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Re: Making unidirectional VLAN and PVLAN jumping bidirectional
From: Clayton Kossmeyer <ckossmey () cisco com>
Date: Mon, 19 Dec 2005 17:26:16 -0500

Hash: SHA1

Cisco Response

This is Cisco PSIRT's response to the statements made by Arhont
Ltd. in their message: Making unidirectional VLAN and PVLAN jumping
bidirectional, posted on 2005-Dec-19. An archived version of the
report can be found here:


Cisco confirms the statements made.

We would like to thank Arhont Ltd. for reporting this issue to us.

We greatly appreciate the opportunity to work with researchers on
security vulnerabilities, and welcome the opportunity to review and
assist in product reports.

Additional Information

Cisco is aware of VLAN spoofing attacks and recommends that customers
apply best practices where possible to reduce the impact of such
attacks on their networks. Many best practices are discussed in Cisco's
SAFE Blueprint for Layer 2 security:


As mentioned in the Arhont advisory, this is a protocol issue with
802.1q VLANS, and not a vendor-specific issue. However, there are
techniques available on Cisco devices that may allow you to reduce your
exposure to the mentioned attacks.

The Cisco SAFE Blueprint for Layer 2 security discusses double tagging
attacks here:


The recommended configuration is to disable 802.1q trunking everywhere
it is not required so that tagged frames are discarded on ports not
configured for trunking.

The publication by Arhont also leverages an IP spoofing component to
enable the attack. Cisco recommends IP anti-spoofing techniques and
features such as Unicast Reverse Path Forwarding (uRPF) to guard
against spoofed IP packets.

The Unicast Reverse Path Forwarding (Unicast RPF) feature helps to
mitigate problems that are caused by spoofed IP source addresses. It is
available on Cisco routers and firewalls. For further details, please
refer to:


By enabling Unicast Reverse Path Forwarding (uRPF), all spoofed packets
will be dropped at the first device. To enable uRPF, use the following

router(config)# ip cef
router(config)# interface
router(config-if)# ip verify unicast reverse-path

Cisco Security Procedures

Complete information on reporting security vulnerabilities in Cisco
products, obtaining assistance with security incidents, and
registering to receive security information from Cisco, is available
on Cisco's worldwide website at
http://www.cisco.com/en/US/products/products_security_vulnerability_policy.html. This
includes instructions for press inquiries regarding Cisco security
notices. All Cisco security advisories are available at
Version: GnuPG v1.4.0 (SunOS)


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