iOSOperating system · Cisco
CVE-2012-3946
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 · uneditedCisco IOS before 15.3(2)S allows remote attackers to bypass interface ACL restrictions in opportunistic circumstances by sending IPv6 packets in an unspecified scenario in which expected packet drops do not occur for "a small percentage" of the packets, aka Bug ID CSCty73682.
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 · moderate confidenceCisco IOS before version 15.3(2)S contains a vulnerability where IPv6 packets can bypass interface ACL (Access Control List) restrictions under specific opportunistic conditions. In certain unspecified scenarios involving IPv6 traffic, a small percentage of packets may not be dropped as expected when they should be blocked by configured ACLs, allowing traffic to pass through that should be denied.
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
<= 15.3CVSS 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
- Authentication
- None
- Confidentiality
- Partial
- Integrity
- None
- Availability
- None
AV:N/AC:L/Au:N/C:P/I:N/A:N
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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Check Cisco IOS versionExecute 'show version' on the device and locate the IOS version string (for example: Cisco IOS Software, Version 15.3(1)T). Compare the version number to the affected range of <= 15.3, noting that versions 15.3(2)S and later are patched.Affected if The device runs Cisco IOS version 15.3 or any version prior to 15.3(2)S
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Verify IPv6 is enabled on interfacesExecute 'show ipv6 interface brief' to list all interfaces with IPv6 enabled. Alternatively, run 'show running-config | include ipv6 enable' to see global IPv6 configuration.Affected if IPv6 is enabled on one or more interfaces (the vulnerability only applies to IPv6 traffic)
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Identify configured IPv6 ACLsExecute 'show ipv6 access-list' to display all IPv6 access control lists configured on the device, or review 'show running-config' for 'ipv6 access-list' entries.Affected if IPv6 ACLs are configured that should filter or block specific traffic flows
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Confirm ACL is applied to an IPv6 interfaceExecute 'show ipv6 interface <interface-id>' and look for the 'IPv6 interface ACL' field in the output, or review the interface configuration with 'show running-config interface <interface-id>' for 'ipv6 traffic-filter' or 'ipv6 access-group' statements.Affected if An IPv6 ACL is applied (inbound or outbound) to an interface that also has IPv6 enabled
The device is affected if it runs Cisco IOS version 15.3 or earlier AND has IPv6 enabled with an ACL applied to an IPv6 interface, as only then can the ACL bypass condition for IPv6 packets be triggered.
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.
From vendor dataUpgrade Cisco IOS to version 15.3(2)S or later to resolve the vulnerability. Until upgraded, implement additional monitoring and logging on IPv6 traffic to detect any ACL bypass attempts.
Fix this in iOS
Scoped from the published advisory
- Consultation4.0 h
- Implementation3.0 h
- Testing4.0 h
- Review / QA2.0 h
An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $3,680.
Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2012-3946 — 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-2012-3946 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
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