iOSOperating system · Cisco
CVE-2017-12240
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 · uneditedThe DHCP relay subsystem of Cisco IOS 12.2 through 15.6 and Cisco IOS XE Software contains a vulnerability that could allow an unauthenticated, remote attacker to execute arbitrary code and gain full control of an affected system. The attacker could also cause an affected system to reload, resulting in a denial of service (DoS) condition. The vulnerability is due to a buffer overflow condition in the DHCP relay subsystem of the affected software. An attacker could exploit this vulnerability by sending a crafted DHCP Version 4 (DHCPv4) packet to an affected system. A successful exploit could allow the attacker to execute arbitrary code and gain full control of the affected system or cause the affected system to reload, resulting in a DoS condition. Cisco Bug IDs: CSCsm45390, CSCuw77959.
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 confidenceBuffer overflow vulnerability in Cisco IOS and IOS XE DHCP relay subsystem allows unauthenticated remote attackers to execute arbitrary code or cause device reload by sending crafted DHCPv4 packets. This is a critical pre-authentication remote code execution flaw affecting DHCP relay functionality.
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
>= 12.2, <= 15.6CVSS 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
- High
- Integrity
- High
- Availability
- High
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/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 checksWork through these to decide whether this CVE applies to you.
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Determine the Cisco iOS versionExecute the 'show version' command on the device console or via SSH. Locate the version number in the output (typically in the first few lines, shown as something like 'Version 15.2(4)E' or similar).Affected if The version falls within the range 12.2 to 15.6 (inclusive)
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Verify DHCP relay is configuredExecute 'show running-config | include ip helper-address' or 'show running-config | include dhcp relay' to check for DHCP relay configuration. Also check 'show ip interface' for helper addresses on relevant interfaces.Affected if DHCP relay is enabled (ip helper-address commands are present or 'ip dhcp relay' is configured)
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Confirm DHCP functionality is activeExecute 'show ip dhcp relay statistics' or check if DHCP relay is listed in 'show ip interface' output for any interface.Affected if The device is actively functioning as a DHCP relay agent
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Identify exposed interfacesReview the interface configurations with 'show running-config' and identify which interfaces have 'ip helper-address' configured, as these are the entry points for crafted DHCPv4 packets.Affected if Any interface has ip helper-address configured and the iOS version is within the affected range
The device is affected if it runs Cisco iOS versions 12.2 through 15.6 and has DHCP relay (ip helper-address) enabled on any interface.
Generated from the published advisory. Verify against your own configuration.
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.
dbcve · scopedApply Cisco security patches for affected IOS/IOS XE versions; if patching is not immediately possible, disable DHCP relay feature if not required and implement network access controls to limit DHCP traffic to trusted sources.
Cisco IOS 16.x or later (specific version depends on hardware platform; consult Cisco IOS version 16.x or 17.x train for your device)
- Check Cisco Security Advisory for CVE-2017-12240 at tools.cisco.com or cisco.com/security-center for specific fixed IOS/IOS XE releases
- Identify the specific Cisco IOS or IOS XE version currently running on affected devices using 'show version'
- Consult Cisco Bug IDs CSCsm45390 and CSCuw77959 in the Cisco Bug Search Tool for patch availability
- Upgrade to the Cisco recommended fixed release for your IOS/IOS XE train (16.x or later typically contains the fix)
- Verify the upgrade in a lab environment before deploying to production
- Apply the upgrade during a maintenance window and have a rollback plan
- After upgrade, verify the DHCP relay functionality is operational
Generated from the published advisory — verify against the referenced sources before acting.
Fix this in iOS
Exploited in the wild — priority engagement
- Consultation6.0 h
- Implementation12.0 h
- Testing6.0 h
- Review / QA3.0 h
An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $7,680.
Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2017-12240 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.
References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.
Primary sourcesPractitioner notes
ContributedPeer-ranked notes from engineers who’ve handled CVE-2017-12240 in production — separate from our analysis above.
The advisory tells you what broke. It rarely tells you what actually worked. If you’ve dealt with this one, that detail is what the next engineer is searching for.
- The version that genuinely resolved it — not the one the vendor claimed
- A config change or rule that shut the vector down
- A gotcha in the upgrade path that cost you an afternoon
No notes yet
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- 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
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