SIP Security Mechanisms Through a secure Software Engineering approach Prajwalan Karanjit

Agenda Introduction Security Issues during phases of SDLC – Requirements Engineering – System Design – Implementation – Testing Open Issues

Introduction SIP  Session Management Protocol, Signalling/Call Control Protocol Many Applications: – VoIP, Distributed Classroom, Virtual Meeting, Shared Whiteboard, Publish/Subscribe based applications etc... Vulnerable to several attacks – Attacks on confidentiality/secrecy (sniffing, traffic analysis) – Attacks on integrity (message tampering, hijack REGISTER) – Attacks on authenticity (spoofing) – Attacks on availability (message fabrication, replay, false proxy ) Countermeasures – TLS/SSL, S/MIME, Digest Authentication, IPSec, and many others...

Introduction But, is just considering countermeasures enough ? – No, Not at all – A systematic and disciplined approach of development – Security aspects need to considered in all phases of Software Development Life Cycle (SDLC) SQUARE (Security Quality Requirements Engineering) UMLSec – Safety critical systems require Spiral Model R. Anderson. Security Engineering: A Guide to Building Dependable Distributed Systems. Wiley, – What about other models ? Waterfall, Agile Development Methods, eXtreme Programming  Software Engineering

Boehm Spiral Model Requirements Engineering System Design Implementation (Coding) System Testing and Customer Review

Requirements Engineering What could an end user expect ? – Call should be established with right callee – Media will remain confidential throughout the path – Third party should not be able to see who is calling whom – Caller is charged correctly – It is possible to make calls when caller wants Security goals – Maintain confidentiality (Encryption) – Integrity protection ( MAC ) – DoS protection ( Deal with replay,... ) – Entities involved are authenticated ( Both caller and callee )

Requirements Engineering Attacks and their impact  Risk ThreatImpact Sniffing the signal messagesLoss of privacy Sniffing the mediaLoss of privacy Message tampering (Impersonating, Hijack REGISTER ) DoS, Loss of Integrity, Incorrect Billing, Unauthorized Access ReplayDoS, Incorrect Billing Spam REGISTER and INVITEDoS Fabricated MessagesDoS, Incorrect Billing, Unauthorized Access Tearing down session (e.g. BYE attack, CANCEL attack) DoS, Incorrect Billing

Requirements Engineering Prioritize the Requirements RequirementPriority Level Connection to correct calleeHigh Proper billingHigh Connection availabilityHigh Signal ProctectionHigh Caller and callee identity protectionHigh / Medium ??? Call QualityMedium Media ProtectionMedium

System Design Several security mechanisms  Which one to go for ? Its important to analyze the tradeoffs associated with each countermeasure – Lets analyze now xyz.com abc.com Trudy

Digest Authentication Authenticating Client – No Encryption  No confidentiality – No guarantee of successful client authenticity  Use Identity Header(RFC 4474) Digest Authentication UA and Proxy Authentication * Source of Picture: Sawda, S., Urien, P.: SIP Security Attacks and Solutions: A state-of-the-art review. IEEE Network, (2006)

Identity Header First Step : Digest Authentication Second Step : From To Call ID Date Cseq Contact SHA - 1 RSA Signature Base 64 Identity Private Key of Proxy INVITE SIP 2.0 From:... To:... Call-ID :... Cseq:... Contact:... Date:... Identity: ”IUSY89KJ%)(HASD=” Identity-Info: ;alg=rsa- sha1 Content-Type:....

TLS/DTLS Provides confidentiality as well as integrity Integrated key management Hop by Hop  Intermediate proxies must be trusted If all the links do not use TLS/DTLS then security may be compromised – ”sips:” URI is used to indicate that TLS must be used. TLS  TCP, DTLS  UDP If signalling is also done over UDP  Only DTLS may be sufficient Using DTLS (without RTP) for media may result into high network overhead PKI Problem

S/MIME Encrypt the MIME body with public key of receiver Provides both confidentiality and integrity Entity authentication Public key of receiver  Obtained through X.509 certificate (signed by trusted CA ) Additional overhead due to PKI All the headers cannot be encrypted  Request URI, Via, Record-Route, To, From, Cseq, Call ID – OR, encrypt all the headers along with body and their hash over header and body + attach unencrypted header End to End

IPSec Network layer security, so protects both TCP and UDP Confidentiality, Integrity, Entity Authentication Integrated Key Management Transparent to application But it is also based on PKI  Deployment challenges – SAD, Administrative Rights, Support by all OS Hop by Hop It seems like application will have to depend on the platform Chang, C., Lu, Y.F., Pang, A.C., Kuo, T.W..: Design and Implementation of SIP Security. LNCS, vol. 3391, pp Springer, Heidelberg (2005)

Secure RTP Basically RTP based media protection Confidentiality  AES in key stream mode Integrity  HMAC-SHA-1 Replay based DoS  Sequence Numbers Low network overhead Key Management Issue – Have to depend on additional SIP signal + TLS or S/MIME – ZRTP  key agreement protocol which performs Diffie-Hellman key exchange for SRTP, No need of PKI, SAS (Short Authentication String) What if media is not RTP ?

So Which one to go with ? Digest + Identity ... S/MIME  PKI problem IPSec  Deployment issues + PKI problem TLS  Trusted Proxies and all intermediate proxis must support it SRTP  Key Distribution issues and supports only RTP DTLS  TLS problems + network overhead if RTP is not used. RequirementSolution Connection to correct calleeTLS/DTLS, S/MIME, IPSec Proper billingDigest, TLS/DTLS, S/MIME, IPSec Connection availability??? Signal ProctectionTLS/DTLS, S/MIME, IPSec Caller and callee identity protectionDigest, TLS/DTLS, S/MIME, IPSec Call Quality??? Media ProtectionDTLS, SRTP, IPSec

Implementation Configuration of different servers such as DNS, Proxies etc... Developing custom proxies, UAs – JAIN SIP + JAIN SDP from Sun Microsystems and NIST – SIP Servlet – SIP API for J2ME from Sun Microsystems and Nokia – As per recent documentation, none provides API support for above security mechanisms  Different API needed – Open SSL  Strictly C based with Unix libraries Java and.NET based API Visual C++ (Win 32 and MFC) – Write Responsible codes

SIP Security Testing Should at least focus on security requirements identified in Requirements Engineering phase Should be performed by sending malformed SIP requests, Sending a lot of authenticated and unauthenticated requests Check of TLS or S/MIME or DTLS or SRTP support Check if deprecated technologies are used RTP or media tampering Buffer overflow and SQL Injection  ???

SIP Security Testing Tools: – SIPp: A free Open Source test tool / traffic generator for the SIP protocol. – PROTOS SIP Fuzzer: - Tool that sends a set of malformed SIP messages. – SiVuS: A SIP Vulnerability Scanner that scans for SIP targets and attacks them – SIPNess: A basic tool to construct, send or receive SIP messages – SIPBomber: A sip-protocol [RFC3261] testing tool – SFTF: A SIP Forum test framework to evaluate and report the security and robustness of a SIP user agent

Open Issues Denial of Service Attacks – Against UA, Proxies – Flooding  Set threshold for each user in the proxy How to know that proxy is authorized for a particular domain – Attribute Certificate – Existing X.509 fields ?? Will the method described in this paper really result into a secure SIP based product ?

Summary SIP is highly vulnerable to several attacks Security Mechanims are there, but each of them have their own tradeoffs – Mainly implementation issues and practical problems Security should be considered through all the phases of SDLC – From Requirements Engineering to Testing