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SIP. Greg Nelson Duc Pham. SIP Introduction. Application-layer (signaling) control protocol for initiating a session among users Registrations, invitations, acceptations, and disconnections. SIP Diagram. A. Proxy. Proxy. B. ……. ……. ……. (Registrar local services). Register. Register.

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slide1

SIP

Greg Nelson

Duc Pham

sip introduction
SIP Introduction
  • Application-layer (signaling) control protocol for initiating a session among users
  • Registrations, invitations, acceptations, and disconnections
sip diagram
SIP Diagram

A

Proxy

Proxy

B

…….

…….

…….

(Registrar local services)

Register

Register

200 Ok

200 Ok

Invite

Invite

100 Trying

Invite

100 Trying

180 Ringing

180 Ringing

180 Ringing

200 Ok

200 Ok

200 Ok

Ack

Media Session

Bye

200 Ok

sip acronyms
SIP Acronyms
  • UAC: a user agent client making requests.
  • UAS: a user agent server responding to the requests (the roles of UAC and UAS are logical entities)
  • SS: flexibly preferred to any middle servers: registrar, proxy servers
sip vulnerabilities
SIP Vulnerabilities
  • Proxy Impersonation
  • Message Tampering
  • Session Teardown
    • Spoofed BYEs
  • Denial of Service
    • Malformed packets
    • REGISTER and INVITE flooding
  • Registration Hijacking
sip security
SIP Security
  • Registration hijacking
    • Authenticate originators of requests
  • Proxy impersonation
    • Authenticate servers
  • Message tampering
    • Secure body and certain headers end-to-end
  • Session teardown
    • Authenticate sender of BYE
    • Confidentiality so attacker can’t learn To, From tags
  • Denial of Service
    • Authenticate and authorize registrations
objectives
Objectives
  • Use AVISPA to model basic protocol.
  • Model SIP URI registration and look for registration hijack attacks.
  • Model interdomain session setup and look for message tampering and proxy impersonation attacks.
  • Add proxy-to-proxy authentication and secrecy (TLS) to model.
  • Discuss other vulnerabilities that we weren’t able to model.
sip model 1
SIP Model (1)

Simplified message formats:

  • REGISTER
  • OK
  • INVITE
  • BYE
  • ACK
sip model 2
SIP Model (2)
  • Register:

UAC -> SS: {sipregister.Ns}

SS -> UAC: {sipok}

  • Invite, connect, bye:

UAC -> SS: {sipinvite.UAC.UAS.{Ni}}

SS -> UAS: {sipinvite.UAC.UAS.SS.{Ni}}

UAS -> SS: {sipok.UAS.UAC.SS.{Nj}}

SS -> UAC: {sipok.UAS.UAC.{Nj}}

UAC -> UAS: {sipack.UAC.UAS}

UAC -> UAS: {sipbye.UAS.UAC}

UAS -> UAC: {sipok.UAC.UAS}

discussion authentication
Discussion --- Authentication
  • Server Authentication: using TLS: server offers a certificate to the UA, preventing proxy impersonating
  • User Authentication: using HTTP digest: server challenges a user with a 401 Proxy Authentication, preventing registration hijacking
discussion interdomain authentication
Discussion --- Interdomain Authentication
  • Trust relationship needed: client-server, server-server

UAC <-> SS <-> UAS

UAC <-> SS <-> Evil

UAC <-> SS <-> SS <->Evil

  • More infrastructures required for absolute interdomain authentication: signature verifications, voice recognitions
discussion message secrecy
Discussion --- Message Secrecy
  • Mechanisms that rely on existence of end-user certificates are seriously limited (S/MIME).
  • May use self-signed certificates
    • Susceptible to obvious MITM attack, but…
    • Attacker can only exploit on initial key exchange.
    • Difficult for attacker to remain in path of all future dialogs.
    • Same vulnerability in SSH => key fingerprints.
    • For VoIP, users could read off key fingerprint.
  • Or, use preconfigured certificates when there is an established trust between all SIP entities.
discussion dos attacks
Discussion --- DoS Attacks
  • Floods of messages directed at proxies can lock up resources on the server.
  • UAs and proxies should challenge questionable requests.
  • Mutual authentication of proxies (TLS)
    • Reduces potential for intermediaries to introduce falsified requests or responses.
    • Harder for attackers to make innocent SIP nodes into agents of amplification.
conclusions
Conclusions
  • AVISPA is easy to use, but difficult to model something besides secrecy and authentication, such as DoS.
  • Registration hijacks are easy to prevent with server authentication (TLS).
  • TLS prevents MITM, but does nothing if proxy is evil; need end-to-end encryption.
  • Simple protocol becomes very complex when addressing vulnerabilities.
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