Internet Engineering Task Force (IETF) J. Seedorf
Request for Comments: 6404 S. Niccolini
Category: Informational NEC
ISSN: 2070-1721 E. Chen
NTT
H. Scholz
VOIPFUTURE
November 2011
Session PEERing for Multimedia INTerconnect (SPEERMINT)
Security Threats and Suggested Countermeasures
Abstract
The Session PEERing for Multimedia INTerconnect (SPEERMINT) working
group (WG) provides a peering framework that leverages the building
blocks of existing IETF-defined protocols such as SIP and ENUM for
the interconnection between SIP Service Providers (SSPs). The
objective of this document is to identify and enumerate SPEERMINT-
specific threat vectors and to give guidance for implementers on
selecting appropriate countermeasures. Security requirements for
SPEERMINT that have been derived from the threats detailed in this
document can be found in RFC 6271; this document provides concrete
countermeasures to meet those SPEERMINT security requirements. In
this document, the different security threats related to SPEERMINT
are classified into threats to the Lookup Function (LUF), the
Location Routing Function (LRF), the Signaling Function (SF), and the
Media Function (MF) of a specific SIP Service Provider. Various
instances of the threats are briefly introduced inside the
classification. Finally, existing security solutions for SIP and
RTP/RTCP (Real-time Transport Control Protocol) are presented to
describe countermeasures currently available for such threats. Each
SSP may have connections to one or more remote SSPs through peering
or transit contracts. A potentially compromised remote SSP that
attacks other SSPs is out of the scope of this document; this
document focuses on attacks on an SSP from outside the trust domain
such an SSP may have with other SSPs.
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Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6404.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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Table of Contents
1. Introduction ....................................................4
2. Security Threats Relevant to SPEERMINT ..........................5
2.1. Threats to the Lookup Function (LUF) .......................5
2.1.1. Threats to LUF Confidentiality ......................5
2.1.2. Threats to LUF Integrity ............................6
2.1.3. Threats to LUF Availability .........................6
2.2. Threats to the Location Routing Function (LRF) .............6
2.2.1. Threats to LRF Confidentiality ......................6
2.2.2. Threats to LRF Integrity ............................7
2.2.3. Threats to LRF Availability .........................7
2.3. Threats to the Signaling Function (SF) .....................7
2.3.1. Threats to SF Confidentiality .......................7
2.3.2. Threats to SF Integrity .............................8
2.3.3. Threats to SF Availability .........................10
2.4. Threats to the Media Function (MF) ........................10
2.4.1. Threats to MF Confidentiality ......................10
2.4.2. Threats to MF Integrity ............................10
2.4.3. Threats to MF Availability .........................11
3. Security Requirements ..........................................11
3.1. Security Requirements from SPEERMINT Requirements
Document ..................................................11
3.2. How to Fulfill the Security Requirements for SPEERMINT ....11
4. Suggested Countermeasures ......................................12
4.1. Database Security BCPs ....................................14
4.2. DNSSEC ....................................................14
4.3. DNS Replication ...........................................15
4.4. Cross-Domain Privacy Protection ...........................15
4.5. Secure Exchange of SIP Messages ...........................15
4.6. Ingress Filtering / Reverse-Path Filtering ................16
4.7. Strong Identity Assertion .................................16
4.8. Reliable Border Element Pooling ...........................17
4.9. Rate limit ................................................17
4.10. Topology Hiding ..........................................17
4.11. Border Element Hardening .................................17
4.12. Securing Session Establishment Data ......................18
4.13. Encryption and Integrity Protection of Media Stream ......18
5. Conclusions ....................................................18
6. Security Considerations ........................................18
7. Acknowledgements ...............................................19
8. Informative References .........................................19
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1. Introduction
With Voice over IP (VoIP), the need for security is compounded
because there is the need to protect both the control plane and the
data plane. In a legacy telephone system, security is a more valid
assumption. Intercepting conversations requires either physical
access to telephone lines or a compromise to the Public Switched
Telephone Network (PSTN) nodes or the office Private Branch eXchanges
(PBXs). Only particularly security-sensitive organizations bother to
encrypt voice traffic over traditional telephone lines. In contrast,
the risk of sending unencrypted data across the Internet is more
significant (e.g., dual-tone multi-frequency (DTMF) tones
corresponding to the credit card number). An additional security
threat to Internet Telephony comes from the fact that the signaling
devices may be addressed directly by attackers as they use the same
underlying networking technology as the multimedia data; traditional
telephone systems have the signaling network separated from the data
network. This is an increased security threat since a hacker could
attack the signaling network and its servers with increased damage
potential (call hijacking, call drop, Denial-of-Service (DoS) attacks
[RFC4732], etc.). Therefore, there is a need to investigate the
different security threats, to extract security-related requirements,
and to highlight potential solutions on how to protect against such
threats.
The Session PEERing for Multimedia INTerconnect (SPEERMINT) working
group provides a peering framework that leverages the building blocks
of existing IETF-defined protocols such as SIP and ENUM for the
interconnection between SIP servers [RFC5486]. The objective of this
document is to identify and enumerate SPEERMINT-specific threat
vectors and to give guidance for implementers on selecting
appropriate countermeasures. Security requirements for SPEERMINT can
be found in RFC 6271 "Requirements for SIP-Based Session Peering"
[RFC6271]. These security requirements for SPEERMINT are derived
from the threats that are detailed in this document; they have been
moved from an earlier version of this document to the SPEERMINT
requirements document [RFC6271]. In addition to being the base for
those security requirements, this document provides to implementers
advice and examples for concrete countermeasures on how to meet these
security requirements for SPEERMINT with technical means. The
SPEERMINT terminology outlined in [RFC5486] is used throughout this
document.
In this document, the different security threats related to SPEERMINT
are classified into threats to the Lookup Function (LUF), the
Location Routing Function (LRF), the Signaling Function (SF), and the
Media Function (MF) of a specific SIP Service Provider (SSP).
Various instances of the threats are briefly introduced inside the
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classification. Finally, existing security solutions for SIP and
RTP/RTCP are presented to describe countermeasures currently
available for such threats. Each SSP may have connections to one or
more remote SSPs through peering or transit contracts. A potentially
compromised remote SSP that attacks other SSPs is out of the scope of
this document; this document focuses on attacks on an SSP from
outside the trust domain such an SSP may have with other SSPs.
2. Security Threats Relevant to SPEERMINT
This section enumerates potential security threats relevant to
SPEERMINT. A taxonomy of VoIP security threats is defined in
[VOIPSATAXONOMY]. This taxonomy is comprehensive and also takes into
account non-VoIP-specific threats (e.g., loss of power, etc.).
Threats relevant to the boundaries of Layer 5 SIP networks are
extracted from this taxonomy and mapped to the functions of the
SPEERMINT architecture as defined in [RFC6406]. Moreover, additional
threats for the SPEERMINT architecture are listed and detailed under
the same classification of SPEERMINT functions and according to the
CIA (Confidentiality, Integrity, and Availability) triad:
o Lookup Function (LUF);
o Location Routing Function (LRF);
o Signaling Function (SF);
o Media Function (MF).
2.1. Threats to the Lookup Function (LUF)
For a given request, the LUF provides a mechanism to determine the
identity of the requested resource on the terminating domain. The
returned identity can be used to look up Session Establishment Data
(SED) using the Location Routing Function (LRF). In direct peerings,
the LUF is usually hosted locally, whereas in a federation context,
this function may be offered by a third party.
If the LUF is hosted locally, it is vulnerable to the same threats
that affect database systems in general. If the SSP relies on a
remote third party to provide the LUF functionality, confidentiality,
integrity, and authenticity of the responses are at risk.
2.1.1. Threats to LUF Confidentiality
For a given request, the Lookup Function (LUF) determines the target
domain to which the request should be routed. The following attacks
are relevant with respect to eavesdropping on LUF messages:
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o SIP URI and peering domain harvesting - an attacker can exploit
this weakness if the underlying database has a weak authentication
system or if SIP messages are sent unencrypted, and then use the
gained knowledge to launch other kinds of attacks.
o Third-party information - a LUF providing information to multiple
companies / third parties can be attacked to obtain information
about third party peering configurations and possible contracts.
2.1.2. Threats to LUF Integrity
The underlying database or LUF messages could be vulnerable to input/
output message modification attacks:
o Injection attack - an attacker could manipulate statements
performed on the database LUF messages sent to a third party. A
specific version of this attack is known as "SQL injection". An
SQL injection is a code insertion into the LUF due to incorrect
input validation.
2.1.3. Threats to LUF Availability
The underlying database or third party LUF service could be
vulnerable to:
o Denial-of-Service attacks - For example, an attacker makes
incomplete requests causing the server to create an idle state for
each of them, which causes memory to be exhausted.
2.2. Threats to the Location Routing Function (LRF)
The LRF determines the location of the Signaling Function (SF) for
the target domain of a given request. Optionally, it may return
additional SED.
2.2.1. Threats to LRF Confidentiality
Similar to the LUF, the following attacks are related to
eavesdropping on LRF messages:
o URI harvesting - the attacker harvests URIs and IP addresses of
the existing User Endpoints (UEs) by issuing a multitude of
location requests. Direct intrusion against vulnerable UEs or
telemarketing are possible attack scenarios that would use the
gained knowledge.
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o SIP device enumeration - the attacker discovers the IP address of
each intermediate signaling device by looking at the Via and
Record-Route headers of a SIP message. Targeting the discovered
devices with subsequent attacks is a possible attack scenario.
2.2.2. Threats to LRF Integrity
An attacker may modify messages, e.g., by feeding bogus information
to the LRF, if the routing data is not correctly validated or sent
unencrypted. Dynamic call routing discovery and establishment, as in
the scope of SPEERMINT, introduce opportunities for attacks such as
the following:
o Man-in-the-Middle attacks - the attacker inserts or has already
inserted an unauthorized node in the signaling path modifying the
SED. The result is that the attacker is then able to read,
insert, and modify the multimedia communications.
o Incorrect destinations - the attacker redirects the calls to an
incorrect destination with the purpose of establishing fraud
communications like voice phishing or DoS attacks.
2.2.3. Threats to LRF Availability
The LRF can be the object of DoS attacks. DoS attacks to the LRF can
be carried out by sending a large number of queries to the LRF or
LUF, with the result of preventing an Originating SSP from looking up
call routing data of any URI outside its administrative domain. As
an alternative, the attacker could target the DNS to disable
resolution of SIP addresses.
2.3. Threats to the Signaling Function (SF)
The Signaling Function involves a great number of sensitive
information. Through the Signaling Function, User Agents (UAs)
assert identities and operators authorize billable resources.
Correct and trusted operation of Signaling Function is essential for
service providers. This section discusses potential security threats
to the Signaling Function to detail the possible attack vectors.
2.3.1. Threats to SF Confidentiality
SF traffic is vulnerable to eavesdropping, in particular, when the
data is moved across multiple SSPs having different levels of
security policies. Threats for the SF confidentiality are listed
here:
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o Call pattern analysis - the attacker tracks the call patterns of
the users violating his/her privacy (e.g., revealing the social
network of various users, the daily phone usage, etc.); also,
rival SSPs may infer information about the customer base of other
SSPs in this way;
o Password cracking - the challenge-response authentication
mechanism of SIP Digest can be attacked with offline dictionary
attacks. With such attacks, an attacker tries to exploit weak
passwords that are used by incautious users.
o Network discovery - the attacker may learn information about the
internal network structure of a peering partner that is directly
or indirectly connected by looking at SIP routing information
(i.e, Record-Route, Via or Contact headers).
2.3.2. Threats to SF Integrity
The integrity of the SF can be violated using SIP request spoofing,
SIP reply spoofing, and SIP message tampering.
2.3.2.1. SIP Request Spoofing
Most SIP request spoofing attacks first require SIP message
eavesdropping. However, some of these attacks can be also performed
by estimating certain fields in SIP headers (e.g., by exploiting the
fact that weak implementations may generate predictable SIP Dialog
parameters) or exploiting broken implementations that do not properly
verify the content of certain headers. Threats in this category are
as follows:
o session teardown - an attacker can send CANCEL/BYE messages in
order to tear down an existing call at the SIP layer; for such an
attack, the attacker either needs to know (e.g., by eavesdropping
a SIP INVITE message) the SIP Dialog of the call to be hijacked
(To-tag, From-tag, Call-ID) or alternatively may rely on SIP
implementations that do not properly authenticate requests based
on the SIP Dialog;
o Billing fraud - the attacker can modify and replay an intercepted
INVITE request in order to bill a call to a victim UE and avoid
paying for the phone call;
o User ID spoofing - SSPs are responsible for asserting the
legitimacy of a user ID; if an SSP fails to achieve the level of
identity assertion that the federation to which it belongs
expects, it may create an entry point for attackers to conduct
user ID spoofing attacks;
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o Unwanted requests - the attacker sends requests to interfere with
regular operation, e.g., by sending a REGISTER request in order to
hijack calls. The SPEERMINT architecture as defined in [RFC6406]
does not require registrations between the Signaling Functions
(SFs) of the connected SSPs. Hence, superfluous requests like
REGISTERs should be rejected.
2.3.2.2. SIP Reply Spoofing
Threats in this category are as follows:
o Forged 199 Response - the attacker sends a forged 199 response to
terminate an early dialog. The forged response will not terminate
the entire session but may alter the direction of the session;
o Forged 200 Response - having seen the contents of an INVITE
request, an eavesdropper can inject a 200 response, affecting the
processing of the transaction of all proxies between the injection
point and the originating UA and at the originating UA itself. In
the extreme case, this can result in a hijacked call. In many
cases, however, such an attack will leave signaling artifacts that
may allow it to be detected (e.g., the element receiving the
forged 200 response may also receive other SIP reply messages from
the actual terminating UE);
o Forged 302 Response - having seen the contents of an INVITE
request, an eavesdropper could also inject a forged "302 Moved
Temporarily" reply, affecting the processing of the transaction at
intermediate entities and the originating UA. This may allow the
attacker to successfully redirect the call to any destination UE
of his choosing;
o Forged 404 Response - having seen the contents of an INVITE
request, an eavesdropper could also inject a forged "404 Not
Found" reply, affecting the processing of the transaction at
intermediate entities and the originating UA. Such an attack may
result in disrupting the call establishment.
2.3.2.3. SIP Message Tampering
This threat involves the alteration of important field values in a
SIP message or in the Session Description Protocol (SDP) body.
Examples of this threat could be the dropping or modification of
handshake packets in order to avoid the establishment of a secure RTP
session (SRTP). The same approach could be used to degrade the
quality of media session by letting a UE negotiate a poor quality
codec.
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2.3.3. Threats to SF Availability
o Flooding attack - a Signaling Path Border Element (SBE) is
susceptible to message flooding attacks that may come from
interconnected SSPs;
o Session blackholing - the attacker (assumed to be able to make
Man-in-the-Middle attacks) intentionally drops essential packets,
e.g., INVITEs, to prevent certain calls from being established;
o SIP Fuzzing attack - fuzzing tests and software can be used by
attackers to discover and exploit vulnerabilities of a SIP entity.
This attack may result in crashing a SIP entity.
2.4. Threats to the Media Function (MF)
The Media Function (MF) is responsible for the actual delivery of
multimedia communication between the users and carries sensitive
information. Through the media function, the UE can establish secure
communications and monitor the quality of conversations. Correct and
trusted operations of MF is essential for privacy and service-
assurance issues. This section discusses potential security threats
to the MF to detail the possible attack vectors.
2.4.1. Threats to MF Confidentiality
The MF is vulnerable to eavesdropping in which the attacker may
reconstruct the voice conversation or sensitive information (e.g.,
PINs from DTMF tones). Some SRTP key exchange mechanisms (e.g.,
[RFC4568]) are vulnerable to bid-down attacks, where an attacker
selectively changes key exchange protocol fields in order to enforce
the establishment of a less secure or even non-secure communication.
2.4.2. Threats to MF Integrity
Both RTP and RTCP are vulnerable to integrity violation in many ways:
o Media injection - if an attacker can somehow detect an ongoing
media session and eavesdrop a few RTP packets, he can start
sending bogus RTP packets to one of the UEs involved using the
same codec. If the bogus RTP packets have consistently greater
timestamps and sequence numbers (but within the acceptable range)
than the legitimate RTP packets, the recipient UE may accept the
bogus RTP packets and discard the legitimate ones.
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o Media session teardown - the attacker sends bogus RTCP BYE
messages to a target UE signaling to tear down the media
communication; please note that RTCP messages are normally not
authenticated.
o Quality-of-Service (QoS) degradation - the attacker sends wrong
RTCP reports advertising more packet loss or more jitter than
actually experimented resulting in the usage of a poor quality
codec degrading the overall quality of the call experience.
2.4.3. Threats to MF Availability
o Malformed messages - the attacker tries to cause a crash or a
reboot of the Data Path Border Element (DBE)/UE by sending RTP/
RTCP malformed messages;
o Messages flooding - the attacker tries to exhaust the resources of
the DBE/UE by sending many RTP/RTCP messages.
3. Security Requirements
3.1. Security Requirements from SPEERMINT Requirements Document
The security requirements for SPEERMINT have been moved from an
earlier version of this document to the SPEERMINT requirements
[RFC6271]. The security requirements for SPEERMINT are the
following, from [RFC6271]:
o Requirement #15: The protocols used to query the Lookup and
Location Routing Functions SHOULD support mutual authentication.
o Requirement #16: The protocols used to query the Lookup and
Location Routing Functions SHOULD provide support for data
confidentiality and integrity.
o Requirement #17: The protocols used to enable session peering MUST
NOT interfere with the exchanges of media security attributes in
SDP. Media attribute lines that are not understood by SBEs must
be ignored and passed along the signaling path untouched.
3.2. How to Fulfill the Security Requirements for SPEERMINT
Requirements #15 and #16 state that the LUF and LRF should support
mutual authentication, data confidentiality, and integrity. In
principle, these requirements can be fulfilled technically with
Transport Layer Security (TLS) or Datagram TLS (DTLS) [RFC5246]
[RFC4347] or IP layer security (IPsec) [RFC4301]. From a pure
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security perspective both solutions fulfill the security requirements
for SPEERMINT, just on a different layer, and both solutions are
widely deployed.
However, from a more practical perspective, transport layer security
(i.e., TLS or DTLS) has the advantage that the application using it
is aware of whether or not security (or rather the corresponding
security features) is enabled. For instance, using TLS has the
consequence that the connection fails if the corresponding connection
endpoint cannot authenticate properly.
While IPsec fulfills the same requirements from a security
perspective, IPsec is somewhat de-coupling security from the
application using it. For instance, IPsec is often provided by
dedicated entities in such a way that from the application layer, it
cannot be recognized whether or not IPsec or certain security
features are turned on ("bump-in-the-wire").
In summary, TLS (or DTLS) has some notable advantages over IPsec for
addressing the SPEERMINT security requirements. In particular,
transport layer security is preferable over IPsec for SPEERMINT
because with TLS (or DTLS) security is more closely coupled to the
LUF or LRF. From a mere technical perspective, however, both
solutions (transport layer security or IPsec) fulfill the SPEERMINT
security requirements, and there may be particular cases where IPsec
is a preferable solution.
4. Suggested Countermeasures
This section describes implementer-specific countermeasures against
the threats described in the previous sections and for addressing the
SPEERMINT security requirements described in [RFC6271]. The
countermeasures listed in this section are not meant to be
exhaustive; rather, the suggested countermeasures are aimed to serve
as starting points and to give guidance for implementers that are
trying to select appropriate countermeasures against certain threats.
The following table provides a map of the relationships between
threats and countermeasures. The suggested countermeasures are
discussed in detail in the subsequent subsections.
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+-------+---------------+-------------------------------------------+
| Group | Threat | Suggested Countermeasure |
+-------+---------------+-------------------------------------------+
| LUF | Unauthorized | database security BCPs (Section 4.1), |
| | access | Secure Exchange of SIP messages |
| | | (Section 4.5) |
| | SQL injection | database security BCPs (Section 4.1), |
| | | Secure Exchange of SIP messages |
| | | (Section 4.5) |
| | DoS to LUF | database security BCPs (Section 4.1), |
| | | Secure Exchange of SIP messages |
| | | (Section 4.5) |
| LRF | URI | privacy protection (Section 4.4), Secure |
| | harvesting | Exchange of SIP messages (Section 4.5) |
| | SIP equipment | privacy protection (Section 4.4), Secure |
| | enumeration | Exchange of SIP messages (Section 4.5) |
| | MitM attack | DNSSEC (Section 4.2), Secure Exchange of |
| | | SIP messages (Section 4.5) |
| | Incorrect | DNSSEC (Section 4.2), Secure Exchange of |
| | destinations | SIP messages (Section 4.5) |
| | DoS to LRF | DNS replication (Section 4.3) |
| SF | Call pattern | Secure Exchange of SIP messages |
| | analysis | (Section 4.5), Securing Session |
| | | Establishment Data (Section 4.12) |
| | Password | Secure Exchange of SIP messages |
| | cracking | (Section 4.5) |
| | Network | Securing Session Establishment Data |
| | discovery | (Section 4.12), Topology Hiding |
| | | (Section 4.10) |
| | Session | Secure Exchange of SIP messages |
| | teardown | (Section 4.5), ingress filtering |
| | | (Section 4.6) |
| | Billing fraud | strong identity assertion (Section 4.7) |
| | User ID | strong identity assertion (Section 4.7) |
| | spoofing | |
| | Forged 200 | Secure Exchange of SIP messages |
| | Response | (Section 4.5), ingress filtering |
| | | (Section 4.6) |
| | Forged 302 | Secure Exchange of SIP messages |
| | Response | (Section 4.5), ingress filtering |
| | | (Section 4.6) |
| | Forged 404 | Secure Exchange of SIP messages |
| | Response | (Section 4.5), ingress filtering |
| | | (Section 4.6) |
| | Flooding | reliable border element pooling |
| | attack | (Section 4.8), rate limit (Section 4.9) |
| | Session | DNSSEC (Section 4.2) |
| | blackholing | |
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| | SIP fuzzing | border element hardening (Section 4.11) |
| | attack | |
| MF | Eavesdropping | Encryption and Integrity Protection of |
| | | Media Stream (Section 4.13) |
| | Media | Encryption and Integrity Protection of |
| | injection | Media Stream (Section 4.13) |
| | Media session | Encryption and Integrity Protection of |
| | teardown | Media Stream (Section 4.13) |
| | QoS | Encryption and Integrity Protection of |
| | degradation | Media Stream (Section 4.13) |
| | Malformed | border element hardening (Section 4.11) |
| | messages | |
| | Message | rate limit (Section 4.9) |
| | flooding | |
+-------+---------------+-------------------------------------------+
4.1. Database Security BCPs
Adequate security measures must be applied to the LUF to prevent it
from being a target of attacks often seen on common database systems.
Common security Best Current Practices (BCPs) for database systems
include the use of strong passwords to prevent unauthorized access,
parameterized statements to prevent SQL injections, and server
replication to prevent any database from being a single point of
failure. [DBSEC] is one of many existing documents that describe BCPs
in this area.
4.2. DNSSEC
If DNS is used by the LRF, it is recommended to deploy the recent
version of Domain Name System Security Extensions (informally called
"DNSSEC-bis") defined by [RFC4033], [RFC4034], and [RFC4035]. DNSSEC
has been designed to protect DNS against well-known attacks such as
DNS cache poisoning or Man-in-the-Middle (MitM) attacks on DNS
queries. Essentially, DNSSEC is a set of public key cryptography
extensions to DNS that provide authentication of DNS data, integrity
protection for DNS entries, and authenticated denial of existence
regarding non-existing DNS entries. In the context of SSP peering,
DNSSEC can provide authentication and integrity regarding the
location of a Signaling Function (SF) entity retrieved via DNS.
Using DNSSEC can thus help to defend against MitM attacks on DNS
queries invoked by the LRF, session blackholing and other attacks
that lead traffic to incorrect destinations.
DNSSEC has been deployed at the root level and in several top-level
domains (e.g., .com and .net). Although, at the time of this
writing, DNSSEC is still not yet widely deployed on the Internet,
even limited deployment can add significant integrity protection and
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authentication to the LRF for Signaling Function locations received
via DNS entries. Neither end users nor terminals are involved in the
DNS resolution process of the LRF. Hence, if a) the sending SSP uses
a DNS resolver that supports DNSSEC extensions, b) the receiving SSP
stores the location of its Signaling Function cryptographically
signed (using DNSSEC extensions) in the DNS, and c) the sending SSP
can obtain an authentication chain (i.e., a series of linked DS and
DNSKEY records) to the receiving SSP, the LRF can be secured with
DNSSEC. In the context of SPEERMINT, all three of these requirements
can be fulfilled even in the case of partial DNSSEC deployment. In
particular, even without Internet-wide deployment of DNSSEC, it may
be possible for a sending SSP to obtain a suitable trust anchor for
verifying the receiving SSP's public key. For instance, a suitable
trust anchor could be configured for that specific SSP's top-level
domain or for the particular SSP's domain directly. If the sending
and the receiving SSP use a common ENUM tree, DNSSEC use with the
ENUM tree's trust anchor is "straightforward".
4.3. DNS Replication
DNS replication is a very important countermeasure to mitigate DoS
attacks on the LRF. Simultaneously bringing down multiple DNS
servers that support the LRF is much more challenging than attacking
a sole DNS server (single point of failure).
4.4. Cross-Domain Privacy Protection
Stripping Via and Record-Route headers, replacing the Contact header,
and even changing Call-IDs are the mechanisms described in [RFC3323]
to protect SIP privacy. This practice allows an SSP to hide its SIP
network topology, prevents intermediate signaling equipment from
becoming the target of DoS attacks, as well as protects the privacy
of UEs according to their preferences. This practice is effective in
preventing SIP equipment enumeration that exploits LRF.
4.5. Secure Exchange of SIP Messages
SIP can be used on top of UDP or TCP as transport protocol [RFC3261].
However, look-up and SED data should be exchanged securely (see
security requirements (Section 3.2)), e.g., to increase the
difficulty of performing session teardown and forging responses (200,
302, 404, etc). If UDP is used to carry SIP messages, DTLS should be
used to secure SIP message exchange between SSPs. If TCP is used as
a transport protocol, it can be secured with TLS. Therefore,
depending on the underlying transport protocol, SSPs should use
either DTLS or TLS to secure SIP message delivery.
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In general, encryption and integrity protection of signaling messages
can be achieved on the transport layer (with TLS or DTLS) or on the
network layer (with IPsec). Both solutions are technically sound,
but transport layer security has some advantages. Please refer to
the subsection on fulfilling the SPEERMINT security requirements
(Section 3.2) for a discussion on using TLS/DTLS or IPsec for
protecting the confidentiality and integrity of signaling messages.
Similar to strong identity assertion, a Public Key Infrastructure
(PKI) is assumed to be in place for TLS/DTLS (or IPsec) deployment so
that SSPs can obtain and trust the keys necessary to decrypt messages
and verify signatures sent by other SSPs.
Message-oriented protection such as [RFC3261] authentication does not
fulfill the SPEERMINT requirements (e.g., mutual authentication).
4.6. Ingress Filtering / Reverse-Path Filtering
Ingress filtering, i.e., blocking all traffic coming from a host that
has a source address different than the addresses that have been
assigned to that host (see [RFC2827]), can effectively prevent UEs
from sending packets with a spoofed source IP address. This can be
achieved by reverse-path filtering, i.e., only accepting ingress
traffic if responses would take the same path. This practice is
effective in preventing session teardown and forged SIP replies (200,
302, 404, etc.), if the recipient correctly verifies the source IP
address for the authenticity of each incoming SIP message.
4.7. Strong Identity Assertion
"Caller ID spoofing" can be achieved thanks to the weak identity
assertion on the From URI of an INVITE request. In a single SSP
domain, strong identity assertion can be easily achieved by
authenticating each INVITE request. However, in the context of
SPEERMINT, only the Originating SSP is able to verify the identity
directly. In order to overcome this problem, there are currently
only two major approaches: transitive trust and cryptographic
signature. The transitive trust approach builds a chain of trust
among different SSP domains. One example of this approach is a
combined mechanism specified in [RFC3324] and [RFC3325]. Using this
approach in a transit peering network scenario, the terminating SSP
must establish a trust relationship with all SSP domains on the path,
which can be seen as an underlying weakness. The use of
cryptographic signatures is an alternative approach. "Session
Initiation Protocol (SIP) Authenticated Identity Body (AIB) Format"
is specified in [RFC3893]. [RFC4474] introduces two new header
fields, IDENTITY and IDENTITY-INFO, that allow a SIP server in the
Originating SSP to digitally sign an INVITE request after
authenticating the sending UE. The terminating SSP can verify if the
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INVITE request is signed by a trusted SSP domain. Although this
approach does not require the terminating SSP to establish a trust
relationship with all transit SSPs on the path, a PKI is assumed to
be in place.
4.8. Reliable Border Element Pooling
It is advisable to implement reliable pooling on border elements. An
architecture and protocols for the management of server pools
supporting mission-critical applications are addressed in the
RSERPOOL WG. Using such mechanisms and protocols (see [RFC5351]
[RFC5352] [RFC5353] for details), a UE can effectively increase its
capacity in handling flooding attacks.
4.9. Rate limit
Flooding attacks on SFs and MFs can also be mitigated by limiting the
rate of incoming traffic through policing or queuing. In this way,
legitimate clients can be denied the service since their traffic may
be discarded. Rate limiting can also be applied on a per-source-IP
basis under the assumption that the source IP of each attack packet
is not spoofed dynamically. Limitations related to NAT and mobility
issues apply and may result in false positives (i.e., source IP
addresses blocked) when multiple legitimate clients are located
behind the same NAT IP address. It may be preferable to limit the
number of concurrent 'sessions', i.e., ongoing calls instead of the
messaging associated with it (since sessions use more resources on
backend-systems). When calculating rate limits, all entities along
the session path should be taken into account. SIP entities on the
receiving end of a call may be the limiting factor (e.g., the number
of ISDN channels on PSTN gateways) rather than the ingress limiting
device.
4.10. Topology Hiding
Topology hiding applies to both the signaling and media plane and
consists of limiting the amount of topology information exposed to
peering partners. Topology hiding requires back-to-back user agent
(B2BUA) functionality. The most common way is the use of a Session
Border Controller (SBC) as SBE. Topology hiding is explained in
[RFC5853].
4.11. Border Element Hardening
To prevent attacks that exploit vulnerabilities (such as buffer
overflows, format string vulnerabilities, etc.) in SPEERMINT border
elements, these implementations should be security hardened. For
instance, fuzz testing is a common black box testing technique used
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in software engineering. Also, security vulnerability tests can be
carried out preventively to assure a UE/SBE/DBE can handle unexpected
data correctly without crashing. [RFC4475] and [PROTOS] are examples
of torture test cases specific for SIP devices and freely available
security testing tools, respectively. These type of tests needs to
be carried out before product release and in addition throughout the
product life cycle.
4.12. Securing Session Establishment Data
Session Establishment Data (SED) contains critical information for
the routing of SIP sessions. In order to prevent attacks such as
service hijacking and denial of service that exploit SED, SSPs should
adopt a secure transport protocol that provides authentication,
confidentiality and integrity to exchange SED among themselves.
Further details can be found in [DRINKS-SPPROV].
4.13. Encryption and Integrity Protection of Media Stream
The Secure Real-time Transport Protocol (SRTP) [RFC3711] prevents
eavesdropping on plain RTP by encrypting the data flow. It uses AES
as the default cipher and defines two modes of operation (Segmented
Integer Counter Mode and f8-mode), which is agreed upon after
negotiation. It also uses HMAC-SHA1 and index keeping to enable
message authentication/integrity and replay protection required to
prevent media injection attacks. Secure RTCP (SRTCP) provides the
same security-related features to RTCP as SRTP does for RTP. SRTCP
is described in [RFC3711] as optional. In order to prevent media
session teardown, it is recommended to turn this feature on. The
choice of the external key management protocol is left to the
deployment, a PKI is necessary to implement the security requirements
of the SPEERMINT requirements document.
5. Conclusions
This document presented the different SPEERMINT security threats
classified in groups related to the LUF, LRF, SF, and MF,
respectively. The multiple instances of the threats were presented
with a brief explanation. Finally, suggested countermeasures for
SPEERMINT were outlined together with possible mitigation of the
existing threats by means of them.
6. Security Considerations
This document is entirely focused on the security threats for
SPEERMINT.
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7. Acknowledgements
This document was originally inspired by the VOIPSA VoIP Security and
Privacy Threat Taxonomy. The authors would like to thank VOIPSA for
having produced a comprehensive taxonomy as the starting point of
this document. Additionally, the authors would like to thank Cullen
Jennings, Jon Peterson, David Schwartz, Hadriel Kaplan, Peter Koch,
Daryl Malas, Jason Livingood, and Robert Sparks for useful comments
to previous editions of this document on the mailing list as well as
during IETF meetings.
Jan Seedorf and Saverio Niccolini are partially supported by the
DEMONS project, a research project supported by the European
Commission under its 7th Framework Program (contract no. 257315).
The views and conclusions contained herein are those of the authors
and should not be interpreted as necessarily representing the
official policies or endorsements, either expressed or implied, of
the DEMONS project or the European Commission.
8. Informative References
[DBSEC] Gertz, M. and S. Jajodia, "Handbook of Database Security:
Applications and Trends", Springer, 2008.
[DRINKS-SPPROV]
Mule, J., Cartwright, K., Ali, S., and A. Mayrhofer,
"Session Peering Provisioning Protocol", Work in Progress,
September 2011.
[PROTOS] Wieser, C., Laakso, M., and H. Schulzrinne, "SIP
Robustness Testing for Large-Scale Use", First
International Workshop on Software Quality (SOQUA 2004),
September 2004.
[RFC2827] Ferguson, P. and D. Senie, "Network Ingress Filtering:
Defeating Denial of Service Attacks which employ IP Source
Address Spoofing", BCP 38, RFC 2827, May 2000.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
June 2002.
[RFC3323] Peterson, J., "A Privacy Mechanism for the Session
Initiation Protocol (SIP)", RFC 3323, November 2002.
[RFC3324] Watson, M., "Short Term Requirements for Network Asserted
Identity", RFC 3324, November 2002.
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[RFC3325] Jennings, C., Peterson, J., and M. Watson, "Private
Extensions to the Session Initiation Protocol (SIP) for
Asserted Identity within Trusted Networks", RFC 3325,
November 2002.
[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, March 2004.
[RFC3893] Peterson, J., "Session Initiation Protocol (SIP)
Authenticated Identity Body (AIB) Format", RFC 3893,
September 2004.
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements",
RFC 4033, March 2005.
[RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Resource Records for the DNS Security Extensions",
RFC 4034, March 2005.
[RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Protocol Modifications for the DNS Security
Extensions", RFC 4035, March 2005.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.
[RFC4347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security", RFC 4347, April 2006.
[RFC4474] Peterson, J. and C. Jennings, "Enhancements for
Authenticated Identity Management in the Session
Initiation Protocol (SIP)", RFC 4474, August 2006.
[RFC4475] Sparks, R., Hawrylyshen, A., Johnston, A., Rosenberg, J.,
and H. Schulzrinne, "Session Initiation Protocol (SIP)
Torture Test Messages", RFC 4475, May 2006.
[RFC4568] Andreasen, F., Baugher, M., and D. Wing, "Session
Description Protocol (SDP) Security Descriptions for Media
Streams", RFC 4568, July 2006.
[RFC4732] Handley, M., Rescorla, E., and IAB, "Internet Denial-of-
Service Considerations", RFC 4732, December 2006.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
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RFC 6404 SPEERMINT Threats and Countermeasures November 2011
[RFC5351] Lei, P., Ong, L., Tuexen, M., and T. Dreibholz, "An
Overview of Reliable Server Pooling Protocols", RFC 5351,
September 2008.
[RFC5352] Stewart, R., Xie, Q., Stillman, M., and M. Tuexen,
"Aggregate Server Access Protocol (ASAP)", RFC 5352,
September 2008.
[RFC5353] Xie, Q., Stewart, R., Stillman, M., Tuexen, M., and A.
Silverton, "Endpoint Handlespace Redundancy Protocol
(ENRP)", RFC 5353, September 2008.
[RFC5486] Malas, D. and D. Meyer, "Session Peering for Multimedia
Interconnect (SPEERMINT) Terminology", RFC 5486,
March 2009.
[RFC5853] Hautakorpi, J., Camarillo, G., Penfield, R., Hawrylyshen,
A., and M. Bhatia, "Requirements from Session Initiation
Protocol (SIP) Session Border Control (SBC) Deployments",
RFC 5853, April 2010.
[RFC6271] Mule, J-F., "Requirements for SIP-Based Session Peering",
RFC 6271, June 2011.
[RFC6406] Malas, D., Ed. and J. Livingood, Ed., "Session PEERing for
Multimedia INTerconnect (SPEERMINT) Architecture",
RFC 6406, November 2011.
[VOIPSATAXONOMY]
Zar, J. and et al, "VOIPSA VoIP Security and Privacy
Threat Taxonomy, Public Release 1.0",
http://www.voipsa.org/Activities/taxonomy.php,
October 2005.
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Authors' Addresses
Jan Seedorf
NEC Laboratories Europe, NEC Europe, Ltd.
Kurfuersten-Anlage 36
Heidelberg 69115
Germany
Phone: +49 (0) 6221 4342 221
EMail: jan.seedorf@neclab.eu
URI: http://www.neclab.eu
Saverio Niccolini
NEC Laboratories Europe, NEC Europe, Ltd.
Kurfuersten-Anlage 36
Heidelberg 69115
Germany
Phone: +49 (0) 6221 4342 118
EMail: saverio.niccolini@.neclab.eu
URI: http://www.neclab.eu
Eric Chen
Information Sharing Platform Laboratories, NTT
3-9-11 Midori-cho
Musashino, Tokyo 180-8585
Japan
EMail: eric.chen@lab.ntt.co.jp
URI: http://www.ntt.co.jp/index_e.html
Hendrik Scholz
VOIPFUTURE GmbH
Wendenstrasse 4
Hamburg 20097
Germany
Phone: +49 (0) 40 688 900 163
EMail: hendrik.scholz@voipfuture.com
URI: http://voipfuture.com
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RFC, FYI, BCP