Securing Open Source Enterprise VoIP Christian Stredicke/snom

September 10-12, 2007 Los Angeles Convention Center Los Angeles, California 3 SIP is the ketchup of the burger Finally the VoIP industry is splitting up into layers: SIP is the ketchup that makes it a tasty combination Hosting Consulting ITSP Hard Phones SIP PBX The Past: Everything is provided (more or less) by one large company The Future: Specialized vendors offering excellent products in a specific area Problem: Products are getting very complex and it is hard to stay competitive ATA SBC Soft Phones IVR

September 10-12, 2007 Los Angeles Convention Center Los Angeles, California 4 Selling Security There is probably no company without firewall any more –Security for and Web is a must have today –Administrators who don‘t understand that are jobless Offer two contracts –One where you make the customer responsible for all security breaks (system without security) –Another one where they just waive your liability (system with security) They will pick the contract that includes security

September 10-12, 2007 Los Angeles Convention Center Los Angeles, California 5 The Evolution of VoIP Privacy “We got transfer working” Use SRTP (but no TLS) VPN TLS + SRTP

September 10-12, 2007 Los Angeles Convention Center Los Angeles, California 6 How to listen to VoIP calls* Ethernet Switch The LAN is the problem! Tools: arp-sk - ARP Swiss Army Knife Tool arp-scan … ARP * If you are just using plain SIP The PC puts itself into the communication stream by pretending to have same MAC address as the phone (PC are pretty fast these days and respond faster than VoIP phones)

September 10-12, 2007 Los Angeles Convention Center Los Angeles, California 7 SRTP scrambles the voice Ethernet Header IPUDPRTPCodec Ethernet Checksum MAC Ethernet Header IPUDPRTPCodec Ethernet Checksum AES “Counter” X The AES Counter is used for XOR the audio data The MAC is a hash over the codec content and makes sure that only the one who knows the counter value can generate the packet With every packet, the counter is pseudorandomly incremented The key is to negotiate the initial counter value securely

September 10-12, 2007 Los Angeles Convention Center Los Angeles, California 8 Key Exchange Algorithms (so far) Sig. Conf. Forking Media before Answer Shared- key conf. PKI?Rekey Bid-down protectio n MIKEY-PSKNo Yes No*Yes MIKEY-RSANo Yes MIKEY-DHNo Yes MIKEY- DHHMAC No No*Yes MIKEY-RSA-RNoYesNoYes SDESYesYes*NoYesNoYes*No SDES-EMYesYes*Yes NoYesNo EKTYes* Yes NoYes* SDP-DHNo ZRTPNoYes No Yes DTLSNoYes No Yes Source: Dan Wing, Overview of SIP Media Security Options, March 21, 2006 (IETF 65)

September 10-12, 2007 Los Angeles Convention Center Los Angeles, California 9 How TLS works Known from other protocols (https, secure SMTP, …) Looks like TCP from the application point of view Uses strong cryptographical methods (RSA, DH) How can you trust the other side? –Certificates –Must be issued by someone that you trust –Preset list or load the root certificate Problem: –Requires at the very least TCP support (most PBXs don't have this today) –Problems for embedded devices (OpenSSL takes several MB)

September 10-12, 2007 Los Angeles Convention Center Los Angeles, California 10 Is VPN the solution? Very well established Secure Latest generations address latency –UDP or GRE Nice side effects: –No more NAT problems –VPN servers are widely available (OpenVPN) –No more port-playing with national carriers Problems: –Media Relay through the central VPN node –Setup is not as easy as TLS

September 10-12, 2007 Los Angeles Convention Center Los Angeles, California 11 DoS is becoming a pain Brute force attacks: –ping –f (start is several times) –Downloading of s (LOL) –Just don‘t hang up (ENUM) Bad software –INVITE of Death (DoS LOL) –Accepting INVITE without any kind of authentication “If you have Gigabit Ethernet, make sure you can process one million ping packets per second”

September 10-12, 2007 Los Angeles Convention Center Los Angeles, California 12 Simple Steps to Increase Security Put your VoIP network into a VLAN –Give higher priority bits for that LAN –Have a mini-SBC between the LANs –Limit bandwidth on trunk level Set the expectations right –Making phone calls over the public Internet has no QoS –Seriously consider PSTN termination Think about upgrade paths Backup

September 10-12, 2007 Los Angeles Convention Center Los Angeles, California 13 The Bottom Line You must address privacy in the enterprise TLS and SRTP are a good solution VPN is even better as is solves NAT as well Think pessimistic about bandwidth utilization