CVE-2023-4966
CVE-2023-4966 is a security flaw in Citrix NetScaler ADC and NetScaler Gateway, the gateway companies use to let staff reach internal apps from outside. Attackers can corrupt memory in Citrix NetScaler ADC and NetScaler Gateway, which they use to take it over. The US Cybersecurity and Infrastructure Security Agency (CISA) added it to its catalog of flaws known to be used in attacks on 18 Oct 2023, and says ransomware gangs have used it.[1]
Citrix NetScaler ADC and NetScaler Gateway is business software or equipment. Most families won’t have it, but attackers break into it to get inside companies, and from there to what those companies hold about their customers, patients and staff. Fixed in version 8.50, 49.15, 92.19 and 37.164 and others.
Description
CISA calls it a buffer overflow in Citrix NetScaler ADC and NetScaler Gateway. In its words: “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.”[1]
The underlying weakness is memory buffer error (CWE-119), in MITRE’s list of common software weaknesses.[2]
It scores 7.5 out of 10 on the Common Vulnerability Scoring System, which rates it high, scored by NVD using CVSS 3.1.[2]
Affected versions
NVD’s record lists these fixed versions: in each line, everything before the version shown is affected.[2]
| Product | Fixed in |
|---|---|
| NetScaler ADC | 8.50 |
| NetScaler ADC | 49.15 |
| NetScaler ADC | 92.19 |
| NetScaler ADC | 37.164 |
| NetScaler ADC | 55.300 |
| NetScaler Gateway | 8.50 |
| NetScaler Gateway | 49.15 |
| NetScaler Gateway | 92.19 |
Most families won’t have Citrix NetScaler ADC and NetScaler Gateway; it’s for whoever looks after IT at work.
Exploitation
CISA added it to its catalog of known exploited vulnerabilities on 18 Oct 2023, 8 days after it was made public. CISA lists a flaw only when it has reliable evidence of it being exploited, and gave US federal agencies until 8 Nov 2023 to fix it.[1] CISA says ransomware gangs have used it.
FIRST’s Exploit Prediction Scoring System, which estimates how likely a flaw is to be used in attacks in the next 30 days from what it sees across the internet, puts it at over 99%. That’s in the top 1% of the flaws it scores.[4]
CISA assesses it as automatable: attackers can run it against many targets at once.
CISA doesn’t say who was attacked, or how.
Fix
Citrix has published a fix or advice.[3] CISA’s 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.”[1]
- Check whether you use it.Citrix NetScaler ADC and NetScaler Gateway is business software or equipment, so most families won’t have it. If you run a business, or work from home on company equipment, ask whoever looks after your IT.
- Send this page to your IT person.Ask them to confirm it’s fixed (fixed in version 8.50, 49.15, 92.19 and 37.164 and others), or not in use. CISA gave US government agencies until 8 Nov 2023 to fix it.
- If it can’t be fixed, take it offline.CISA’s advice when there’s no fix is to stop using the product, or take it off the internet, until there is one.
You can’t fix someone else’s server, but you can watch for what follows a break-in like this: a breach notice, a password reset you didn’t ask for, or a scam message that knows things it shouldn’t. HARD watches for those for you and your family.
Timeline
| Date | Event |
|---|---|
| The flaw is made public in the National Vulnerability Database, with a severity score of 7.5.[2] | |
| CISA adds it to its catalog of flaws known to be used in attacks, noting that ransomware gangs have used it.[1] | |
| CISA’s deadline for US government agencies to fix it.[1] | |
| FIRST puts the odds of it being used in attacks in the next 30 days at over 99%.[4] |
See also
Other Citrix NetScaler ADC and NetScaler Gateway flaws
- CVE-2026-8452: improper restriction of operations within the bounds of a memory buffer ·
- CVE-2023-6549: buffer overflow ·
- CVE-2023-6548: code injection ·
- CVE-2023-3519: code injection ·
Other Citrix flaws
- CVE-2026-88779, Citrix NetScaler: improper restriction of operations within the bounds of a memory buffer ·
- CVE-2026-88772, Citrix NetScaler: improper restriction of operations within the bounds of a memory buffer ·
- CVE-2026-88771, Citrix NetScaler: improper input validation ·
- CVE-2026-19490, Citrix NetScaler: authentication bypass using an alternate path or channel ·
- CVE-2026-3055, Citrix NetScaler: out-of-bounds read ·
Other memory buffer error flaws
- CVE-2009-3459, Adobe Acrobat and Reader: heap-based buffer overflow ·
- CVE-2025-31277, Apple devices: buffer overflow ·
- CVE-2026-3910, Chrome and other Chromium browsers: improper restriction of operations within the bounds of a memory buffer ·
- CVE-2008-0015, Microsoft Windows: video activeX control remote code execution ·
- CVE-2026-20700, Apple devices: multiple buffer overflow ·
Every Citrix flaw on CISA’s list · Everything added in October 2023
References
- ^ Cybersecurity and Infrastructure Security Agency. “Citrix NetScaler ADC and NetScaler Gateway Buffer Overflow Vulnerability”. Known Exploited Vulnerabilities Catalog, added 18 Oct 2023.
- ^ National Institute of Standards and Technology. “CVE-2023-4966 Detail”. National Vulnerability Database, published 10 Oct 2023.
- ^ Citrix. Advisories and fixes: netscaler.com, support.citrix.com, support.citrix.com.
- ^ FIRST. “Exploit Prediction Scoring System”. EPSS 1.000, percentile 100.0, 31 Oct 2026.
- CVE Program. “CVE-2023-4966”. The official CVE record.
This article is built from CISA’s catalog (version 2026.10.30), NVD’s record and FIRST’s scores, kept in HARD’s database and checked every morning. The plain English is HARD’s.