CVE-2018-0171
Cisco IOS and IOS XE Software Smart Install Remote Code Execution Vulnerability
Description
CVE-2018-0171 is a CRITICAL severity remote code execution vulnerability in the Smart Install feature of Cisco IOS and Cisco IOS XE Software, classified under CWE-20 (Improper Input Validation) and CWE-787 (Out-of-bounds Write). The vulnerability stems from improper validation of packet data in the Smart Install protocol, allowing an unauthenticated, remote attacker to send a crafted message to TCP port 4786 and trigger a buffer overflow. This can result in a device reload (denial of service), arbitrary code execution, or an indefinite loop causing a watchdog crash. With a CVSS 3.1 score of 9.8 and an EPSS score of 0.93123 (99.79th percentile), this vulnerability poses an extreme exploitation risk. CISA added it to the KEV catalog on November 3, 2021.
KEV Information
CVSS Score
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:HOpen in CalculatorAffected Products
| Vendor | Product | Version |
|---|---|---|
| cisco | ios | 15.2\(5\)e |
Multiple CVSS Assessments
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
References
- http://www.securityfocus.com/bid/103538(Third Party Advisory, VDB Entry)
- http://www.securitytracker.com/id/1040580(Third Party Advisory, VDB Entry)
- https://ics-cert.us-cert.gov/advisories/ICSA-18-107-04(Third Party Advisory, US Government Resource)
- https://ics-cert.us-cert.gov/advisories/ICSA-18-107-05(Third Party Advisory, US Government Resource)
- https://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20180328-smi2(Vendor Advisory)
- https://www.darkreading.com/perimeter/attackers-exploit-cisco-switch-issue-as-vendor-warns-of-yet-another-critical-flaw/d/d-id/1331490(Press/Media Coverage)
- https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2018-0171(US Government Resource)
Weakness Type
CWE-20: Improper Input Validation
Improper Input Validation occurs when a product receives input or data but does not validate or incorrectly validates that the input has the properties required to process the data safely and correctly. In network protocol implementations, this often means failing to verify packet lengths, field boundaries, or data types before processing, which can lead to memory corruption vulnerabilities.
In the context of CVE-2018-0171, the Cisco Smart Install protocol implementation in IOS and IOS XE Software does not properly validate packet data received on TCP port 4786. The Smart Install feature is designed to simplify initial switch deployment by allowing zero-touch configuration. However, the protocol handler fails to validate the size and content of incoming Smart Install messages before processing them. An attacker can craft a malicious Smart Install message with oversized or malformed fields that trigger a buffer overflow (CWE-787) in the device memory, leading to code execution with elevated privileges or a device crash.
Learn more: CWE-20 — Improper Input Validation
Impact Analysis
CVE-2018-0171 carries a CVSS 3.1 base score of 9.8 (CRITICAL) with network-exploitable attack characteristics requiring no authentication or user interaction.
Confidentiality (High): Successful exploitation allows arbitrary code execution on the affected Cisco device. This grants the attacker access to all configuration data including routing tables, SNMP community strings, TACACS/RADIUS shared secrets, VPN configurations, and any credentials stored on the device. The attacker can intercept and inspect all network traffic passing through the compromised switch or router.
Integrity (High): With code execution on the network device, an attacker can modify routing configurations to redirect traffic, alter access control lists to open unauthorized network paths, install persistent backdoors in the device firmware, and manipulate spanning tree or VLAN configurations to reshape the network topology for further attacks.
Availability (High): The vulnerability can directly cause device reloads and denial of service. An attacker can repeatedly crash affected devices, causing sustained network outages. The indefinite loop condition triggers a watchdog crash, making the device completely unresponsive. In environments with many Cisco switches using Smart Install, a single attacker could systematically take down critical network infrastructure.
Scope (Unchanged): While the scope remains unchanged in the CVSS assessment, compromising a core network device effectively impacts all systems and services dependent on that device for connectivity. The EPSS score of 0.93123 (99.79th percentile) places this among the most likely vulnerabilities to be exploited.
Exploit Maturity
Active Exploitation: CVE-2018-0171 is actively exploited in the wild. CISA added this vulnerability to the Known Exploited Vulnerabilities catalog on November 3, 2021, with a remediation deadline of May 3, 2022. The Smart Install protocol has been a persistent target for nation-state and criminal threat actors targeting network infrastructure.
Public Exploit Availability: Exploit code and proof-of-concept tools for CVE-2018-0171 are publicly available. The Smart Install Exploitation Tool (SIET) can be used to scan for and exploit vulnerable devices. Multiple security researchers have published detailed analyses of the vulnerability and its exploitation.
Nation-State Activity: This vulnerability has been associated with campaigns targeting critical infrastructure. The U.S. ICS-CERT issued advisories (ICSA-18-107-04 and ICSA-18-107-05) warning of exploitation against industrial control systems that rely on Cisco networking equipment.
EPSS Context: The EPSS score of 0.93123 (99.79th percentile) places this vulnerability in the top 0.21% of all CVEs for exploitation probability, reflecting widespread active exploitation and readily available attack tools.
Attack Surface: The Smart Install feature listens on TCP port 4786 by default on many Cisco switches, and the feature is often enabled without administrators being aware. Internet scanning has revealed hundreds of thousands of devices with this port exposed.
Remediation
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Apply Cisco software updates immediately. Install the patched IOS and IOS XE versions as described in Cisco Security Advisory cisco-sa-20180328-smi2. Prioritize devices exposed to untrusted networks.
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Disable Smart Install if not actively used. On devices where Smart Install is not required, disable the feature entirely using the command
no vstackin global configuration mode. This eliminates the attack surface completely. -
Block TCP port 4786 at network boundaries. If Smart Install cannot be immediately patched or disabled, implement access control lists to block TCP port 4786 from untrusted sources. Only permit Smart Install traffic from authorized management stations.
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Audit network devices for Smart Install exposure. Use network scanning tools to identify all devices with TCP port 4786 open. Verify whether Smart Install is actually needed on each device and disable it where unnecessary.
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Monitor for exploitation attempts. Implement intrusion detection signatures for malformed Smart Install packets on TCP port 4786. Review device logs for unexpected reloads, configuration changes, or Smart Install activity from unauthorized sources.
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Implement network segmentation for management traffic. Ensure that management protocols like Smart Install are only accessible from dedicated management VLANs and not from general user or internet-facing networks.
Technical Details
CVE-2018-0171 is a buffer overflow vulnerability in the Smart Install client feature of Cisco IOS and IOS XE Software. Smart Install is a Cisco proprietary protocol designed to simplify the deployment and configuration of new switches by providing zero-touch installation capabilities. The protocol operates on TCP port 4786 and allows a Smart Install director to push configurations and firmware images to client switches.
The vulnerability exists in the processing of Smart Install messages received by the client. When a Cisco device running IOS or IOS XE receives a specially crafted Smart Install message on TCP port 4786, the protocol handler copies packet data into a fixed-size buffer without properly checking the data length. This results in a stack-based buffer overflow that overwrites adjacent memory regions.
The CVSS vector (AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H) reflects that the attack is network-based, requires low complexity, needs no privileges or user interaction, and impacts all three CIA triad components at the highest level. The attack requires only that TCP port 4786 be reachable on the target device.
Exploitation can produce three distinct outcomes depending on the attacker's payload: a device reload causing denial of service, arbitrary code execution allowing complete device takeover, or an indefinite processing loop that triggers the IOS watchdog timer and forces a crash. The Cisco Bug ID for this vulnerability is CSCvg76186.
Frequently Asked Questions
Is CVE-2018-0171 being actively exploited?
Yes, CVE-2018-0171 is actively exploited in the wild. CISA added it to the Known Exploited Vulnerabilities catalog on November 3, 2021. The vulnerability has been used in campaigns targeting critical infrastructure, and ICS-CERT issued specific advisories about its exploitation. The EPSS score of 0.93123 (99.79th percentile) confirms extremely high exploitation activity.
What products are affected by CVE-2018-0171?
CVE-2018-0171 affects Cisco devices running IOS and IOS XE Software with the Smart Install client feature enabled. This includes many Cisco Catalyst switches and other devices where Smart Install is active by default on TCP port 4786. The vulnerability is not limited to specific hardware models but depends on the IOS/IOS XE software version.
How do I fix CVE-2018-0171?
The most effective immediate mitigation is to disable Smart Install using the no vstack command if the feature is not actively used. For environments requiring Smart Install, apply the patched Cisco IOS/IOS XE versions from the Cisco security advisory. Additionally, block TCP port 4786 from untrusted networks using access control lists.
How severe is CVE-2018-0171?
CVE-2018-0171 is rated CRITICAL with a CVSS score of 9.8, the highest severity tier. It allows unauthenticated remote code execution on network infrastructure devices with no user interaction required. The EPSS score of 0.93123 places it in the top 0.21% of all CVEs for exploitation likelihood. Compromising a core network device can cascade into widespread network disruption.
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