VulnerabilityPublished 10 Oct 2023Updated 21 Oct 2025

CVE-2023-4966: Citrix NetScaler ADC · Memory Buffer Overflow

Affects Citrix NetScaler ADCSource CVE Database V5

Exploited in the wild · used in ransomware campaigns · KEV due 8 Nov 2023 · public exploit available (Metasploit) · 99% EPSS : apply the vendor's fix or mitigations to Citrix NetScaler ADC now.

Data as of 26 Oct 2026, 14:05 UTC

CVSS 3.1 base score

9.4Critical

How severe the flaw is on the 0–10 CVSS scale.

KEV deadline: 8 Nov 2023 US federal agencies must patch by this date; a sensible deadline for everyone else. Other KEV due dates
Ransomware: Used in ransomware campaigns
EPSS: 99.9% chance of exploitation in 30 days – higher than 99% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:L

Attack Vector: Network
Attack Complexity: Low
Privileges Required: None
User Interaction: None
Scope: Unchanged
Confidentiality: High
Integrity: High
Availability: Low

Bold values are the worst case: easiest to exploit or largest impact.

Attack Vector: Network: The vulnerable component is bound to the network stack and the set of possible attackers extends beyond the other options listed, up to and including the entire Internet. Such a vulnerability is often termed 'remotely exploitable' and can be thought of as an attack being exploitable at the protocol level one or more network hops away (e.g., across one or more routers).
Attack Complexity: Low: Specialized access conditions or extenuating circumstances do not exist. An attacker can expect repeatable success against the vulnerable component.
Privileges Required: None: The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files to carry out an attack.
User Interaction: None: The vulnerable system can be exploited without any interaction from any user.
Scope: Unchanged: An exploited vulnerability can only affect resources managed by the same security authority. In this case, the vulnerable component and the impacted component are either the same, or both are managed by the same security authority.
Confidentiality: High: There is total loss of confidentiality, resulting in all resources within the impacted component being divulged to the attacker. Alternatively, access to only some restricted information is obtained, but the disclosed information presents a direct, serious impact.
Integrity: High: There is a total loss of integrity, or a complete loss of protection. For example, the attacker is able to modify any/all files protected by the impacted component. Alternatively, only some files can be modified, but malicious modification would present a direct, serious consequence to the impacted component.
Availability: Low: Performance is reduced or there are interruptions in resource availability. Even if repeated exploitation of the vulnerability is possible, the attacker does not have the ability to completely deny service to legitimate users. The resources in the impacted component are either partially available all of the time, or fully available only some of the time, but overall there is no direct, serious consequence to the impacted component.

Definitions: FIRST CVSS v3.1 specification.

Exploitation evidence

t = 0 is the CVE’s publication on cve.org

Disclosed: evidence found.
CVE published 10 Oct 2023
Detectable: evidence found.
nuclei template: Citrix Bleed - Leaking Session Tokens
Weaponised: evidence found.
1 Metasploit module
Exploited in the wildExploited: evidence found.
Listed in CISA KEV 8 days after disclosure
Observed by OffSeq MirageSeen by Mirage: evidence found.
15 honeypot hits, first 3.0 years after disclosure

CVE ID reserved
by Citrix Systems, Inc.
14 Sep 202315:51 UTC, 26 days before disclosure
CVE published
on cve.org
10 Oct 202313:12 UTC, t = 0
Added to CISA KEV (opens in a new tab)
exploitation confirmed · ransomware use known
18 Oct 2023, 8 days after disclosure
KEV remediation due
US federal deadline
8 Nov 2023, 29 days after disclosure
First seen by OffSeq Mirage
from 3 attacker networks; activity in United States, Turkey, Netherlands
22 Oct 202618:04 UTC, 3.0 years after disclosure
Last seen by OffSeq Mirage (opens in a new tab)
25 Oct 2026, 23:46 UTC
25 Oct 202623:46 UTC, 3.0 years after disclosure

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), CISA KEV (opens in a new tab), OffSeq Mirage (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 26 Oct 2026.

Disclosed: The CVE record is public on cve.org. Its publication is t = 0.
Detectable: A public nuclei template can find exposed instances, for defenders and attackers alike.
Weaponised: Public exploit code exists in Metasploit or Exploit-DB, or CISA SSVC or the CVSS exploit maturity reports a public proof of concept.
Exploited in the wild: CISA KEV lists it, CISA SSVC rates exploitation active, or a source reports attacks.
Observed by OffSeq Mirage: OffSeq Mirage honeypot sensors recorded attack traffic for this CVE, or found exposed hosts without attack traffic.

Moments are spaced evenly, not to scale; a dashed line marks a gap of six months or more. Deltas count from t = 0. Sources that give only a date (CISA KEV, Exploit-DB, or a time of exactly 00:00Z) read “same day” rather than hours. Metasploit modules carry the flaw’s disclosure date, not their release date, and nuclei templates carry no date, so both appear above but not on the timeline.

What to do

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.

Affected software

Product	Affected	Fixed in
Citrix NetScaler ADC	Affected 12.1-FIPS – 14.1 6 affected builds	Fixed in–

Check the firmware or software version for Citrix NetScaler ADC in the device's admin interface against the vendor advisory.

Description

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.

Impact

For European organizations, the impact of CVE-2023-4966 is substantial due to the widespread deployment of Citrix NetScaler ADC in corporate and governmental remote access infrastructures. Successful exploitation can lead to the disclosure of sensitive information such as authentication credentials, session tokens, or other confidential data, potentially enabling further attacks like unauthorized access, lateral movement, or data exfiltration. This is particularly concerning for sectors with stringent data protection requirements, such as finance, healthcare, and public administration. The vulnerability undermines the confidentiality and integrity of remote access services, which are critical for business continuity and secure teleworking, especially in the post-pandemic environment where VPN usage remains high. Additionally, the exposure of AAA virtual servers could compromise identity and access management systems, increasing the risk of widespread credential compromise. The low impact on availability means systems may remain operational while being silently compromised, complicating detection. European organizations face regulatory risks under GDPR if sensitive personal data is exposed due to this vulnerability. The absence of known exploits in the wild provides a window for proactive mitigation, but the critical severity demands immediate attention to prevent potential targeted attacks.

Mitigation

1. Monitor Citrix communications and official advisories closely for patches addressing CVE-2023-4966 and apply them immediately upon release. 2. Until patches are available, restrict network access to NetScaler ADC and Gateway management interfaces and VPN endpoints using firewalls and access control lists to limit exposure to trusted IPs only. 3. Implement strict network segmentation to isolate NetScaler ADC devices from general user networks and sensitive backend systems. 4. Enable and review detailed logging and monitoring on NetScaler appliances to detect unusual access patterns or anomalous behavior indicative of exploitation attempts. 5. Conduct regular vulnerability scanning and penetration testing focused on remote access infrastructure to identify potential exploitation. 6. Enforce multi-factor authentication (MFA) on all remote access services to reduce the impact of credential disclosure. 7. Educate security teams on the specifics of this vulnerability to improve incident response readiness. 8. Review and harden AAA virtual server configurations to minimize unnecessary exposure and privilege escalation opportunities. 9. Consider deploying network intrusion detection/prevention systems (IDS/IPS) with updated signatures to detect exploitation attempts once available. 10. Maintain an incident response plan tailored to remote access infrastructure compromise scenarios.

CWE-119Improper Restriction of Operations within the Bounds of a Memory Buffer (opens in a new tab)
Class weaknessLikelihood of exploit: High
The product performs operations on a memory buffer, but it reads from or writes to a memory location outside the buffer's intended boundary. This may result in read or write operations on unexpected memory locations that could be linked to other variables, data structures, or internal program data.
MITRE discourages mapping vulnerabilities to this broad entry; the root cause is usually a more specific weakness.
Requirements · Language Selection
Use a language that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid.
Architecture and Design · Libraries or Frameworks
Use a vetted library or framework that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid.
Operation · Build and Compilation · Environment Hardening
Use automatic buffer overflow detection mechanisms that are offered by certain compilers or compiler extensions. Examples include: the Microsoft Visual Studio /GS flag, Fedora/Red Hat FORTIFY_SOURCE GCC flag, StackGuard, and ProPolice, which provide various mechanisms including canary-based detection and range/index…
OWASP Cheat Sheets
XML Security (opens in a new tab)
Mass Assignment (opens in a new tab)
OWASP ASVS 4.0.3
V13.3.1 (opens in a new tab)
V5.1.4 (opens in a new tab)
V5.1.3 (opens in a new tab)
Attack patterns
Buffer Overflow via Environment Variables (opens in a new tab)
Overflow Buffers (opens in a new tab)
Client-side Injection-induced Buffer Overflow (opens in a new tab)
Filter Failure through Buffer Overflow (opens in a new tab)
MIME Conversion (opens in a new tab)
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).

References

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Exploitation evidence

Timeline

What to do

Affected software

Description

AI analysisMachine-generated, updated 21 Oct 2025, 20:02 UTC

Public exploits and detection1 Metasploit module, a Nuclei detection template

WeaknessesCWE-119

Technical details

References

Community reviews

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