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iOSOperating system · Cisco

CVE-1999-0162

MEDIUM · 5.0 CVSS v2.0 Published 1998-09-01
Mitigation only
No fix yet — a mitigation exists. There is no fixed release. A documented workaround reduces exposure in the meantime.
See remediation →
59/100
Remediation priority · Elevated
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
The "established" keyword in some Cisco IOS software allowed an attacker to bypass filtering.

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 confidence

The 'established' keyword in Cisco IOS ACLs performs only a basic TCP flag check (specifically the ACK bit) to determine if a packet belongs to an existing connection. An attacker could craft packets with the ACK flag set to bypass ACLs that rely on the 'established' keyword, effectively circumventing the intended return-traffic-only filtering logic.

MitigationMigrate from the 'established' keyword in ACLs to proper stateful firewall inspection (like Cisco's Context-Based Access Control or zone-based policy firewall) to verify actual connection state rather than relying solely on TCP flag inspection.

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:= 11.2

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
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 checks

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

  1. Identify the Cisco IOS version
    Run 'show version' on the Cisco device to obtain the exact IOS version number
    Affected if The device runs Cisco IOS version 11.2 exactly
  2. Locate ACLs using the established keyword
    Run 'show ip access-lists' or examine the running configuration to search for the word 'established' in any ACL entries
    Affected if Any ACL entries contain the 'established' keyword
  3. Verify if vulnerable ACLs are applied to interfaces
    Run 'show ip interfaces' or review interface configurations to determine if ACLs containing 'established' are actively bound to inbound interfaces
    Affected if ACLs with the 'established' keyword are applied to one or more interfaces

A device is affected only if it runs Cisco IOS version 11.2 and has ACLs containing the 'established' keyword actively applied to interfaces, enabling return traffic based solely on the ACK flag check.

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

Migrate from the 'established' keyword in ACLs to proper stateful firewall inspection (like Cisco's Context-Based Access Control or zone-based policy firewall) to verify actual connection state rather than relying solely on TCP flag inspection.

Fix this in iOS Scoped from the published advisory
  • Consultation3.0 h
  • Implementation4.0 h
  • Testing3.0 h
  • Review / QA2.0 h
12.0 hours of engineering $2,130
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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-1999-0162 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