CriticalCVE-2018-0171

    Cisco IOS and IOS XE Software Smart Install Remote Code Execution Vulnerability (CVE-2018-0171)

    Published In CISA KEV since Sources: SingCERT, NCSC NZ, CVE.org

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    What is affected, what to do

    A vulnerability in the Smart Install feature of Cisco IOS Software and Cisco IOS XE Software could allow an unauthenticated, remote attacker to trigger a reload of an affected device, resulting in a denial of service (DoS) condition, or to execute arbitrary code on an affected device. The vulnerability is due to improper validation of packet data.

    The vulnerability is being actively exploited; treat it as a priority. The available sources name no fixed version. Check the vendor guidance in the references below. Immediate actions, mitigation and detection rules are in the runbook below.

    Affected
    • Cisco IOS and IOS XE eq Cisco IOS and IOS XE..Cisco IOS and IOS XE

    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: SingCERT, NCSC NZ, CVE.org

    99.48 %

    EPSS score

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

    Source: FIRST EPSS, percentile 99.94

    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
    228.90 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: CSA/SingCERT 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
    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
    99.48 %
    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
    99.94
    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
    CISA assessment (SSVC)
    Exploitation: active · automatable · Technical impact: total
    Source: CISA Vulnrichment, as of 8 November 2024
    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-20

    Response and mitigation

    Why act now?

    Prioritisation rationale

    This flaw is network-reachable without authentication and needs no user interaction. The Smart Install client is enabled by default on many access switches, and public tooling has abused the protocol for years to steal configurations, force reloads, and execute code. It sits among the CVEs most likely to be exploited; CISA does not flag known ransomware use. Typical targets are enterprise and operator networks running ageing switch estates, including critical-infrastructure environments, because compromised network infrastructure gives persistent, low-visibility access and the ability to redirect traffic. Treat any device with TCP/4786 reachable from untrusted networks as critical and act immediately; internally isolated devices with no vstack applied can follow the regular patch cycle.

    1. Runbook step 1

      Immediate response (0-24 h)

      • Assess exposure: Inventory every Cisco IOS/IOS XE switch and run show vstack config on each one. If the Smart Install client is enabled and TCP/4786 is listening, treat the device as affected. IOS 15.2(5)E is explicitly listed; check other releases against bug ID CSCvg76186 using the Cisco Software Checker.
      • Disable Smart Install: On every device that does not actively use Smart Install (in practice, nearly all of them), apply no vstack and persist it with write memory. This is the fastest and most effective immediate fix.
      • Apply the patch: A fixed release is available from the vendor. Look up the exact version in the Cisco advisory for CSCvg76186 or in the Cisco Software Checker. Upgrade internet-facing devices and core/distribution switches first.
      • Close the perimeter: Block TCP/4786 immediately via ACL at the internet edge, at VPN boundaries, and between user and management zones. Also scan externally (e.g. nmap -p 4786 <own prefixes>) to find forgotten exposures.
      • Rule out compromise: On every device that was reachable, diff the running config against a known-good golden config. Also check:
        • show version (unexpected reloads, uptime)
        • dir flash: and show boot (unknown images or files)
        • verify /md5 of the image against Cisco's published hash
      • Rotate credentials: Smart Install abuse allows configuration retrieval. On any potentially exposed device, rotate enable secrets, local users, SNMP communities, TACACS+/RADIUS shared secrets, and VPN and routing-protocol keys.

    2. Runbook step 2

      Mitigation layers

      • Network segmentation: Restrict TCP/4786 to a dedicated management VLAN, reachable only from the Smart Install director. An infrastructure ACL (iACL) on all L3 interfaces looks like this:
        • deny tcp any any eq 4786, preceded by a permit for the director IP only
      • Control-plane protection: Use Control Plane Policing (CoPP) or a management-plane ACL to drop TCP/4786 from untrusted sources. Disable Telnet and HTTP on management interfaces and allow only SSHv2 and HTTPS.
      • IPS/firewall rules: Enable vendor signatures for Smart Install protocol abuse on Suricata, Snort, or the NGFW. At minimum, deploy a generic rule: alert tcp !$SMI_DIRECTOR any -> $NETWORK_DEVICES 4786 (msg:"Smart Install access from non-director"; flow:to_server,established; sid:<local>;).
      • Egress control: Block TFTP (UDP/69) and FTP from network devices to the internet. A common Smart Install abuse pattern is copying the config out, or pulling a new image in, over TFTP.
      • Configuration management: Version configs with a backup tool (e.g. Oxidized, RANCID) and alert on drift. Add no vstack to the hardening baseline and to compliance checks.
      • Lifecycle: Prioritise replacing end-of-support devices that have no fixed release. Until they are replaced, isolate them strictly and enforce no vstack.

    3. Runbook step 3

      Detection rules

      • Network telemetry (NetFlow/Zeek conn.log): Any TCP/4786 session whose source is not the Smart Install director is suspicious.
        • SPL: index=netflow dest_port=4786 NOT src_ip IN ("<director_ip>") | stats count by src_ip, dest_ip
        • KQL: CommonSecurityLog | where DestinationPort == 4786 and SourceIP != "<director_ip>"
      • Outbound TFTP/FTP from switches: Check Zeek tftp/conn.log or firewall logs for UDP/69 or TCP/21 sourced from the network-device address range. This pattern indicates config exfiltration or image replacement via Smart Install.
      • Device syslog: Look for unexpected reloads, watchdog or crash messages, and config changes without a change ticket.
        • Messages: %SYS-5-RELOAD, %SYS-5-CONFIG_I, %SYS-2-WATCHDOG
        • Sigma shape: logsource: product: cisco, service: syslog with selection: message|contains: ['%SYS-5-RELOAD', '%SYS-2-WATCHDOG'], correlated with TCP/4786 traffic in the same time window.
      • Integrity drift: The config-backup diff shows new local users, altered boot variables, new ACL exceptions, GRE tunnels, SPAN/ERSPAN sessions, or changed AAA servers.
      • External scanning: Repeated SYNs to TCP/4786 from the internet in edge firewall logs indicate mass scanning. Use these sources as indicators and correlate any hits on reachable devices.

    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

    A vulnerability in the Smart Install feature of Cisco IOS Software and Cisco IOS XE Software could allow an unauthenticated, remote attacker to trigger a reload of an affected device, resulting in a denial of service (DoS) condition, or to execute arbitrary code on an affected device. The vulnerability is due to improper validation of packet data. An attacker could exploit this vulnerability by sending a crafted Smart Install message to an affected device on TCP port 4786. A successful exploit could allow the attacker to cause a buffer overflow on the affected device, which could have the following impacts: Triggering a reload of the device, Allowing the attacker to execute arbitrary code on the device, Causing an indefinite loop on the affected device that triggers a watchdog crash. Cisco Bug IDs: CSCvg76186.

    Source: CSA/SingCERT, NCSC NZ (CC BY 4.0), NVD (NIST), cvelistv5, CISA KEV Catalog

    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
    —Cisco IOS and IOS XE
    • Cisco IOS and IOS XE
    • —

    Vendors and products

    Affected operating systems

    Affected operating systems
    Mobileciscoios15.2\(5\)e

    Weakness classes (CWE)

    • CWE-20Improper Input Validation

      Class

      The product receives input or data, but it does not validate or incorrectly validates that the input has the properties that are required to process the data safely and correctly.

      Entry at MITRE CWE-20

    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
    CSA/SingCERTActive Exploitation of Cisco Smart Install Client Vulnerability (opens in a new tab)CSA/SingCERTactively exploited

    References & sources

    • ics-cert.us-cert.govadvisories/ICSA-18-107-04x_refsource_MISC, Third-party advisory, US government resource
    • securitytracker.com1040580Vulnerability database, x_refsource_SECTRACK, Third-party advisory, Vulnerability database
    • tools.cisco.comsecurity/center/content/CiscoSecurityAdvisory/cisco-sa-20180328…x_refsource_CONFIRM, Vendor advisory
    • darkreading.comperimeter/attackers-exploit-cisco-switch-issue-as-vendor-warns-…x_refsource_MISC, Press coverage
    • ics-cert.us-cert.govadvisories/ICSA-18-107-05x_refsource_MISC, Third-party advisory, US government resource
    • securityfocus.com103538Vulnerability database, x_refsource_BID, Third-party advisory, Vulnerability database
    • cisa.govknown-exploited-vulnerabilities-catalog?field_cve=CVE-2018-0171government-resource, US government resource
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