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Actively exploited in the wild. This CVE is on the CISA Known Exploited Vulnerabilities list — treat remediation as urgent. Federal remediation due by 17 Mar 2022.
iOSOperating system · Cisco

CVE-2018-0156

HIGH · 7.5 CVSS v3.1 Published 2018-03-28
Mitigation only
No fix yet — a mitigation exists. There is no fixed release. A documented workaround reduces exposure in the meantime.
See remediation →
100/100
Remediation priority · Urgent
In the wild Remotely reachable No privileges Zero-click

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
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. The vulnerability is due to improper validation of packet data. An attacker could exploit this vulnerability by sending a crafted packet to an affected device on TCP port 4786. Only Smart Install client switches are affected. Cisco devices that are configured as a Smart Install director are not affected by this vulnerability. Cisco Bug IDs: CSCvd40673.

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis · high confidence

The Smart Install feature in Cisco IOS and IOS XE Software contains a vulnerability in packet data validation on TCP port 4786. An unauthenticated remote attacker can send a crafted packet to an affected Smart Install client switch, causing the device to reload and result in a denial of service. Only devices configured as Smart Install clients are vulnerable; Smart Install directors are not affected.

MitigationIf Smart Install is not required, disable the feature using the 'no vstack' command or block TCP port 4786 at network boundaries. Otherwise, apply the relevant Cisco software patches for CSCvd40673.

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.

NVD · CPE data
iOSOperating system
Affected:= 15.2\(2\)e4= 15.2\(2a\)ja
Ios XeOperating system
Affected:= 15.2\(2\)e4= 15.2\(2a\)ja

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Network
Complexity
Low
Privileges
None
User interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
None
Availability
High

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

dbcve checks

Work through these to decide whether this CVE applies to you.

  1. Check Cisco IOS/IOS XE version
    Run 'show version' and locate the version string in the output
    Affected if Version is exactly 15.2(2)e4 or 15.2(2a)ja on iOS, or exactly 15.2(2)e4 or 15.2(2a)ja on IOS XE
  2. Verify Smart Install feature is enabled
    Run 'show vstack' or examine running configuration with 'show run | include vstack'
    Affected if The output shows Smart Install is active or 'vstack' command appears in the configuration
  3. Confirm device role as Smart Install client
    Run 'show vstack detail' and check if the device is acting as a client rather than a director
    Affected if Device is configured as a Smart Install client (not a director)
  4. Check if TCP port 4786 is listening
    Run 'show control-plane host open-ports' or 'show tcp brief' and look for port 4786
    Affected if TCP port 4786 is shown as listening or open

A device is affected if it runs an exact vulnerable version (15.2(2)e4 or 15.2(2a)ja), has Smart Install enabled, is configured as a Smart Install client, and has TCP port 4786 listening.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

From vendor data
Mitigation available No clean upgrade yet — mitigate in the meantime
Mitigation

If Smart Install is not required, disable the feature using the 'no vstack' command or block TCP port 4786 at network boundaries. Otherwise, apply the relevant Cisco software patches for CSCvd40673.

Fix this in iOS Exploited in the wild — priority engagement
  • Consultation4.0 h
  • Implementation8.0 h
  • Testing4.0 h
  • Review / QA2.0 h
18.0 hours of engineering $3,200
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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2018-0156 in production — separate from our analysis above.

No notes yet

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What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here
  • Verified mitigations, workarounds, and config changes
  • Version or environment caveats, and links to real fixes
  • No weaponised exploit code, or anything meant to cause harm
  • No spam, self-promotion, credentials, or personal data