26 Security Considerations: Threat Model and Security Usage
SIP is not an easy protocol to secure. Its use of intermediaries,
its multi-faceted trust relationships, its expected usage between
elements with no trust at all, and its user-to-user operation make
security far from trivial. Security solutions are needed that are
deployable today, without extensive coordination, in a wide variety
of environments and usages. In order to meet these diverse needs,
several distinct mechanisms applicable to different aspects and
usages of SIP will be required.
Note that the security of SIP signaling itself has no bearing on the
security of protocols used in concert with SIP such as RTP, or with
the security implications of any specific bodies SIP might carry
(although MIME security plays a substantial role in securing SIP).
Any media associated with a session can be encrypted end-to-end
independently of any associated SIP signaling. Media encryption is
outside the scope of this document.
The considerations that follow first examine a set of classic threat
models that broadly identify the security needs of SIP. The set of
security services required to address these threats is then detailed,
followed by an explanation of several security mechanisms that can be
used to provide these services. Next, the requirements for
implementers of SIP are enumerated, along with exemplary deployments
in which these security mechanisms could be used to improve the
security of SIP. Some notes on privacy conclude this section.
26.1 Attacks and Threat Models
This section details some threats that should be common to most
deployments of SIP. These threats have been chosen specifically to
illustrate each of the security services that SIP requires.
The following examples by no means provide an exhaustive list of the
threats against SIP; rather, these are "classic" threats that
demonstrate the need for particular security services that can
potentially prevent whole categories of threats.
These attacks assume an environment in which attackers can
potentially read any packet on the network - it is anticipated that
SIP will frequently be used on the public Internet. Attackers on the
network may be able to modify packets (perhaps at some compromised
intermediary). Attackers may wish to steal services, eavesdrop on
communications, or disrupt sessions.
26.1.1 Registration Hijacking
The SIP registration mechanism allows a user agent to identify itself
to a registrar as a device at which a user (designated by an address
of record) is located. A registrar assesses the identity asserted in
the From header field of a REGISTER message to determine whether this
request can modify the contact addresses associated with the
address-of-record in the To header field. While these two fields are
frequently the same, there are many valid deployments in which a
third-party may register contacts on a user's behalf.
The From header field of a SIP request, however, can be modified
arbitrarily by the owner of a UA, and this opens the door to
malicious registrations. An attacker that successfully impersonates
a party authorized to change contacts associated with an address-of-
record could, for example, de-register all existing contacts for a
URI and then register their own device as the appropriate contact
address, thereby directing all requests for the affected user to the
This threat belongs to a family of threats that rely on the absence
of cryptographic assurance of a request's originator. Any SIP UAS
that represents a valuable service (a gateway that interworks SIP
requests with traditional telephone calls, for example) might want to
control access to its resources by authenticating requests that it
receives. Even end-user UAs, for example SIP phones, have an
interest in ascertaining the identities of originators of requests.
This threat demonstrates the need for security services that enable
SIP entities to authenticate the originators of requests.
26.1.2 Impersonating a Server
The domain to which a request is destined is generally specified in
the Request-URI. UAs commonly contact a server in this domain
directly in order to deliver a request. However, there is always a
possibility that an attacker could impersonate the remote server, and
that the UA's request could be intercepted by some other party.
For example, consider a case in which a redirect server at one
domain, chicago.com, impersonates a redirect server at another
domain, biloxi.com. A user agent sends a request to biloxi.com, but
the redirect server at chicago.com answers with a forged response
that has appropriate SIP header fields for a response from
biloxi.com. The forged contact addresses in the redirection response
could direct the originating UA to inappropriate or insecure
resources, or simply prevent requests for biloxi.com from succeeding.
This family of threats has a vast membership, many of which are
critical. As a converse to the registration hijacking threat,
consider the case in which a registration sent to biloxi.com is
intercepted by chicago.com, which replies to the intercepted
registration with a forged 301 (Moved Permanently) response. This
response might seem to come from biloxi.com yet designate chicago.com
as the appropriate registrar. All future REGISTER requests from the
originating UA would then go to chicago.com.
Prevention of this threat requires a means by which UAs can
authenticate the servers to whom they send requests.
26.1.3 Tampering with Message Bodies
As a matter of course, SIP UAs route requests through trusted proxy
servers. Regardless of how that trust is established (authentication
of proxies is discussed elsewhere in this section), a UA may trust a
proxy server to route a request, but not to inspect or possibly
modify the bodies contained in that request.
Consider a UA that is using SIP message bodies to communicate session
encryption keys for a media session. Although it trusts the proxy
server of the domain it is contacting to deliver signaling properly,
it may not want the administrators of that domain to be capable of
decrypting any subsequent media session. Worse yet, if the proxy
server were actively malicious, it could modify the session key,
either acting as a man-in-the-middle, or perhaps changing the
security characteristics requested by the originating UA.
This family of threats applies not only to session keys, but to most
conceivable forms of content carried end-to-end in SIP. These might
include MIME bodies that should be rendered to the user, SDP, or
encapsulated telephony signals, among others. Attackers might
attempt to modify SDP bodies, for example, in order to point RTP
media streams to a wiretapping device in order to eavesdrop on
subsequent voice communications.
Also note that some header fields in SIP are meaningful end-to-end,
for example, Subject. UAs might be protective of these header fields
as well as bodies (a malicious intermediary changing the Subject
header field might make an important request appear to be spam, for
example). However, since many header fields are legitimately
inspected or altered by proxy servers as a request is routed, not all
header fields should be secured end-to-end.
For these reasons, the UA might want to secure SIP message bodies,
and in some limited cases header fields, end-to-end. The security
services required for bodies include confidentiality, integrity, and
authentication. These end-to-end services should be independent of
the means used to secure interactions with intermediaries such as
26.1.4 Tearing Down Sessions
Once a dialog has been established by initial messaging, subsequent
requests can be sent that modify the state of the dialog and/or
session. It is critical that principals in a session can be certain
that such requests are not forged by attackers.
Consider a case in which a third-party attacker captures some initial
messages in a dialog shared by two parties in order to learn the
parameters of the session (To tag, From tag, and so forth) and then
inserts a BYE request into the session. The attacker could opt to
forge the request such that it seemed to come from either
participant. Once the BYE is received by its target, the session
will be torn down prematurely.
Similar mid-session threats include the transmission of forged re-
INVITEs that alter the session (possibly to reduce session security
or redirect media streams as part of a wiretapping attack).
The most effective countermeasure to this threat is the
authentication of the sender of the BYE. In this instance, the
recipient needs only know that the BYE came from the same party with
whom the corresponding dialog was established (as opposed to
ascertaining the absolute identity of the sender). Also, if the
attacker is unable to learn the parameters of the session due to
confidentiality, it would not be possible to forge the BYE. However,
some intermediaries (like proxy servers) will need to inspect those
parameters as the session is established.
26.1.5 Denial of Service and Amplification
Denial-of-service attacks focus on rendering a particular network
element unavailable, usually by directing an excessive amount of
network traffic at its interfaces. A distributed denial-of-service
attack allows one network user to cause multiple network hosts to
flood a target host with a large amount of network traffic.
In many architectures, SIP proxy servers face the public Internet in
order to accept requests from worldwide IP endpoints. SIP creates a
number of potential opportunities for distributed denial-of-service
attacks that must be recognized and addressed by the implementers and
operators of SIP systems.
Attackers can create bogus requests that contain a falsified source
IP address and a corresponding Via header field that identify a
targeted host as the originator of the request and then send this
request to a large number of SIP network elements, thereby using
hapless SIP UAs or proxies to generate denial-of-service traffic
aimed at the target.
Similarly, attackers might use falsified Route header field values in
a request that identify the target host and then send such messages
to forking proxies that will amplify messaging sent to the target.
Record-Route could be used to similar effect when the attacker is
certain that the SIP dialog initiated by the request will result in
numerous transactions originating in the backwards direction.
A number of denial-of-service attacks open up if REGISTER requests
are not properly authenticated and authorized by registrars.
Attackers could de-register some or all users in an administrative
domain, thereby preventing these users from being invited to new
sessions. An attacker could also register a large number of contacts
designating the same host for a given address-of-record in order to
use the registrar and any associated proxy servers as amplifiers in a
denial-of-service attack. Attackers might also attempt to deplete
available memory and disk resources of a registrar by registering
huge numbers of bindings.
The use of multicast to transmit SIP requests can greatly increase
the potential for denial-of-service attacks.
These problems demonstrate a general need to define architectures
that minimize the risks of denial-of-service, and the need to be
mindful in recommendations for security mechanisms of this class of
26.2 Security Mechanisms
From the threats described above, we gather that the fundamental
security services required for the SIP protocol are: preserving the
confidentiality and integrity of messaging, preventing replay attacks
or message spoofing, providing for the authentication and privacy of
the participants in a session, and preventing denial-of-service
attacks. Bodies within SIP messages separately require the security
services of confidentiality, integrity, and authentication.
Rather than defining new security mechanisms specific to SIP, SIP
reuses wherever possible existing security models derived from the
HTTP and SMTP space.
Full encryption of messages provides the best means to preserve the
confidentiality of signaling - it can also guarantee that messages
are not modified by any malicious intermediaries. However, SIP
requests and responses cannot be naively encrypted end-to-end in
their entirety because message fields such as the Request-URI, Route,
and Via need to be visible to proxies in most network architectures
so that SIP requests are routed correctly. Note that proxy servers
need to modify some features of messages as well (such as adding Via
header field values) in order for SIP to function. Proxy servers
must therefore be trusted, to some degree, by SIP UAs. To this
purpose, low-layer security mechanisms for SIP are recommended, which
encrypt the entire SIP requests or responses on the wire on a hop-
by-hop basis, and that allow endpoints to verify the identity of
proxy servers to whom they send requests.
SIP entities also have a need to identify one another in a secure
fashion. When a SIP endpoint asserts the identity of its user to a
peer UA or to a proxy server, that identity should in some way be
verifiable. A cryptographic authentication mechanism is provided in
SIP to address this requirement.
An independent security mechanism for SIP message bodies supplies an
alternative means of end-to-end mutual authentication, as well as
providing a limit on the degree to which user agents must trust
26.2.1 Transport and Network Layer Security
Transport or network layer security encrypts signaling traffic,
guaranteeing message confidentiality and integrity.
Oftentimes, certificates are used in the establishment of lower-layer
security, and these certificates can also be used to provide a means
of authentication in many architectures.
Two popular alternatives for providing security at the transport and
network layer are, respectively, TLS  and IPSec .
IPSec is a set of network-layer protocol tools that collectively can
be used as a secure replacement for traditional IP (Internet
Protocol). IPSec is most commonly used in architectures in which a
set of hosts or administrative domains have an existing trust
relationship with one another. IPSec is usually implemented at the
operating system level in a host, or on a security gateway that
provides confidentiality and integrity for all traffic it receives
from a particular interface (as in a VPN architecture). IPSec can
also be used on a hop-by-hop basis.
In many architectures IPSec does not require integration with SIP
applications; IPSec is perhaps best suited to deployments in which
adding security directly to SIP hosts would be arduous. UAs that
have a pre-shared keying relationship with their first-hop proxy
server are also good candidates to use IPSec. Any deployment of
IPSec for SIP would require an IPSec profile describing the protocol
tools that would be required to secure SIP. No such profile is given
in this document.
TLS provides transport-layer security over connection-oriented
protocols (for the purposes of this document, TCP); "tls" (signifying
TLS over TCP) can be specified as the desired transport protocol
within a Via header field value or a SIP-URI. TLS is most suited to
architectures in which hop-by-hop security is required between hosts
with no pre-existing trust association. For example, Alice trusts
her local proxy server, which after a certificate exchange decides to
trust Bob's local proxy server, which Bob trusts, hence Bob and Alice
can communicate securely.
TLS must be tightly coupled with a SIP application. Note that
transport mechanisms are specified on a hop-by-hop basis in SIP, thus
a UA that sends requests over TLS to a proxy server has no assurance
that TLS will be used end-to-end.
The TLS_RSA_WITH_AES_128_CBC_SHA ciphersuite  MUST be supported at
a minimum by implementers when TLS is used in a SIP application. For
purposes of backwards compatibility, proxy servers, redirect servers,
and registrars SHOULD support TLS_RSA_WITH_3DES_EDE_CBC_SHA.
Implementers MAY also support any other ciphersuite.
26.2.2 SIPS URI Scheme
The SIPS URI scheme adheres to the syntax of the SIP URI (described
in 19), although the scheme string is "sips" rather than "sip". The
semantics of SIPS are very different from the SIP URI, however. SIPS
allows resources to specify that they should be reached securely.
A SIPS URI can be used as an address-of-record for a particular user
- the URI by which the user is canonically known (on their business
cards, in the From header field of their requests, in the To header
field of REGISTER requests). When used as the Request-URI of arequest, the SIPS scheme signifies that each hop over which therequest is forwarded, until the request reaches the SIP entityresponsible for the domain portion of the Request-URI, must besecured with TLS; once it reaches the domain in question it ishandled in accordance with local security and routing policy, quitepossibly using TLS for any last hop to a UAS. When used by theoriginator of a request (as would be the case if they employed a SIPSURI as the address-of-record of the target), SIPS dictates that theentire request path to the target domain be so secured.
When used as the Request-URI of a request, the SIPS scheme
signifies that each hop over which the request is forwarded, until
the request reaches the resource identified by the Request-URI, is
secured with TLS. When used by the originator of a request (as
would be the case if they employed a SIPS URI as the address-of-
record of the target), SIPS dictates that the entire request path
to the target domain be so secured.
The SIPS scheme is applicable to many of the other ways in which SIP
URIs are used in SIP today in addition to the Request-URI, including
in addresses-of-record, contact addresses (the contents of Contact
headers, including those of REGISTER methods), and Route headers. In
each instance, the SIPS URI scheme allows these existing fields to
designate secure resources. The manner in which a SIPS URI is
dereferenced in any of these contexts has its own security properties
which are detailed in .
The use of SIPS in particular entails that mutual TLS authentication
SHOULD be employed, as SHOULD the ciphersuite
TLS_RSA_WITH_AES_128_CBC_SHA. Certificates received in the
authentication process SHOULD be validated with root certificates
held by the client; failure to validate a certificate SHOULD result
in the failure of the request.
Note that in the SIPS URI scheme, transport is independent of TLS,
and thus "sips:email@example.com;transport=tcp" and
"sips:firstname.lastname@example.org;transport=sctp" are both valid (although
note that UDP is not a valid transport for SIPS). The use of
"transport=tls" has consequently been deprecated, partly because
it was specific to a single hop of the request. This is a change
since RFC 2543.
Users that distribute a SIPS URI as an address-of-record may elect to
operate devices that refuse requests over insecure transports.
26.2.3 HTTP Authentication
SIP provides a challenge capability, based on HTTP authentication,
that relies on the 401 and 407 response codes as well as header
fields for carrying challenges and credentials. Without significant
modification, the reuse of the HTTP Digest authentication scheme in
SIP allows for replay protection and one-way authentication.
The usage of Digest authentication in SIP is detailed in Section 22.
As is discussed above, encrypting entire SIP messages end-to-end for
the purpose of confidentiality is not appropriate because network
intermediaries (like proxy servers) need to view certain header
fields in order to route messages correctly, and if these
intermediaries are excluded from security associations, then SIP
messages will essentially be non-routable.
However, S/MIME allows SIP UAs to encrypt MIME bodies within SIP,
securing these bodies end-to-end without affecting message headers.
S/MIME can provide end-to-end confidentiality and integrity for
message bodies, as well as mutual authentication. It is also
possible to use S/MIME to provide a form of integrity and
confidentiality for SIP header fields through SIP message tunneling.
The usage of S/MIME in SIP is detailed in Section 23.
26.3 Implementing Security Mechanisms
26.3.1 Requirements for Implementers of SIP
Proxy servers, redirect servers, and registrars MUST implement TLS,
and MUST support both mutual and one-way authentication. It is
strongly RECOMMENDED that UAs be capable initiating TLS; UAs MAY also
be capable of acting as a TLS server. Proxy servers, redirect
servers, and registrars SHOULD possess a site certificate whose
subject corresponds to their canonical hostname. UAs MAY have
certificates of their own for mutual authentication with TLS, but no
provisions are set forth in this document for their use. All SIP
elements that support TLS MUST have a mechanism for validating
certificates received during TLS negotiation; this entails possession
of one or more root certificates issued by certificate authorities
(preferably well-known distributors of site certificates comparable
to those that issue root certificates for web browsers).
All SIP elements that support TLS MUST also support the SIPS URI
Proxy servers, redirect servers, registrars, and UAs MAY also
implement IPSec or other lower-layer security protocols.
When a UA attempts to contact a proxy server, redirect server, or
registrar, the UAC SHOULD initiate a TLS connection over which it
will send SIP messages. In some architectures, UASs MAY receive
requests over such TLS connections as well.
Proxy servers, redirect servers, registrars, and UAs MUST implement
Digest Authorization, encompassing all of the aspects required in 22.
Proxy servers, redirect servers, and registrars SHOULD be configured
with at least one Digest realm, and at least one "realm" string
supported by a given server SHOULD correspond to the server's
hostname or domainname.
UAs MAY support the signing and encrypting of MIME bodies, and
transference of credentials with S/MIME as described in Section 23.
If a UA holds one or more root certificates of certificate
authorities in order to validate certificates for TLS or IPSec, it
SHOULD be capable of reusing these to verify S/MIME certificates, as
appropriate. A UA MAY hold root certificates specifically for
validating S/MIME certificates.
Note that is it anticipated that future security extensions may
upgrade the normative strength associated with S/MIME as S/MIME
implementations appear and the problem space becomes better
26.3.2 Security Solutions
The operation of these security mechanisms in concert can follow the
existing web and email security models to some degree. At a high
level, UAs authenticate themselves to servers (proxy servers,
redirect servers, and registrars) with a Digest username and
password; servers authenticate themselves to UAs one hop away, or to
another server one hop away (and vice versa), with a site certificate
delivered by TLS.
On a peer-to-peer level, UAs trust the network to authenticate one
another ordinarily; however, S/MIME can also be used to provide
direct authentication when the network does not, or if the network
itself is not trusted.
The following is an illustrative example in which these security
mechanisms are used by various UAs and servers to prevent the sorts
of threats described in Section 26.1. While implementers and network
administrators MAY follow the normative guidelines given in the
remainder of this section, these are provided only as example
When a UA comes online and registers with its local administrative
domain, it SHOULD establish a TLS connection with its registrar
(Section 10 describes how the UA reaches its registrar). The
registrar SHOULD offer a certificate to the UA, and the site
identified by the certificate MUST correspond with the domain in
which the UA intends to register; for example, if the UA intends to
register the address-of-record 'email@example.com', the site
certificate must identify a host within the atlanta.com domain (such
as sip.atlanta.com). When it receives the TLS Certificate message,
the UA SHOULD verify the certificate and inspect the site identified
by the certificate. If the certificate is invalid, revoked, or if it
does not identify the appropriate party, the UA MUST NOT send the
REGISTER message and otherwise proceed with the registration.
When a valid certificate has been provided by the registrar, the
UA knows that the registrar is not an attacker who might redirect
the UA, steal passwords, or attempt any similar attacks.
The UA then creates a REGISTER request that SHOULD be addressed to a
Request-URI corresponding to the site certificate received from the
registrar. When the UA sends the REGISTER request over the existing
TLS connection, the registrar SHOULD challenge the request with a 401
(Proxy Authentication Required) response. The "realm" parameter
within the Proxy-Authenticate header field of the response SHOULD
correspond to the domain previously given by the site certificate.
When the UAC receives the challenge, it SHOULD either prompt the user
for credentials or take an appropriate credential from a keyring
corresponding to the "realm" parameter in the challenge. The
username of this credential SHOULD correspond with the "userinfo"
portion of the URI in the To header field of the REGISTER request.
Once the Digest credentials have been inserted into an appropriate
Proxy-Authorization header field, the REGISTER should be resubmitted
to the registrar.
Since the registrar requires the user agent to authenticate
itself, it would be difficult for an attacker to forge REGISTER
requests for the user's address-of-record. Also note that since
the REGISTER is sent over a confidential TLS connection, attackers
will not be able to intercept the REGISTER to record credentials
for any possible replay attack.
Once the registration has been accepted by the registrar, the UA
SHOULD leave this TLS connection open provided that the registrar
also acts as the proxy server to which requests are sent for users in
this administrative domain. The existing TLS connection will be
reused to deliver incoming requests to the UA that has just completed
Because the UA has already authenticated the server on the other
side of the TLS connection, all requests that come over this
connection are known to have passed through the proxy server -
attackers cannot create spoofed requests that appear to have been
sent through that proxy server.
18.104.22.168 Interdomain Requests
Now let's say that Alice's UA would like to initiate a session with a
user in a remote administrative domain, namely "firstname.lastname@example.org". We
will also say that the local administrative domain (atlanta.com) has
a local outbound proxy.
The proxy server that handles inbound requests for an administrative
domain MAY also act as a local outbound proxy; for simplicity's sake
we'll assume this to be the case for atlanta.com (otherwise the user
agent would initiate a new TLS connection to a separate server at
this point). Assuming that the client has completed the registration
process described in the preceding section, it SHOULD reuse the TLS
connection to the local proxy server when it sends an INVITE request
to another user. The UA SHOULD reuse cached credentials in the
INVITE to avoid prompting the user unnecessarily.
When the local outbound proxy server has validated the credentials
presented by the UA in the INVITE, it SHOULD inspect the Request-URI
to determine how the message should be routed (see ). If the
"domainname" portion of the Request-URI had corresponded to the local
domain (atlanta.com) rather than biloxi.com, then the proxy server
would have consulted its location service to determine how best to
reach the requested user.
Had "email@example.com" been attempting to contact, say,
"firstname.lastname@example.org", the local proxy would have proxied to the
request to the TLS connection Alex had established with the
registrar when he registered. Since Alex would receive this
request over his authenticated channel, he would be assured that
Alice's request had been authorized by the proxy server of the
local administrative domain.
However, in this instance the Request-URI designates a remote domain.
The local outbound proxy server at atlanta.com SHOULD therefore
establish a TLS connection with the remote proxy server at
biloxi.com. Since both of the participants in this TLS connection
are servers that possess site certificates, mutual TLS authentication
SHOULD occur. Each side of the connection SHOULD verify and inspect
the certificate of the other, noting the domain name that appears in
the certificate for comparison with the header fields of SIP
messages. The atlanta.com proxy server, for example, SHOULD verify
at this stage that the certificate received from the remote side
corresponds with the biloxi.com domain. Once it has done so, and TLS
negotiation has completed, resulting in a secure channel between the
two proxies, the atlanta.com proxy can forward the INVITE request to
The proxy server at biloxi.com SHOULD inspect the certificate of the
proxy server at atlanta.com in turn and compare the domain asserted
by the certificate with the "domainname" portion of the From header
field in the INVITE request. The biloxi proxy MAY have a strict
security policy that requires it to reject requests that do not match
the administrative domain from which they have been proxied.
Such security policies could be instituted to prevent the SIP
equivalent of SMTP 'open relays' that are frequently exploited to
This policy, however, only guarantees that the request came from the
domain it ascribes to itself; it does not allow biloxi.com to
ascertain how atlanta.com authenticated Alice. Only if biloxi.com
has some other way of knowing atlanta.com's authentication policies
could it possibly ascertain how Alice proved her identity.
biloxi.com might then institute an even stricter policy that forbids
requests that come from domains that are not known administratively
to share a common authentication policy with biloxi.com.
Once the INVITE has been approved by the biloxi proxy, the proxy
server SHOULD identify the existing TLS channel, if any, associated
with the user targeted by this request (in this case
"email@example.com"). The INVITE should be proxied through this channel
to Bob. Since the request is received over a TLS connection that had
previously been authenticated as the biloxi proxy, Bob knows that the
From header field was not tampered with and that atlanta.com has
validated Alice, although not necessarily whether or not to trust
Before they forward the request, both proxy servers SHOULD add a
Record-Route header field to the request so that all future requests
in this dialog will pass through the proxy servers. The proxy
servers can thereby continue to provide security services for the
lifetime of this dialog. If the proxy servers do not add themselves
to the Record-Route, future messages will pass directly end-to-end
between Alice and Bob without any security services (unless the two
parties agree on some independent end-to-end security such as
S/MIME). In this respect the SIP trapezoid model can provide a nice
structure where conventions of agreement between the site proxies can
provide a reasonably secure channel between Alice and Bob.
An attacker preying on this architecture would, for example, be
unable to forge a BYE request and insert it into the signaling
stream between Bob and Alice because the attacker has no way of
ascertaining the parameters of the session and also because the
integrity mechanism transitively protects the traffic between
Alice and Bob.
22.214.171.124 Peer-to-Peer Requests
Alternatively, consider a UA asserting the identity
"firstname.lastname@example.org" that has no local outbound proxy. When Carol
wishes to send an INVITE to "email@example.com", her UA SHOULD initiate
a TLS connection with the biloxi proxy directly (using the mechanism
described in  to determine how to best to reach the given
Request-URI). When her UA receives a certificate from the biloxi
proxy, it SHOULD be verified normally before she passes her INVITE
across the TLS connection. However, Carol has no means of proving
her identity to the biloxi proxy, but she does have a CMS-detached
signature over a "message/sip" body in the INVITE. It is unlikely in
this instance that Carol would have any credentials in the biloxi.com
realm, since she has no formal association with biloxi.com. The
biloxi proxy MAY also have a strict policy that precludes it from
even bothering to challenge requests that do not have biloxi.com in
the "domainname" portion of the From header field - it treats these
users as unauthenticated.
The biloxi proxy has a policy for Bob that all non-authenticated
requests should be redirected to the appropriate contact address
registered against 'firstname.lastname@example.org', namely <sip:email@example.com>.
Carol receives the redirection response over the TLS connection she
established with the biloxi proxy, so she trusts the veracity of the
Carol SHOULD then establish a TCP connection with the designated
address and send a new INVITE with a Request-URI containing the
received contact address (recomputing the signature in the body as
the request is readied). Bob receives this INVITE on an insecure
interface, but his UA inspects and, in this instance, recognizes the
From header field of the request and subsequently matches a locally
cached certificate with the one presented in the signature of the
body of the INVITE. He replies in similar fashion, authenticating
himself to Carol, and a secure dialog begins.
Sometimes firewalls or NATs in an administrative domain could
preclude the establishment of a direct TCP connection to a UA. In
these cases, proxy servers could also potentially relay requests
to UAs in a way that has no trust implications (for example,
forgoing an existing TLS connection and forwarding the request
over cleartext TCP) as local policy dictates.
126.96.36.199 DoS Protection
In order to minimize the risk of a denial-of-service attack against
architectures using these security solutions, implementers should
take note of the following guidelines.
When the host on which a SIP proxy server is operating is routable
from the public Internet, it SHOULD be deployed in an administrative
domain with defensive operational policies (blocking source-routed
traffic, preferably filtering ping traffic). Both TLS and IPSec can
also make use of bastion hosts at the edges of administrative domains
that participate in the security associations to aggregate secure
tunnels and sockets. These bastion hosts can also take the brunt of
denial-of-service attacks, ensuring that SIP hosts within the
administrative domain are not encumbered with superfluous messaging.
No matter what security solutions are deployed, floods of messages
directed at proxy servers can lock up proxy server resources and
prevent desirable traffic from reaching its destination. There is a
computational expense associated with processing a SIP transaction at
a proxy server, and that expense is greater for stateful proxy
servers than it is for stateless proxy servers. Therefore, stateful
proxies are more susceptible to flooding than stateless proxy
UAs and proxy servers SHOULD challenge questionable requests with
only a single 401 (Unauthorized) or 407 (Proxy Authentication
Required), forgoing the normal response retransmission algorithm, and
thus behaving statelessly towards unauthenticated requests.
Retransmitting the 401 (Unauthorized) or 407 (Proxy Authentication
Required) status response amplifies the problem of an attacker
using a falsified header field value (such as Via) to direct
traffic to a third party.
In summary, the mutual authentication of proxy servers through
mechanisms such as TLS significantly reduces the potential for rogue
intermediaries to introduce falsified requests or responses that can
deny service. This commensurately makes it harder for attackers to
make innocent SIP nodes into agents of amplification.
Although these security mechanisms, when applied in a judicious
manner, can thwart many threats, there are limitations in the scope
of the mechanisms that must be understood by implementers and network
26.4.1 HTTP Digest
One of the primary limitations of using HTTP Digest in SIP is that
the integrity mechanisms in Digest do not work very well for SIP.
Specifically, they offer protection of the Request-URI and the method
of a message, but not for any of the header fields that UAs would
most likely wish to secure.
The existing replay protection mechanisms described in RFC 2617 also
have some limitations for SIP. The next-nonce mechanism, for
example, does not support pipelined requests. The nonce-count
mechanism should be used for replay protection.
Another limitation of HTTP Digest is the scope of realms. Digest is
valuable when a user wants to authenticate themselves to a resource
with which they have a pre-existing association, like a service
provider of which the user is a customer (which is quite a common
scenario and thus Digest provides an extremely useful function). By
way of contrast, the scope of TLS is interdomain or multirealm, since
certificates are often globally verifiable, so that the UA can
authenticate the server with no pre-existing association.
The largest outstanding defect with the S/MIME mechanism is the lack
of a prevalent public key infrastructure for end users. If self-
signed certificates (or certificates that cannot be verified by one
of the participants in a dialog) are used, the SIP-based key exchange
mechanism described in Section 23.2 is susceptible to a man-in-the-
middle attack with which an attacker can potentially inspect and
modify S/MIME bodies. The attacker needs to intercept the first
exchange of keys between the two parties in a dialog, remove the
existing CMS-detached signatures from the request and response, and
insert a different CMS-detached signature containing a certificate
supplied by the attacker (but which seems to be a certificate for the
proper address-of-record). Each party will think they have exchanged
keys with the other, when in fact each has the public key of the
It is important to note that the attacker can only leverage this
vulnerability on the first exchange of keys between two parties - on
subsequent occasions, the alteration of the key would be noticeable
to the UAs. It would also be difficult for the attacker to remain in
the path of all future dialogs between the two parties over time (as
potentially days, weeks, or years pass).
SSH is susceptible to the same man-in-the-middle attack on the first
exchange of keys; however, it is widely acknowledged that while SSH
is not perfect, it does improve the security of connections. The use
of key fingerprints could provide some assistance to SIP, just as it
does for SSH. For example, if two parties use SIP to establish a
voice communications session, each could read off the fingerprint of
the key they received from the other, which could be compared against
the original. It would certainly be more difficult for the man-in-
the-middle to emulate the voices of the participants than their
signaling (a practice that was used with the Clipper chip-based
The S/MIME mechanism allows UAs to send encrypted requests without
preamble if they possess a certificate for the destination address-
of-record on their keyring. However, it is possible that any
particular device registered for an address-of-record will not hold
the certificate that has been previously employed by the device's
current user, and that it will therefore be unable to process an
encrypted request properly, which could lead to some avoidable error
signaling. This is especially likely when an encrypted request is
The keys associated with S/MIME are most useful when associated with
a particular user (an address-of-record) rather than a device (a UA).
When users move between devices, it may be difficult to transport
private keys securely between UAs; how such keys might be acquired by
a device is outside the scope of this document.
Another, more prosaic difficulty with the S/MIME mechanism is that it
can result in very large messages, especially when the SIP tunneling
mechanism described in Section 23.4 is used. For that reason, it is
RECOMMENDED that TCP should be used as a transport protocol when
S/MIME tunneling is employed.
The most commonly voiced concern about TLS is that it cannot run over
UDP; TLS requires a connection-oriented underlying transport
protocol, which for the purposes of this document means TCP.
It may also be arduous for a local outbound proxy server and/or
registrar to maintain many simultaneous long-lived TLS connections
with numerous UAs. This introduces some valid scalability concerns,
especially for intensive ciphersuites. Maintaining redundancy of
long-lived TLS connections, especially when a UA is solely
responsible for their establishment, could also be cumbersome.
TLS only allows SIP entities to authenticate servers to which they
are adjacent; TLS offers strictly hop-by-hop security. Neither TLS,
nor any other mechanism specified in this document, allows clients to
authenticate proxy servers to whom they cannot form a direct TCP
26.4.4 SIPS URIs
Actually using TLS on every segment of a request path entails thatthe terminating UAS must be reachable over TLS (perhaps registeringwith a SIPS URI as a contact address). This is the preferred use ofSIPS. Many valid architectures, however, use TLS to secure part ofthe request path, but rely on some other mechanism for the final hopto a UAS, for example. Thus SIPS cannot guarantee that TLS usagewill be truly end-to-end. Note that since many UAs will not acceptincoming TLS connections, even those UAs that do support TLS may berequired to maintain persistent TLS connections as described in theTLS limitations section above in order to receive requests over TLSas a UAS.
Actually using TLS on every segment of a request path entails that
the terminating UAS is reachable over TLS (by registering with a
SIPS URI as a contact address). The SIPS scheme implies
transitive trust. Obviously, there is nothing that prevents
proxies from cheating. Thus, SIPS cannot guarantee that TLS usage
will be truly respected end-to-end on each segment of a request
path. Note that since many UAs will not accept incoming TLS
connections, even those UAs that do support TLS will be required
to maintain persistent TLS connections as described in the TLS
limitations section above in order to receive requests over TLS as
Location services are not required to provide a SIPS binding for a
SIPS Request-URI. Although location services are commonly populated
by user registrations (as described in Section 10.2.1), various other
protocols and interfaces could conceivably supply contact addresses
for an AOR, and these tools are free to map SIPS URIs to SIP URIs as
appropriate. When queried for bindings, a location service returns
its contact addresses without regard for whether it received a
request with a SIPS Request-URI. If a redirect server is accessing
the location service, it is up to the entity that processes the
Contact header field of a redirection to determine the propriety of
the contact addresses.
Ensuring that TLS will be used for all of the request segments up to
the target domainUAS is somewhat complex. It is possible that
cryptographically authenticated proxy servers along the way that are
non-compliant or compromised may choose to disregard the forwarding
rules associated with SIPS (and the general forwarding rules in
Section 16.6). Such malicious intermediaries could, for example,
retarget a request from a SIPS URI to a SIP URI in an attempt to
Alternatively, an intermediary might legitimately retarget a requestfrom a SIP to a SIPS URI. Recipients of a request whose Request-URIuses the SIPS URI scheme thus cannot assume on the basis of theRequest-URI alone that SIPS was used for the entire request path(from the client onwards).
To address these concerns, it is RECOMMENDED that recipients of a
request whose Request-URI contains a SIP or SIPS URI inspect the To
header field value to see if it contains a SIPS URI (though note that
it does not constitute a breach of security if this URI has the same
scheme but is not equivalent to the URI in the To header field).
Although clients may choose to populate the Request-URI and To header
field of a request differently, when SIPS is used this disparity
could be interpreted as a possible security violation, and the
request could consequently be rejected by its recipient. Recipients
MAY also inspect the Via header chain in order to double-check
whether or not TLS was used for the entire request path until the
local administrative domain was reached. S/MIME may also be used bythe originating UAC to help ensure that the original form of the Toheader field is carried end-to-end.
S/MIME or, preferably, [RFC4474] may also be used by the
originating UAC to help ensure that the original form of the To
header field is carried end-to-end.
If the UAS has reason to believe that the scheme of the Request-URI
has been improperly modified in transit, the UA SHOULD notify its
user of a potential security breach.
As a further measure to prevent downgrade attacks, entities that
accept only SIPS requests MAY also refuse connections on insecure
End users will undoubtedly discern the difference between SIPS and
SIP URIs, and they may manually edit them in response to stimuli.
This can either benefit or degrade security. For example, if an
attacker corrupts a DNS cache, inserting a fake record set that
effectively removes all SIPS records for a proxy server, then any
SIPS requests that traverse this proxy server may fail. When a user,
however, sees that repeated calls to a SIPS AOR are failing, they
could on some devices manually convert the scheme from SIPS to SIP
and retry. Of course, there are some safeguards against this (if the
destination UA is truly paranoid it could refuse all non-SIPS
requests), but it is a limitation worth noting. On the bright side,
users might also divine that 'SIPS' would be valid even when they are
presented only with a SIP URI.
SIP messages frequently contain sensitive information about their
senders - not just what they have to say, but with whom they
communicate, when they communicate and for how long, and from where
they participate in sessions. Many applications and their users
require that this sort of private information be hidden from any
parties that do not need to know it.
Note that there are also less direct ways in which private
information can be divulged. If a user or service chooses to be
reachable at an address that is guessable from the person's name and
organizational affiliation (which describes most addresses-of-
record), the traditional method of ensuring privacy by having an
unlisted "phone number" is compromised. A user location service can
infringe on the privacy of the recipient of a session invitation by
divulging their specific whereabouts to the caller; an implementation
consequently SHOULD be able to restrict, on a per-user basis, what
kind of location and availability information is given out to certain
classes of callers. This is a whole class of problem that is
expected to be studied further in ongoing SIP work.
In some cases, users may want to conceal personal information in
header fields that convey identity. This can apply not only to the
From and related headers representing the originator of the request,
but also the To - it may not be appropriate to convey to the final
destination a speed-dialing nickname, or an unexpanded identifier for
a group of targets, either of which would be removed from the
Request-URI as the request is routed, but not changed in the To