CriticalCVE-2023-6549

    Citrix NetScaler ADC and NetScaler Gateway Buffer Overflow Vulnerability (CVE-2023-6549)

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

    What is affected, what to do

    Improper Restriction of Operations within the Bounds of a Memory Buffer in NetScaler ADC and NetScaler Gateway allows Unauthenticated Denial of Service and Out-Of-Bounds Memory Read

    The vulnerability is being actively exploited; treat it as a priority. Update Cloud Software Group NetScaler ADC to version 12.35, Cloud Software Group NetScaler ADC to version 51.15, Cloud Software Group NetScaler ADC to version 92.21. Immediate actions, mitigation and detection rules are in the runbook below.

    Affected
    • Cloud Software Group NetScaler ADC lt 14.1..12.35
    • Cloud Software Group NetScaler ADC lt 13.1..51.15
    • Cloud Software Group NetScaler ADC lt 13.0..92.21
    • Cloud Software Group NetScaler ADC lt 13.1-FIPS..37.176
    • Cloud Software Group NetScaler ADC lt 12.1-NDcPP..55.302
    Fixed in
    • Cloud Software Group NetScaler ADC 12.35
    • Cloud Software Group NetScaler ADC 51.15
    • Cloud Software Group NetScaler ADC 92.21
    • Cloud Software Group NetScaler ADC 37.176
    • Cloud Software Group NetScaler ADC 55.302

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

    Metrics

    8.2

    Priority

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

    Source: Vendor (CNA), CVSS v3

    8.2

    CVSS v3

    Severity High on the scale from 0 to 10.

    Source: pwn.watch, CVE.org

    57.6 %

    EPSS score

    High — model estimates ≥ 50% chance of real-world exploitation within 30 days.

    Source: FIRST EPSS, percentile 99.09

    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
    223.86 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 exploited
    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
    8.2 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
    8.2
    EPSS score

    The EPSS score is the absolute model output for an individual vulnerability: the estimated probability of real-world exploitation within the next 30 days, expressed as a value between 0 and 1 (i.e. 0 % to 100 %). A score of 0.42 means „the model estimates a 42 % chance of real-world exploitation within 30 days“.

    Read in glossary
    57.6 %
    High — model estimates ≥ 50% chance of real-world exploitation within 30 days.
    EPSS percentile

    The EPSS percentile is the relative position of a vulnerability within all EPSS-scored CVEs, expressed as a value between 0 and 1. A percentile of 0.90 means „this CVE has a higher EPSS score than 90 % of all other scored CVEs“. Unlike the EPSS score, it is not a probability — but a positioning within a ranking.

    Read in glossary
    99.09
    Critical — this CVE ranks in the top fifth of all CVEs scored today (rank ≥ 80%).
    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

    CVE-2023-6549 hits the component that carries all remote access for many organisations. It is network-reachable without authentication or user interaction and sits among the CVEs most likely to be exploited. The primary impact is availability: knocking the Gateway over cuts VPN, ICA and RDP access in one go, which for KRITIS and NIS2 entities can quickly become a reportable operational incident. The accompanying memory over-read adds residual session-theft risk, given the track record of earlier NetScaler bugs. CISA does not flag known ransomware use. Even so, treat every internet-facing NetScaler with a Gateway or AAA configuration as critical and patch it within 24 hours. Internal-only ADC instances without those vServer types can follow the normal patch cycle.

    1. Runbook step 1

      Immediate response (0-24 h)

      • Inventory and check configuration: Inventory every NetScaler ADC and NetScaler Gateway instance (MPX, VPX, SDX instances, cloud). Check firmware with show version, then grep ns.conf for add vpn vserver and add authentication vserver. Only boxes configured as Gateway (VPN vServer, ICA Proxy, CVPN, RDP Proxy) or as an AAA vServer are vulnerable. Put those at the top of the queue.

      • Patch: Upgrade to the fixed builds:

        • 14.1-12.35 or later
        • 13.1-51.15 or later
        • 13.0-92.21 or later
        • 13.1-FIPS 13.1-37.176 or later
        • 12.1-FIPS/12.1-NDcPP 12.1-55.302 or later

        Standard 12.1 without FIPS or NDcPP is end-of-life. Move it to a supported branch.

      • Roll HA pairs safely: Patch the secondary node first, fail over, then patch the former primary. Confirm sync with show ha node. This keeps remote access up throughout.

      • Kill sessions as a precaution: The flaw also includes an out-of-bounds memory read. As a precaution, terminate sessions after patching: kill aaa session -all, kill icaconnection -all, kill rdp connection -all and clear lb persistentSessions. This invalidates any session tokens that may have leaked.

      • Preserve evidence before upgrading: Before upgrading, preserve /var/core/, /var/log/ns.log* and /var/nslog/ for later crash triage.

    2. Runbook step 2

      Mitigation layers

      • Network segmentation and exposure: Where the business allows, restrict Gateway and AAA vServer reachability to known source ranges or countries, using upstream firewall or geo-IP ACLs. The management interface (NSIP) must never be internet-facing. Keep it on a dedicated management VLAN.
      • Upstream DDoS protection and rate limiting: Enable provider or scrubbing-service DDoS protection for the Gateway on 443/TCP. On the NetScaler itself, add per-client-IP rate-limit selectors on Gateway vServers. This makes repeated crash attempts harder.
      • Availability architecture: Ensure an HA pair or multi-site GSLB is in place. Document and test a fallback remote-access path (break-glass VPN). This also supports NIS2 Art. 21 business-continuity obligations.
      • Reduce configuration surface: Disable Gateway features you don't use, such as idle CVPN, RDP Proxy or AAA vServers. Each one removed directly shrinks the attack surface.
      • No full workaround: The supplied context names no vendor workaround that replaces the patch. The controls above reduce risk only. Upgrading remains mandatory.

    3. Runbook step 3

      Detection rules

      • Packet engine crashes: New core dumps under /var/core/<n>/ (NSPPE-*) and unexpected packet engine restarts point to exploitation attempts. Example SPL: index=netscaler ("NSPPE" OR "core" OR "PPE crash") | stats count by host, _time
      • Unexpected HA failovers: Correlate HA failover and state-change events from ns.log and SNMP traps. Repeated failovers in a short window with no change ticket strongly indicate active DoS exploitation.
      • Web and Gateway log anomalies: Hunt for suspicious requests to Gateway and AAA endpoints (/vpn/, /logon/LogonPoint/, /nf/auth/, /oauth/). Look for oversized headers or URIs, malformed requests, and request bursts from single sources immediately preceding a crash.
      • Network telemetry (Zeek/Suricata): Watch the Gateway VIP for TCP reset and TLS-abort bursts and for mass client disconnects. Example: zeek conn.log | where id.resp_h == <VIP> and conn_state in ("RSTO","RSTR") per minute, alerting on a threshold.
      • Session hijacking as a follow-on signal: Given the out-of-bounds read, check whether the same AAA session is being reused from changing source IPs or geolocations. Use NetScaler AAA logs joined with IdP sign-in logs in the SIEM.

    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

    Improper Restriction of Operations within the Bounds of a Memory Buffer in NetScaler ADC and NetScaler Gateway allows Unauthenticated Denial of Service and Out-Of-Bounds Memory Read

    Source: pwn.watch, NVD (NIST), cvelistv5, CISA KEV Catalog, 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
    Cloud Software GroupNetScaler ADC
    • 13.0 – < 92.21
    • 12.1-NDcPP – < 55.302
    • 14.1 – < 12.35
    • 13.1-FIPS – < 37.176
    • 13.1 – < 51.15
    • 92.21
    • 55.302
    • 12.35
    • 37.176
    • 51.15
    citrixnetscaler_application_delivery_controller
    • 13.0 – < 13.0-92.21
    • 13.1 – < 13.1-37.176
    • 13.1 – < 13.1-51.15
    • 14.1 – < 14.1-12.35
    • 12.1 – < 12.1-55.302
    • 13.0-92.21
    • 13.1-37.176
    • 13.1-51.15
    • 14.1-12.35
    • 12.1-55.302
    citrixnetscaler_gateway
    • 13.1 – < 13.1-51.15
    • 14.1 – < 14.1-12.35
    • 13.0 – < 13.0-92.21
    • 13.1-51.15
    • 14.1-12.35
    • 13.0-92.21

    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

    References & sources

    • support.citrix.comCitrix advisory: CVE-2023-6548Vendor advisory, pwn-watch
    • cisa.govknown-exploited-vulnerabilities-catalog?field_cve=CVE-2023-6549government-resource, US government resource
    • pwn.watchpwn.watch: Citrix NetScaler ADC and NetScaler Gateway: 2 actively exploited vulnerabilitiespwn-watch
    • NVD (NIST)NVD: CVE-2023-6548pwn-watch
    • NVD (NIST)NVD: CVE-2023-6549pwn-watch
    • cisa.govCISA Known Exploited Vulnerabilities Catalogpwn-watch
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