CVE-2023-4966: Citrix NetScaler ADC and NetScaler Gateway Buffer Overflow Vulnerability

Should you act?

Exploited: patch now if you run it

CISA has confirmed exploitation in the wild. This signal is computed from CISA KEV, EPSS and CVSS data; it is not an editorial verdict.

• Yes, if you run Citrix NetScaler ADC and NetScaler Gateway. CISA added this CVE to its Known Exploited Vulnerabilities catalog on 2023-10-18, which means there is reliable evidence of exploitation in the wild. US federal agencies must remediate by 2023-11-08; treat that as the outer limit, not the target. CISA also lists it as known to be used in ransomware campaigns.
• EPSS 100.0%, percentile 100.0: FIRST’s model estimates a 100.0% chance of exploitation activity in the next 30 days, higher than 100.0% of all scored CVEs (very high). EPSS is a probability, not a verdict.
• Remotely reachable with no login and no user interaction. If the vulnerable service is exposed to the internet, prioritize it.
• Editorial verdict: Act now. Citrix Bleed: NetScaler Gateway Leaks Session Tokens — If a NetScaler configured as a Gateway or AAA server is still on a vulnerable build, assume it is being probed and patch, kill every session, and hunt; patching alone does not evict an attacker holding a stolen session.

What it is

Sensitive information disclosure in NetScaler ADC and NetScaler Gateway when configured as a Gateway (VPN virtual server, ICA Proxy, CVPN, RDP Proxy) or AAA virtual server.

CISA’s summary

Citrix NetScaler ADC and NetScaler Gateway contain a buffer overflow vulnerability that allows for sensitive information disclosure when configured as a Gateway (VPN virtual server, ICA Proxy, CVPN, RDP Proxy) or AAA virtual server.

Weakness type: CWE-119.

Product: Citrix NetScaler ADC and NetScaler Gateway · Assigned by: Citrix · Published: 2023-10-10

Affected and fixed versions

Severity scores

• Attack vector: network (remotely reachable)
• Attack complexity: low
• Privileges required: none (no login)
• User interaction: none
• Scope: unchanged

CVSS measures how bad a flaw is if exploited, not how likely exploitation is. Read it next to the exploitation data below.

Exploitation and urgency

• CISA KEV: added 2023-10-18; federal due date 2023-11-08. Known ransomware use: Known.
• CISA required action: Apply mitigations and kill all active and persistent sessions per vendor instructions [https://www.netscaler.com/blog/news/cve-2023-4966-critical-security-update-now-available-for-netscaler-adc-and-netscaler-gateway/] OR discontinue use of the product if mitigations are unavailable.
• CISA SSVC assessment: exploitation active, automatable yes, technical impact total.
• FIRST EPSS (model of 2026-10-21): 100.00% probability of exploitation activity in the next 30 days; percentile 100.0.

Timeline

• 2023-10-10 — CVE record published by Citrix
• 2023-10-10 — erroneo.us brief: Citrix Bleed: NetScaler Gateway Leaks Session Tokens
• 2023-10-18 — Added to CISA’s Known Exploited Vulnerabilities catalog
• 2023-11-08 — CISA remediation deadline for US federal agencies
• 2026-07-31 — CVE record last updated

Sources and references

Original sources first. Version and fix details should always be confirmed against the vendor or project advisory.

Vendor and project advisories, patches

1. Listed in CISA KEV notes (netscaler.com) netscaler.com
2. Listed in CISA KEV notes (support.citrix.com) support.citrix.com

Vulnerability records

1. CVE.org record CVE-2023-4966 cve.org
2. NVD entry CVE-2023-4966 nvd.nist.gov

Government

1. CISA Known Exploited Vulnerabilities catalog entry cisa.gov

Other references from the CVE record and advisory

1. support.citrix.com support.citrix.com
2. packetstormsecurity.com packetstormsecurity.com

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