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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 24 Mar 2022.
iOSOperating system · Cisco

CVE-2017-12231

HIGH · 7.5 CVSS v3.1 Published 2017-09-29
Fix available
A fix is available. Upgrade to after 15.6 or later.
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 implementation of Network Address Translation (NAT) functionality in Cisco IOS 12.4 through 15.6 could allow an unauthenticated, remote attacker to cause a denial of service (DoS) condition on an affected device. The vulnerability is due to the improper translation of H.323 messages that use the Registration, Admission, and Status (RAS) protocol and are sent to an affected device via IPv4 packets. An attacker could exploit this vulnerability by sending a crafted H.323 RAS packet through an affected device. A successful exploit could allow the attacker to cause the affected device to crash and reload, resulting in a DoS condition. This vulnerability affects Cisco devices that are configured to use an application layer gateway with NAT (NAT ALG) for H.323 RAS messages. By default, a NAT ALG is enabled for H.323 RAS messages. Cisco Bug IDs: CSCvc57217.

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

A vulnerability in Cisco IOS NAT implementation allows unauthenticated remote attackers to cause denial of service by sending crafted H.323 RAS (Registration, Admission, and Status) protocol packets through affected devices. The improper translation of these packets triggers a device crash and reload, exploiting the NAT Application Layer Gateway (NAT ALG) for H.323 RAS, which is enabled by default.

MitigationDisable the NAT ALG for H.323 RAS messages or upgrade to a Cisco IOS version that addresses this vulnerability; alternatively, implement packet filtering to block malicious H.323 RAS traffic at network perimeter.

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:>= 12.4, <= 15.6

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 version
    Run 'show version' and locate the version number in the output (the first line typically shows 'Cisco IOS Software, Version ...')
    Affected if The version number falls within 12.4 to 15.6 (inclusive)
  2. Verify NAT is enabled
    Run 'show running-config | include ip nat' or 'show ip nat statistics'
    Affected if NAT configuration appears in the running config or NAT statistics show active translations
  3. Confirm H.323 ALG is not disabled
    Run 'show running-config | include h323' or check if the device has NAT ALG for H.323 explicitly disabled (commands like 'no ip nat service h323 alg' would indicate it is disabled)
    Affected if The H.323 ALG is not explicitly disabled, meaning it remains enabled (this is the default state when NAT is active)
  4. Identify if H.323 RAS traffic is processed
    Run 'show ip nat translations' or 'show ip nat statistics' to see if there are any NAT translations involving H.323 ports (1718, 1719, 1720)
    Affected if H.323 RAS packets are being translated through the device's NAT

You are affected if you run Cisco IOS version 12.4 through 15.6, have NAT enabled with the default H.323 ALG, and your device processes H.323 RAS traffic through its NAT translation table.

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
Upgrade available Upgrade to a release after 15.6
Interim mitigation

Disable the NAT ALG for H.323 RAS messages or upgrade to a Cisco IOS version that addresses this vulnerability; alternatively, implement packet filtering to block malicious H.323 RAS traffic at network perimeter.

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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-2017-12231 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