Intel

CVE-2018-0171

Cisco IOS and IOS XE — Cisco IOS and IOS XE Software Smart Install Remote Code Execution Vulnerability

criticalKEV: actively exploitedPoCEPSS 99%

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: CISA_KEVNVDCVELISTV5

Response & Mitigation

Why act now?

Prioritisation rationale

CVE-2018-0171 scores CVSS 9.8 (AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H) and sits at the 99.9th EPSS percentile, reflecting the wide availability of public exploit tooling and sustained scanning activity against TCP/4786 across the internet. Unauthenticated remote code execution on network infrastructure is a worst-case scenario for NIS2-scoped organisations — a compromised switch or router invalidates every downstream segmentation control and provides an ideal pivot point for lateral movement and persistent access. Although CISA has not flagged this CVE for known ransomware campaign use, network device compromise is a well-documented precursor to large-scale intrusions. Any organisation with Cisco IOS or IOS XE devices reachable on TCP/4786 from untrusted networks should treat this as a P1 incident: patch and no vstack take precedence over all other remediation work.

Runbook · Step 1

Immediate response (0-24 h)

  • Apply the vendor patch: Cisco released fixes for CVE-2018-0171 under Security Advisory cisco-sa-20180328-smi2. Upgrade all affected IOS and IOS XE devices to the fixed release listed in the advisory immediately (verify the exact version at tools.cisco.com/security/center — do not assume a version number).
  • Disable Smart Install: On every device that does not require a legitimate Smart Install Director, disable the feature immediately: no vstack in global configuration. Confirm with show vstack config — output must read "Role: Client (SmartInstall disabled)".
  • Block TCP port 4786: Deploy ACLs on all edge and core switches and routers to drop inbound traffic to TCP/4786 from untrusted sources. Apply this to management VLANs as well unless a legitimate Director is present.
  • Inventory all affected devices: Run show version and show vstack config across the entire Cisco estate. Any device with an active Smart Install client and no patch applied must be isolated immediately or moved to a quarantine VLAN.
  • Retrospective traffic analysis: Review NetFlow or firewall logs for the past 30 days for inbound connections to TCP/4786 on device IPs — flag any source outside your authorised address space for incident investigation.

Runbook · Step 2

Mitigation layers

  • Network segmentation: Move the management plane of all network devices into a dedicated out-of-band management VLAN. Permit TCP/4786 only from authorised Smart Install Director IPs; deny all other sources via ACL.
  • IPS/IDS signature: Enable Cisco IPS Signature 7880-0 (Smart Install Exploitation) if Cisco IPS or Firepower is deployed. Alternatively, write a Snort rule matching the Smart Install message header pattern on TCP/4786 (first 4 bytes |00 00 00 01| as the type field).
  • Infrastructure ACL (iACL): Implement Cisco's recommended iACL on all routers and Layer-3 switches to block traffic destined for the device's own IP on TCP/4786 from the internet and from unauthorised internal segments.
  • Least-privilege / access control: Ensure no Smart Install Director is reachable from the internet. Restrict VTY access to known management hosts using access-class on VTY lines.
  • TFTP server hardening: Smart Install can be abused to exfiltrate device configurations via TFTP. Disable TFTP servers in the environment or restrict them to authorised hosts to limit configuration theft as a secondary impact.

Runbook · Step 3

Detection rules

  • Network telemetry (NetFlow/Zeek): Alert on any TCP/4786 connection to device IPs originating from external or unauthorised internal sources. Zeek: conn.log | where id.resp_p == 4786 && !id.orig_h in [authorized_directors].
  • Suricata/Snort signature: alert tcp any any -> $NETWORK_DEVICES 4786 (msg:"CVE-2018-0171 Smart Install Exploit Attempt"; flow:to_server,established; content:"|00 00 00 01|"; depth:4; sid:9000171; rev:1;) — tune $NETWORK_DEVICES to your device IP ranges.
  • Cisco syslog monitoring: Forward device syslog to your SIEM and alert on: %SMART_INSTALL-5-DHCP_CONFLICT, %SYS-2-MALLOCFAIL, %SYS-3-CPUHOG, and unexpected reloads (%SYS-5-RELOAD) with no corresponding change-management record.
  • SIEM correlation (SPL example): index=network_devices sourcetype=cisco:ios (message="*vstack*" OR message="*smart install*" OR message="*MALLOCFAIL*") | stats count by host, message | where count > 0 — manually triage all hits.
  • Configuration change monitoring: Monitor for unexpected configuration changes — new local user accounts, modified ACLs, new TFTP server entries — via TACACS+/AAA accounting logs or network configuration management tools such as Cisco DNA Center or RANCID.

Metrics

Severity
critical
Priority score
209.51
KEV status
Actively exploited
Exploit maturity
actively exploited (KEV)
XIEM Effective CVSS
9.8
Source: nvd-v3
CVSS v3
9.8
CVSS v4
—
EPSS percentile
99.9 %
Critical — this CVE ranks in the top fifth of all CVEs scored today (rank ≥ 80%).
EPSS score
99.5 %
High — model estimates ≥ 50% chance of real-world exploitation within 30 days.
Published
2021-11-03 00:00 UTC
CWE
CWE-20

Weakness classes (CWE)

  • CWE-20Class

    Improper Input Validation

    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.

    cwe.mitre.org →

Affected operating systems

  • mobile

    cisco / ios15.2\(5\)e

Public exploit references

Public proof-of-concepts and detection templates for this vulnerability. Maturity ranges from reported PoCs through working detection scripts up to fully weaponized exploit modules. NEOSEC mirrors the code internally for forensic analysis; externally we only link to the original sources.

References & sources

IDCVE-2018-0171
Cisco IOS and IOS XE — Cisco IOS and IOS XE Software Smart Install Remote Code Execution Vulnerability — CVE-2018-0171 | NEOSEC Intel