CriticalCVE-2025-6543

    Memory overflow vulnerability leading to unintended control flow and Denial of Service (CVE-2025-6543)

    Published In CISA KEV since Sources: NCSC NZ, CERT-EU, pwn.watch, CVE.org

    What is affected, what to do

    This vulnerability results in a buffer overflow, causing unintended control flow and Denial of Service in Citrix NetScaler ADC and Gateway when configured as a Gateway (VPN virtual server, ICA Proxy, CVPN, RDP Proxy) or AAA virtual server. An attacker can exploit this to take control of the instance, and it is actively being used in ransomware campaigns since June 2025. The affected products are specific configurations of NetScaler ADC and Gateway. Immediate patching to the latest version is critical to mitigate this security risk.

    The vulnerability is being actively exploited; treat it as a priority. Update NetScaler ADC to version 47.46, NetScaler ADC to version 59.19, NetScaler ADC to version 37.236. Immediate actions, mitigation and detection rules are in the runbook below.

    Affected
    • NetScaler ADC lt 14.1..47.46
    • NetScaler ADC lt 13.1..59.19
    • NetScaler ADC lt 13.1 FIPS and NDcPP..37.236
    • NetScaler Gateway lt 14.1..47.46
    • NetScaler Gateway lt 13.1..59.19
    • NetScaler Gateway lt 13.1 FIPS and NDcPP..37.236
    Fixed in
    • NetScaler ADC 47.46
    • NetScaler ADC 59.19
    • NetScaler ADC 37.236
    • NetScaler Gateway 47.46
    • NetScaler Gateway 59.19
    • NetScaler Gateway 37.236

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

    Metrics

    9.8

    Priority

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

    Source: NVD, CVSS v3

    9.8

    CVSS v3

    Severity Critical on the scale from 0 to 10.

    Source: NCSC NZ, CERT-EU, pwn.watch, CVE.org

    10.6 %

    EPSS score

    Elevated — model estimates 10-50% exploitation likelihood.

    Source: FIRST EPSS, percentile 95.8

    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
    232.04 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: VulnCheck KEV (opens in a new tab), ANSSI / CERT-FR, NCSC NZ 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
    9.8 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
    9.8
    CVSS v4

    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
    9.2
    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
    10.6 %
    Elevated — model estimates 10-50% exploitation likelihood.
    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
    95.8
    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

    The flaw is exploitable over the network without authentication or user interaction, and it hits exactly the components that sit on the perimeter by design: Gateway and AAA vServers. It was exploited before a patch existed. Attackers have used it not just for denial of service but for code execution, persistent webshells and subsequent log cleanup. Exploitation likelihood is very high, though CISA currently does not flag ransomware use. For NIS2/KRITIS organisations running NetScaler as their remote-access or SSO gateway, treat this as an emergency change within 24 hours, paired with a compromise assessment. Instances without a Gateway or AAA role can follow the regular maintenance window.

    1. Runbook step 1

      Immediate response (0-24 h)

      • Scope the exposure: Inventory every NetScaler ADC and Gateway instance, including HA pairs, cloud VPX and SDX instances. Run show ns version to get the build. Run grep -E "add (vpn|authentication) vserver" /nsconfig/ns.conf to confirm whether a Gateway role (VPN, ICA Proxy, CVPN, RDP Proxy) or an AAA vServer is configured. Only those configurations are vulnerable, but they are almost always internet-facing.
      • Patch now per the Citrix Security Bulletin: upgrade 14.1 to 14.1-47.46 or later, 13.1 to 13.1-59.19 or later, and 13.1-FIPS/NDcPP to 13.1-37.236 or later. On HA pairs, upgrade the secondary node first, then fail over.
      • Replace end-of-life builds: Release lines the vendor has retired receive no fix. Pull those instances off the internet immediately or migrate them to a supported line. Verify the EoL status in the vendor advisory.
      • Preserve evidence before patching: Patching does not remove persistence an attacker has already established. Image or collect /var/log/, /var/nslog/, /var/core/, /var/netscaler/logon/, /var/vpn/, /netscaler/ns_gui/, /nsconfig/ns.conf and crontabs first. Take a VM snapshot where possible.
      • Kill sessions after patching: As a precaution, run kill aaa session -all, kill icaconnection -all, kill rdp connection -all and kill pcoipConnection -all.
      • Rotate credentials if compromise indicators are found: This covers nsroot, LDAP bind accounts, RADIUS shared secrets and any private keys stored on the appliance.

    2. Runbook step 2

      Mitigation layers

      • Network segmentation: Restrict the NSIP and management services (SSH 22, HTTP/HTTPS 80/443 on the NSIP) to a dedicated admin VLAN. Default-deny outbound internet traffic from NSIP/SNIP and allow only NTP, DNS, LDAP and RADIUS to known destinations. This breaks reverse shells and exfiltration.
      • Access restriction on the Gateway/AAA vServer: Until patched, limit reachability with geo-IP or source-IP allowlists on the upstream firewall. Disable unused Gateway features (CVPN, RDP Proxy) where operationally acceptable.
      • IPS/WAF: Enable current vendor signatures for CVE-2025-6543 on the upstream NGFW/IPS (Suricata/ET rulesets, Palo Alto, Fortinet). Watch for oversized headers or parameters in requests to /vpn/, /p/u/, /oauth/ and /cgi/. The vendor offers no official workaround, so IPS rules are a compensating control, not a substitute for the patch.
      • Availability: The flaw can trigger denial of service. Verify HA failover and monitoring of the packet engine (nsppe), and document fallback remote-access paths in your BCM runbook.
      • IAM/hardening: Eliminate default nsroot credentials, enforce external authentication with MFA for administration, and forward logs via syslog to a central SIEM so that local log wiping is noticed.

    3. Runbook step 3

      Detection rules

      • Packet-engine crashes: Look for new core dumps in /var/core/ (e.g. NSPPE-*) and unexpected restarts or HA failovers in ns.log and /var/log/messages. SPL shape: index=netscaler ("nsppe" AND ("core" OR "crash" OR "restart")) OR "HA state change".
      • Webshells and unknown files: Run find /netscaler/ns_gui /var/netscaler/logon /var/vpn /var/tmp -type f -newer /nsconfig/ns.conf.0 \( -name "*.php" -o -name "*.pl" -o -name "*.sh" \) and diff the results against a known-clean reference instance. Also check for new cron entries and changes to /flash/nsconfig/rc.netscaler.
      • Process ancestry: Flag shell processes (sh, bash, python, perl, nc, curl) spawned by httpd or nsppe. Use shell history (/var/log/bash.log) or external process monitoring where available.
      • Network telemetry (Zeek/Suricata): Flag outbound connections from NSIP/SNIP to unknown external IPs, especially on unusual ports or long-lived sessions. Zeek shape: conn.log where id.orig_h in netscaler_ips and !local_resp and the destination is not on the allowlist.
      • Log gaps and tampering: Treat sudden breaks in the syslog stream, truncated or zero-byte logs, and mismatches between local and centrally collected logs as compromise indicators.

    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

    Memory overflow vulnerability leading to unintended control flow and Denial of Service in NetScaler ADC and NetScaler Gateway when configured as Gateway (VPN virtual server, ICA Proxy, CVPN, RDP Proxy) OR AAA virtual server

    Source: NCSC NZ (CC BY 4.0), certeu_advisory, 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
    NetScalerADC
    • 14.1 – < 47.46
    • 13.1 FIPS and NDcPP – < 37.236
    • 13.1 – < 59.19
    • 47.46
    • 37.236
    • 59.19
    NetScalerGateway
    • 13.1 FIPS and NDcPP – < 37.236
    • 13.1 – < 59.19
    • 14.1 – < 47.46
    • 37.236
    • 59.19
    • 47.46
    citrixnetscaler_application_delivery_controller
    • 13.1 – < 13.1-37.236
    • 13.1 – < 13.1-59.19
    • 14.1 – < 14.1-47.46
    • 13.1-37.236
    • 13.1-59.19
    • 14.1-47.46
    citrixnetscaler_gateway
    • 13.1 – < 13.1-59.19
    • 14.1 – < 14.1-47.46
    • 13.1-59.19
    • 14.1-47.46

    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
    NCSC NZCVE-2025-6543 affecting Citrix Netscaler products (opens in a new tab)NCSC NZactively exploited
    ANSSI / CERT-FRLicence Ouverte 2.0, as of CERTFR-2025-ALE-009 – Multiples vulnérabilités dans Citrix NetScaler ADC et NetScaler Gateway (clôturée 2025-08-18) (opens in a new tab)cert.cyber.gouv.fractively exploited

    References & sources

    • support.citrix.comCitrix advisory: Citrix NetScaler ADC and GatewayVendor advisory, pwn-watch
    • cisa.govknown-exploited-vulnerabilities-catalog?field_cve=CVE-2025-6543government-resource, US government resource
    • cert.cyber.gouv.frANSSI / CERT-FR: CERTFR-2025-ALE-009 – Multiples vulnérabilités dans Citrix NetScaler ADC et NetScaler Gateway (clôturée 2025-08-18)cert-fr
    • pwn.watchpwn.watch: Citrix NetScaler ADC and Gateway: NetScaler ADC and Gateway Buffer Overflow Vulnerabilitypwn-watch
    • netscaler.comCitrix advisory: Citrix NetScaler ADC and Gatewaypwn-watch
    • NVD (NIST)NVD: CVE-2025-6543pwn-watch
    • cisa.govCISA Known Exploited Vulnerabilities Catalogpwn-watch
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