the Dutch National Cyber Security Center (NCSC-NL) had issued alerts to local organizations in the country.

The US Cybersecurity and Infrastructure Security Agency (CISA) has ordered federal agencies to patch by Wednesday, 30 September.

It’s not clear who is behind the exploitation attempts but in 2025, a cyber intrusion linked to China-based group Salt Typhoon targeted a Citrix zero day.

The Remaining Five Vulnerabilities

The rest of the CVEs published by Citrix include:

• CVE-2026-88774: a feature policy bypass due to improper HTTP URL based expression usage (CVSS 7) 
• CVE-2026-88775: a memory overflow vulnerability leading to unpredictable or erroneous behavior or denial of service (CVSS 8.8)
• CVE-2026-88776: a memory overflow vulnerability leading to unpredictable or erroneous behavior or denial of service (CVSS 8.8)
• CVE-2026-88777: a memory overflow vulnerability leading to unpredictable or erroneous behavior or denial of service (CVSS 8.8)
• CVE-2026-88778: a TCP Initial Sequence Number (ISN) prediction flaw with a (CVSS 8.8)

“This bulletin only applies to customer-managed Citrix NetScaler ADC and Citrix NetScaler Gateway,” the vendor confirmed. “Cloud Software Group upgrades the Citrix-managed cloud services and Citrix-managed Adaptive Authentication with the necessary software updates.”