CriticalCVE-2023-4966

    Unauthenticated sensitive information disclosure (CVE-2023-4966)

    Published In CISA KEV since Sources: pwn.watch, CVE.org

    What is affected, what to do

    A buffer overflow vulnerability in Citrix NetScaler ADC and NetScaler Gateway allows for sensitive information disclosure when configured as a Gateway (VPN, ICA Proxy, CVPN, RDP Proxy) or AAA server. Attackers can exploit this vulnerability to leak sensitive data, compromising the integrity and confidentiality of affected systems. The vulnerability is actively exploited in ransomware campaigns since October 2023. Affected versions range from 12.1 to 14.1 of NetScaler products. Organizations should urgently update to the latest patched versions to mitigate the risk of exploitation.

    The vulnerability is being actively exploited; treat it as a priority. CISA lists it as used in ransomware campaigns. Update Citrix NetScaler ADC to version 8.50, Citrix NetScaler ADC to version 49.15, Citrix NetScaler ADC to version 92.19. Immediate actions, mitigation and detection rules are in the runbook below.

    Affected
    • Citrix NetScaler ADC lt 14.1..8.50
    • Citrix NetScaler ADC lt 13.1..49.15
    • Citrix NetScaler ADC lt 13.0..92.19
    • Citrix NetScaler ADC lt 13.1-FIPS..37.164
    • Citrix NetScaler ADC lt 12.1-NDcPP..55.300
    • Citrix NetScaler Gateway lt 14.1..8.50
    • Citrix NetScaler Gateway lt 13.1..49.15
    • Citrix NetScaler Gateway lt 13.0..92.19
    Fixed in
    • Citrix NetScaler ADC 8.50
    • Citrix NetScaler ADC 49.15
    • Citrix NetScaler ADC 92.19
    • Citrix NetScaler ADC 37.164
    • Citrix NetScaler ADC 55.300
    • Citrix NetScaler Gateway 8.50
    • Citrix NetScaler Gateway 49.15
    • Citrix NetScaler Gateway 92.19

    Not sure whether and how you are affected? Ask a question about this vulnerability

    Metrics

    9.4

    Priority

    XIEM Effective CVSS: highest available CVSS rating from NVD and vendor data.

    Source: NVD, CVSS v3

    9.4

    CVSS v3

    Severity Critical on the scale from 0 to 10.

    Source: pwn.watch, CVE.org

    —

    EPSS score

    No value is available for this vulnerability.

    Show all metrics
    Severity
    Critical
    Priority (0–325)

    The priority score is a consolidated metric that condenses several individual signals into a single, action-guiding ranking: technical severity (CVSS), exploitation likelihood (EPSS), known-exploited status (KEV) and the context of the affected asset. The aim is to answer the only truly important question in vulnerability management: what first?

    Read in glossary
    255.63 Methodology
    Exploit maturity

    An exploit is code or a technique that specifically uses a concrete vulnerability to force unintended behaviour — such as executing code or escalating privileges. It is the tool that turns a theoretical flaw into a practical attack.

    Read in glossary
    actively exploitedSource: CERT.at, ANSSI / CERT-FR How we classify
    XIEM Effective CVSS

    XIEM® is the tiered managed-security operating model from NEOSEC and a registered trademark of NEOSEC GmbH. It extends classic SIEM — the central collection, normalisation and correlation of security-relevant events — with endpoint forensics, network visibility and deception technology, and consolidates them into three building tiers: Sentry, Orchestrate and Command.

    Read in glossary
    9.4 Methodology
    CVSS v3

    CVSS is the globally established, open standard for assessing the technical severity of a vulnerability on a scale from 0 to 10. It describes how easily a flaw can be exploited and what damage it can cause — but not how likely its actual exploitation is. CVSS and the complementary EPSS deliberately answer different questions.

    Read in glossary
    9.4
    KEV status

    The KEV catalogue is a list maintained by the US authority CISA of vulnerabilities for which active exploitation in the wild has been confirmed. It answers not the question „how severe?“ but „is it already being attacked?“ — thereby sharply prioritising patching.

    Read in glossary
    Actively exploited
    Published
    In CISA KEV since

    The KEV catalogue is a list maintained by the US authority CISA of vulnerabilities for which active exploitation in the wild has been confirmed. It answers not the question „how severe?“ but „is it already being attacked?“ — thereby sharply prioritising patching.

    Read in glossary
    CWE

    CWE is a catalogue of typical weakness types in software and hardware — such as flawed input validation, unsafe memory handling or poor authentication. Where a CVE names a concrete vulnerability in a concrete product, a CWE describes the underlying class of defect.

    Read in glossary
    CWE-119

    Response and mitigation

    Why act now?

    Prioritisation rationale

    "Citrix Bleed" is exploitable over the network without authentication and leaks valid session tokens. Attackers can therefore skip passwords and MFA entirely and walk straight into an authenticated session. It sits among the CVEs most likely to be exploited, and CISA flags confirmed use in ransomware campaigns. Typical victims include government, healthcare, finance, logistics and manufacturing, which are exactly the KRITIS/NIS2 profiles that rely on Citrix remote access. Treat any unpatched, internet-facing appliance with a Gateway or AAA role as a P1 incident: patch it, terminate all sessions and open a compromise assessment. Internal-only instances without those roles rank considerably lower.

    1. Runbook step 1

      Immediate response (0-24 h)

      • Patch now: Upgrade NetScaler ADC/Gateway to a fixed build: 14.1-8.50, 13.1-49.15, 13.0-92.19, 13.1-FIPS 13.1-37.164, 12.1-NDcPP 12.1-55.300 (or later). Plain 12.1 is end-of-life and receives no fix. Migrate those systems or pull them off the internet.
      • Kill every session after patching. The fix does not invalidate tokens that were already stolen. Run the following on each appliance: kill aaa session -all, kill icaconnection -all, kill rdp connection -all, kill pcoipConnection -all, clear lb persistentSessions.
      • Scope exposure: Enumerate every appliance running a Gateway vServer (VPN, ICA Proxy, CVPN, RDP Proxy) or an AAA vServer, including HA peers and DR nodes. Confirm their builds with show version.
      • Assume compromise if the appliance was internet-facing while unpatched. Rotate credentials used through the gateway, prioritising privileged AD accounts, service accounts and the LDAP bind account in the NetScaler config.
      • Preserve evidence: Copy ns.log, the /var/log/ archives and /var/nslog/newnslog* before rebooting or upgrading, because rotation erases traces.

    2. Runbook step 2

      Mitigation layers

      • No vendor workaround exists: Citrix states that patching is the only fix. Everything below only reduces risk until the patch is applied.
      • Network segmentation: Temporarily restrict access to Gateway and AAA vServers at the upstream firewall to known source ranges or countries. The management interface (NSIP) must never be internet-reachable.
      • IPS/WAF rule: Drop requests to /oauth/idp/.well-known/openid-configuration that carry an oversized Host header, for example above a few hundred bytes. On-box, you can use a responder policy on HTTP.REQ.HOSTNAME.LENGTH.GT(<threshold>) with a DROP action. Also enable the available Suricata/ET signatures for CVE-2023-4966.
      • Session hardening: Cut VPN/ICA session and idle timeouts aggressively to shrink the hijacking window. MFA offers no protection here, because a stolen token is already a post-authentication session.
      • Internal least privilege: Use internal firewalls to restrict traffic from the VPN pool and SNIP to Tier-0 assets such as DCs, backup servers and hypervisor management. This blunts lateral movement.

    3. Runbook step 3

      Detection rules

      • Exploit attempts (web/network): Look for GETs to /oauth/idp/.well-known/openid-configuration with a multi-KB Host header and unusually large 200 responses. A Zeek/proxy query shape is: uri="/oauth/idp/.well-known/openid-configuration" AND len(host) > 1000.
      • Session hijacking (ns.log): Watch for the same session or user appearing from changing client IPs mid-session, or concurrent sessions for one user from different ASNs or countries. Sessions with no preceding AAA LOGIN event are also suspicious. Correlate on username and client IP in the SIEM.
      • Lateral movement (Windows): Hunt for Event ID 4624 (logon type 3/10), 4648 and 4769 from the VPN pool against servers the user has never touched. Example KQL: SecurityEvent | where EventID==4624 and IpAddress in (vpn_pool) | summarize dcount(Computer) by Account | where dcount_Computer > 10.
      • Post-exploitation (EDR/Sysmon): Use Sysmon EID 1 or EDR telemetry to flag remote-access and exfiltration tools such as AnyDesk, Splashtop, rclone and Atera. Also flag net.exe, nltest /domain_trusts, adfind and vssadmin delete shadows on hosts reached via the gateway.
      • Appliance integrity: Check for unexpected files under /var/netscaler/logon/ or /var/vpn/, new cron entries and webshells. These indicate follow-on persistence.

    Detection rules

    No NEOSEC detection rule is available for this vulnerability yet.

    Classification under NIS2 and CRA

    Rule-based classification from exploitation status and severity. It does not replace a case-by-case assessment.

    NIS2

    NIS2 is EU Directive (EU) 2022/2555 to raise the level of cybersecurity across the Union. It substantially broadens obligations compared with the first NIS directive: more sectors, mandatory risk management, staged reporting duties and personal accountability of senior management.

    Read in glossary
    Actively exploited: treat the vulnerability as a priority in vulnerability handling under Art. 21(2)(e) NIS2. If it is exploited in your environment, check the reporting obligation under Art. 23 (early warning within 24 hours).
    CRA

    The CRA is EU Regulation (EU) 2024/2847 setting horizontal cybersecurity requirements for „products with digital elements“ — from industrial controllers and software to connected consumer devices. For the first time manufacturers must ensure security across the entire product lifecycle, handle vulnerabilities and provide security updates.

    Read in glossary
    Since 11.09.2026, manufacturers of affected products with digital elements must report actively exploited vulnerabilities under Art. 14 CRA, with an early warning within 24 hours via ENISA's single reporting platform.
    ISO/IEC 27001

    ISO/IEC 27001 is the internationally leading standard for an information security management system (ISMS). It describes a risk-based, process-oriented framework to systematically plan, implement, review and improve information security.

    Read in glossary
    Control A.8.8, management of technical vulnerabilities: assess relevance to your systems, plan remediation and keep evidence.

    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.

    Source: pwn.watch, CISA KEV Catalog, cvelistv5, NVD (NIST), CVE.org (MITRE)

    Affected products

    The following versions are affected. Older releases of the same product line are also vulnerable unless stated otherwise.

    Affected products
    Affected versionsFixed in
    CitrixNetScaler ADC
    • 12.1-NDcPP – < 55.300
    • 14.1 – < 8.50
    • 13.1 – < 49.15
    • 13.1-FIPS – < 37.164
    • 13.0 – < 92.19
    • 55.300
    • 8.50
    • 49.15
    • 37.164
    • 92.19
    CitrixNetScaler Gateway
    • 14.1 – < 8.50
    • 13.1 – < 49.15
    • 13.0 – < 92.19
    • 8.50
    • 49.15
    • 92.19
    citrixnetscaler_application_delivery_controller
    • 13.0 – < 13.0-92.19
    • 13.1 – < 13.1-49.15
    • 12.1 – < 12.1-55.300
    • 13.1 – < 13.1-37.164
    • 14.1 – < 14.1-8.50
    • 13.0-92.19
    • 13.1-49.15
    • 12.1-55.300
    • 13.1-37.164
    • 14.1-8.50
    citrixnetscaler_gateway
    • 13.1 – < 13.1-49.15
    • 13.0 – < 13.0-92.19
    • 14.1 – < 14.1-8.50
    • 13.1-49.15
    • 13.0-92.19
    • 14.1-8.50

    Vendors and products

    Weakness classes (CWE)

    • CWE-119Improper Restriction of Operations within the Bounds of a Memory Buffer

      Class

      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.

      Entry at MITRE CWE-119

    Public exploit references

    Publicly reported proof-of-concepts, exploit modules and detection templates for this vulnerability. NEOSEC links to the original sources only and does not provide any exploit code.

    Public exploit references
    CISA KEV CatalogCISA Known Exploited Vulnerability (opens in a new tab)cisa.govactively exploited
    ANSSI / CERT-FRLicence Ouverte 2.0, as of CERTFR-2023-ALE-012 – [MàJ] Vulnérabilité dans Citrix NetScaler ADC et NetScaler Gateway (clôturée 2024-02-16) (opens in a new tab)cert.cyber.gouv.fractively exploited
    Packet StormCitrix Bleed Session Token Leakage Proof Of Concept (opens in a new tab)packetstormsecurity.comPoC (publicly reported)reported, not verified—

    References & sources

    • support.citrix.comarticle/CTX579459
    • packetstormsecurity.comfiles/175323/Citrix-Bleed-Session-Token-Leakage-Proof-Of-Concep…
    • cisa.govknown-exploited-vulnerabilities-catalog?field_cve=CVE-2023-4966government-resource
    • cert.atCERT.at: Update #1: Kritische Sicherheitslücke in Citrix NetScaler ADC und NetScaler Gateway - aktiv ausgenutzt - Updates verfügbarcert-at
    • cert.cyber.gouv.frANSSI / CERT-FR: CERTFR-2023-ALE-012 – [MàJ] Vulnérabilité dans Citrix NetScaler ADC et NetScaler Gateway (clôturée 2024-02-16)cert-fr
    • pwn.watchpwn.watch: Citrix Bleed: NetScaler session token leak (CVE-2023-4966)pwn-watch
    • support.citrix.comNetScaler ADC and NetScaler Gateway Security Bulletin for CVE-2023-4966 and CVE-2023-4967pwn-watch
    • netscaler.comCVE-2023-4966: Critical security update now available for NetScaler ADC and NetScaler Gatewaypwn-watch
    • cloud.google.comRemediation for Citrix NetScaler ADC and Gateway Vulnerability (CVE-2023-4966)pwn-watch
    • cisa.gov#StopRansomware: LockBit 3.0 Ransomware Affiliates Exploit CVE 2023-4966 Citrix Bleed Vulnerability (AA23-325A)pwn-watch
    • cisa.govGuidance for Addressing Citrix NetScaler ADC and Gateway Vulnerability CVE-2023-4966, Citrix Bleedpwn-watch
    • NVD (NIST)NVD: CVE-2023-4966pwn-watch
    • cisa.govCISA Known Exploited Vulnerabilities Catalogpwn-watch

    Linked advisories

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    Badge preview for CVE-2023-4966
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