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VoIP Phones - Cisco 7xxx

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ALT=Cisco7940

Phones and Versions Tested

Cisco 7xxx Versions tested
Model Version Notes
7940 3.8.6


7940G SIP41.8-5-4S Appears stable


7940G SIP41.9-4-2SR1-1S Phone runs sluggish, reboots randomly, not recommended
Feature Notes
Supports 302 Redirect ?
Tested on FireBrick SIP Server Yes
IPv6 Support No


* Important *

Whilst testing SIP41.8-5-4S it appears the phone/firmware is vulnerable to a SIP/UDP amplification attack where crafted SIP 'INVITE' packets sent on UDP 5600 cause the phone to respond to its upstream SIP gateway with multiple SIP 2.0 '404' packets. Measured at a rate of approximately 20 packets/second, whilst not a major cause for concern with a single phone could potentially cause service issues where several phones are in use.

This vulnerability is protected against by correct firewall protection/filtering limiting inbound UDP on SIP port 5600 to the phone to be exclusively from the parent SIP service (e.g. voiceless.aa.net.uk - see the VoIP security/firewall section). It is not recommended to leave these phones connected to the internet without any protection.

Configuration

Firewall & Security

  • You will also want to set up firewall rules, as per the VoIP Firewall page.
  • Also see the VoIP Security page for information about securing your VoIP service.