Citrix and the U.S. Cybersecurity and Infrastructure Security Agency (CISA) report active exploitation of two zero-day vulnerabilities in customer-managed NetScaler ADC and NetScaler Gateway appliances: CVE-2026-88771 and CVE-2026-88772. Treat any affected appliance as urgent: identify its release family and configuration, check for signs of compromise, preserve evidence if compromise is suspected, and update to the vendor-recommended build. The two flaws have different exposure conditions, so patching is necessary but does not by itself establish whether an appliance has been compromised.
What is happening
Citrix says it observed exploitation of CVE-2026-88771 and CVE-2026-88772 on unmitigated NetScaler deployments. CISA added both vulnerabilities to its Known Exploited Vulnerabilities catalog and said it had received reports and partner threat intelligence confirming global exploitation. In its September 27, 2026 alert, CISA wrote: “CISA has received reports and partner threat intelligence confirming that threat actors are actively exploiting these vulnerabilities globally.” Read CISA’s alert and Citrix’s security bulletin.
The bulletin covers eight CVEs, but Citrix identifies only the first two as observed in active exploitation, and CISA’s alert concerns those two. Their vendor-published CVSS v4.0 base scores are both 9.5; those severity ratings describe technical severity, not the number of victims or the probability a particular appliance was compromised.
How the two exploited vulnerabilities differ
| Vulnerability | What it is | Exposure condition stated by Citrix |
|---|---|---|
| CVE-2026-88771 | Improper input validation that can allow unauthenticated remote code execution (RCE). | All customer-managed NetScaler ADC and Gateway deployments are affected, including default configurations. No additional feature or non-default configuration is required. |
| CVE-2026-88772 | A memory overflow that can allow RCE or denial of service. | DTLS must be enabled. Citrix says DTLS is enabled by default on VPN virtual servers unless an operator explicitly disables it. |
These conditions are from Citrix’s bulletin. For CVE-2026-88771, do not treat a default or apparently minimal configuration as protection. For CVE-2026-88772, review the actual virtual-server configuration: Citrix’s advisory describes checking for DTLS virtual servers and VPN virtual servers that do not have -dtls OFF. That setting check helps determine exposure; it is not a complete compromise assessment.
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Which appliances and builds are affected
Citrix’s bulletin applies to customer-managed appliances and gives separate thresholds for supported release families and FIPS/NDcPP variants. A build before the threshold listed for its own family is affected. Confirm the installed release and variant against the current vendor bulletin before choosing an update.
| Appliance release family | Affected versions | Vendor-recommended update |
|---|---|---|
| NetScaler ADC/Gateway 14.1 | Before 14.1-73.37 | 14.1-73.37 or later |
| NetScaler ADC/Gateway 13.1 | Before 13.1-64.23 | 13.1-64.23 or later |
| ADC FIPS 14.1 | Before 14.1-73.37 FIPS | Corresponding 14.1 FIPS update; confirm the applicable build in Citrix’s bulletin |
| ADC FIPS/NDcPP 13.1 | Before 13.1-37.279 | 13.1-37.279 or later |
Citrix also says Secure Private Access Hybrid deployments that use NetScaler instances are affected; those instances need the recommended builds. The bulletin concerns customer-managed appliances. Citrix says it is updating Citrix-managed cloud services and Adaptive Authentication itself, so customers should distinguish those services from appliances they operate. The affected-version thresholds and update guidance may change; verify the latest Citrix advisory and obtain the update from Citrix for the appliance’s exact release family before scheduling a change.
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What administrators should do first
- Inventory exposed instances. Identify customer-managed ADC and Gateway appliances, including NetScaler instances used by Secure Private Access Hybrid, and record each appliance’s release family, build, and FIPS/NDcPP status.
- Check the configuration condition for CVE-2026-88772. Inspect DTLS virtual servers and VPN virtual servers for the
-dtls OFFsetting, following the configuration checks in Citrix’s bulletin. Assume CVE-2026-88771 applies to every customer-managed deployment. - Look for indicators of compromise where possible. CISA recommends assessing exposure and reviewing Citrix’s advisory; Citrix has made indicator-of-compromise material available through NetScaler Console. If you suspect a breach, preserve relevant forensic evidence before updating when circumstances allow: CISA warns that applying updates may reduce forensic visibility.
- Plan patching and containment together. Update affected appliances to the vendor-recommended build for their release family. Mandiant recommends a targeted, phased approach that prioritizes patching while matching containment and compensating controls to risk and operational requirements. Consider the effect of broad internet isolation or strict IP allow-listing on remote access before applying those controls.
- Hunt beyond the appliance if compromise is suspected. Investigate possible lateral movement across the environment, including activity involving privileged access management systems. Patching the appliance does not answer whether an attacker already established access elsewhere.
Coordinate evidence collection, containment, and change windows among incident response, network, and identity teams. Use the appliance-specific procedures and indicators in Citrix’s bulletin and CISA’s alert.
What researchers observed after exploitation
Google Cloud’s Mandiant Consulting and Google Threat Intelligence Group reported on September 29, 2026 that they identified in-the-wild exploitation of CVE-2026-88772 in late September, with activity ongoing since at least early September. They assessed that organizations in North America and Europe across government, finance, technology, education, and legal and professional services were likely impacted. That is the researchers’ assessment, not a confirmed victim count or evidence that every vulnerable appliance was compromised. Their findings are detailed in Mandiant and GTIG’s report.
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The report describes exploitation that bypassed authentication and established root-level initial access after an unhandled termination of the NetScaler Packet Processing Engine. Researchers observed custom PHP web shells, including WHIPSHOT, which concealed Base64-encoded command-and-control payloads in HTTP headers. In at least one intrusion, they also observed SLAPSHOT, a Python tunneler used for internal reconnaissance and credential theft.
Reported persistence examples included changes to web-server handlers and a setuid change to /bin/sh. These are observed campaign behaviors, not a checklist that every compromised appliance will exhibit. Their presence can inform a hunt, but their absence alone does not establish that a system is clean.
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Other vulnerabilities in Citrix’s September bulletin
The bulletin also lists six vulnerabilities that are not identified as the two actively exploited zero-days in the CISA alert. They may still matter during exposure triage; their configuration conditions differ, so do not assume that the two exploited flaws describe all risk in the bulletin.
| CVE | Issue described by Citrix | Published CVSS v4.0 base score |
|---|---|---|
| CVE-2026-88773 | HTTP request smuggling; HTTP configuration required | 9.3 |
| CVE-2026-88774 | Feature policy bypass involving HTTP URL-based expressions | 7.0 |
| CVE-2026-88775 | Memory overflow with configuration-specific preconditions | 8.8 |
| CVE-2026-88776 | Memory overflow with configuration-specific preconditions | 8.8 |
| CVE-2026-88777 | Memory overflow with non-HTTP Layer 7 protocol preconditions | 8.8 |
| CVE-2026-88778 | TCP initial sequence number prediction | 8.8 |
Descriptions and scores are Citrix-published; consult the full bulletin for affected configurations and remediation details. The scores are severity ratings, not a measure of exploitation or prevalence.
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