CVE-1999-1306: Cisco IOS 9.1 and earlier does not properly handle extended IP access lists when the IP route cache is enab...

Security readout for executives and security teams

This is a legacy Cisco IOS filtering flaw. On IOS 9.1 and earlier, certain extended IP access lists could be bypassed when route caching and the “established” keyword were used together. The business concern is mainly unmanaged or forgotten legacy routing equipment enforcing perimeter rules. Exposure is most likely in very old Cisco IOS 9.1 or earlier routers still operating with extended IP ACLs, route caching enabled, and “established” ACL entries. Current exposure cannot be inferred from the bundle because affected CPEs and vendor fix status are not provided. Set priority based on whether legacy Cisco routers still protect important network boundaries. If none exist, business urgency is low. If affected routers remain in production, prioritize remediation because firewall-like filtering may not enforce intended access restrictions. Mitigation focus: Inventory Cisco IOS devices and identify any running IOS 9.1 or earlier.; Check for extended IP ACLs using “established” with IP route cache enabled.; Consult Cisco or CERT guidance before changing routing or cache behavior..

Prepared

Jul 1, 2026, 09:01 UTC (UTC+00:00)

Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.