Network Working Group A. Barbir
Request for Comments: 3837 Nortel Networks
Category: Informational O. Batuner
Independent consultant
B. Srinivas
Nokia
M. Hofmann
Bell Labs/Lucent Technologies
H. Orman
Purple Streak Development
August 2004
Security Threats and Risks for Open Pluggable Edge Services (OPES)
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2004).
Abstract
The document investigates the security threats associated with the
Open Pluggable Edge Services (OPES) and discusses the effects of
security threats on the underlying architecture. The main goal of
this document is threat discovery and analysis. The document does
not specify or recommend any solutions.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. OPES Data Flow Threats . . . . . . . . . . . . . . . . . . . . 4
2.1. OPES Flow Network Level Threats . . . . . . . . . . . . 5
2.1.1. Connection-Flow Denial-of-Service (DoS). . . . . 6
2.1.2. Threats to Network Robustness. . . . . . . . . . 6
2.2. OPES Flow Application Level Threats. . . . . . . . . . . 6
2.2.1. Unauthorized OPES Entities . . . . . . . . . . . 6
2.2.2. Unauthorized Actions of legitimate OPES Entities 7
2.2.3. Unwanted Content Transformations . . . . . . . . 7
2.2.4. Corrupted Content . . . . . . . . . . . . . . . 7
2.2.5. Threats to Message Structure Integrity . . . . . 8
2.2.6. Granularity of Protection . . . . . . . . . . . 8
2.2.7. Risks of Hop-by-Hop Protection . . . . . . . . . 8
2.2.8. Threats to Integrity of Complex Data . . . . . . 8
2.2.9. Denial of Service (DoS) . . . . . . . . . . . . 9
2.2.10. Tracing and Notification Information . . . . . . 9
2.2.11. Unauthenticated Communication in OPES Flow . . . 9
3. Threats to Out-of-Band Data . . . . . . . . . . . . . . . . . 9
3.1. Threats that Endanger the OPES Data Flow . . . . . . . . 10
3.2. Inaccurate Accounting Information . . . . . . . . . . . 10
3.3. OPES Service Request Repudiation . . . . . . . . . . . . 11
3.4. Inconsistent Privacy Policy . . . . . . . . . . . . . . 11
3.5. Exposure of Privacy Preferences . . . . . . . . . . . . 11
3.6. Exposure of Security Settings . . . . . . . . . . . . . 11
3.7. Improper Enforcement of Privacy and Security Policy . . 11
3.8. DoS Attacks . . . . . . . . . . . . . . . . . . . . . . 12
4. Security Considerations . . . . . . . . . . . . . . . . . . . 12
5. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
5.1. Normative References . . . . . . . . . . . . . . . . . . 12
5.2. Informative References . . . . . . . . . . . . . . . . . 12
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12
7. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 13
8. Full Copyright Statement . . . . . . . . . . . . . . . . . . . 14
1. Introduction
The Open Pluggable Edge Services (OPES) [1] architecture enables
cooperative application services (OPES services) between a data
provider, a data consumer, and zero or more OPES processors. The
application services under consideration analyze and possibly
transform application-level messages exchanged between the data
provider and the data consumer. The OPES processor can distribute
the responsibility of service execution by communicating and
collaborating with one or more remote callout servers. The details
of the OPES architecture can be found in [1].
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Security threats with respect to OPES can be viewed from different
angles. There are security risks that affect content consumer
applications, and those that affect the data provider applications.
These threats affect the quality and integrity of data that the
applications either produce or consume. On the other hand, the
security risks can also be categorized into trust within the system
(i.e., OPES service providers) and protection of the system from
threats imposed by outsiders such as hackers and attackers. Insiders
are those parties that are part of the OPES system. Outsiders are
those entities that are not participating in the OPES system.
It must be noted that not everyone in an OPES delivery path is
equally trusted. Each OPES administrative trust domain must protect
itself from all outsiders. Furthermore, it may have a limited trust
relationship with another OPES administrative domain for certain
purposes.
OPES service providers must use authentication as the basis for
building trust relationships between administrative domains.
Insiders can intentionally or unintentionally inflict harm and damage
on the data consumer and data provider applications. This can be
through bad system configuration, execution of bad software or, if
their networks are compromised, by inside or outside hackers.
Depending on the deployment scenario, the trust within the OPES
system is based on a set of transitive trust relationships between
the data provider application, the OPES entities, and the data
consumer application. Threats to OPES entities can be at the OPES
flow level and/or at the network level.
In considering threats to the OPES system, the document will follow a
threat analysis model that identifies the threats from the
perspective of how they will affect the data consumer and the data
provider applications.
The main goal of this document is threat discovery and analysis. The
document does not specify or recommend any solutions.
It is important to mention that the OPES architecture has many
similarities with other so called overlay networks, specifically web
caches and content delivery networks (CDN) (see [2], [4]). This
document focuses on threats that are introduced by the existence of
the OPES processor and callout servers. Security threats specific to
content services that do not use the OPES architecture are considered
out-of-scope of this document. However, this document can be used as
input when considering security implications for web caches and CDNs.
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The document is organized as follows: Section 2 discusses threats to
OPES data flow on the network and application level, section 3
discusses threats to other parts of the system, and section 4
discusses security considerations.
2. OPES Data Flow Threats
Threats to the OPES data flow can affect the data consumer and data
provider applications. At the OPES flow level, threats can occur at
Policy Enforcement Points, and Policy Decision Points [3], and along
the OPES flow path where network elements are used to process the
data.
A serious problem is posed by the very fact that the OPES
architecture is based on widely adopted protocols (HTTP is used as an
example). The architecture document specifically requires that "the
presence of an OPES processor in the data request/response flow SHALL
NOT interfere with the operations of non-OPES aware clients and
servers". This greatly facilitates OPES' deployment, but on the
other hand a vast majority of clients (browsers) will not be able to
exploit any safeguards added as base protocol extensions.
For the usual data consumer, who might have questions such as (Where
does this content come from? Can I get it another way? What is the
difference? Is it legitimate?). Even if there are facilities and
technical expertise present to pursue these questions, such thorough
examination of each result is prohibitively expensive in terms of
time and effort. OPES-aware content providers may try to protect
themselves by adding verification scripts and special page
structures. OPES-aware end users may use special tools. In all
other cases (non-OPES aware clients and servers) protection will rely
on monitoring services and investigation of occasionally discovered
anomalies.
An OPES system poses a special danger as a possible base for
classical man-in-the-middle attacks. One of the reasons why such
attacks are relatively rare is the difficulty in finding an
appropriate base: a combination of a traffic interception point
controlling a large flow of data and an application codebase running
on a high-performance hardware with sufficient performance to analyze
and possibly modify all passing data. An OPES processor meets this
definition. This calls for special attention to protection measures
at all levels of the system.
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Any compromise of an OPES processor or remote callout server can have
a ripple effect on the integrity of the affected OPES services across
all service providers that use the service. To mitigate this threat,
appropriate security procedures and tools (e.g., a firewall) should
be applied.
Specific threats can exist at the network level and at the OPES data
flow level.
2.1. OPES Flow Network Level Threats
OPES processor and callout servers are susceptible to network level
attacks from outsiders or from the networks of other OPES service
providers (i.e., if the network of a contracted OPES service is
compromised).
The OPES architecture is based on common application protocols that
do not provide strong guarantees of privacy, authentication, or
integrity. The IAB considerations [4] require that the IP address of
an OPES processor be accessible to data consumer applications at the
IP addressing level. This requirement limits the ability of service
providers to position the OPES processor behind firewalls and may
expose the OPES processor and remote callout servers to network level
attacks. For example, the use of TCP/IP as a network level protocol
makes OPES processors subject to many known attacks, such as IP
spoofing and session stealing.
The OPES system is also susceptible to a number of security threats
that are commonly associated with network infrastructure. These
threats include snooping, denial of service, sabotage, vandalism,
industrial espionage, and theft of service.
There are best practice solutions to mitigate network level threats.
It is recommended that the security of the OPES entities at the
network level be enhanced using known techniques and methods that
minimize the risks of IP spoofing, snooping, denial of service, and
session stealing.
At the OPES Flow level, connection-level security between the OPES
processor and callout servers is an important consideration. For
example, it is possible to spoof the OPES processor or the remote
callout server. There are threats to data confidentiality between
the OPES processor and the remote callout server in an OPES flow.
The next subsections cover possible DoS attacks on an OPES processor,
remote callout server or data consumer application, and network
robustness.
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2.1.1. Connection-Flow Denial-of-Service (DoS)
OPES processors, callout servers, and data consumer applications can
be vulnerable to DoS attacks. DoS attacks can be of various types.
One example of a DoS attack is the overloading of OPES processors or
callout servers by spurious service requests issued by a malicious
node, which denies the legal data traffic the necessary resources to
render service. The resources include CPU cycles, memory, network
interfaces, etc. A Denial-of-Service attack can be selective,
generic, or random in terms of which communication streams are
affected.
Distributed DoS is also possible when an attacker successfully
directs multiple nodes over the network to initiate spurious service
requests to an OPES processor (or callout server) simultaneously.
2.1.2. Threats to Network Robustness
If OPES implementation violates end-to-end addressing principles, it
could endanger the Internet infrastructure by complicating routing
and connection management. If it does not use flow-control
principles for managing connections, or if it interferes with end-
to-end flow control of connections that it did not originate, then it
could cause Internet congestion.
An implementation that violates the IAB requirement of explicit IP
level addressing (for example, by adding OPES functional capabilities
to an interception proxy) may defeat some of the protective
mechanisms and safeguards built into the OPES architecture.
2.2. OPES Flow Application Level Threats
At the content level, threats to the OPES system can come from
outsiders or insiders. The threat from outsiders is frequently
intentional. Threats from insiders can be intentional or accidental.
Accidents may result from programming or configuration errors that
result in bad system behavior.
Application level problems and threats to the OPES systems are
discussed below:
2.2.1. Unauthorized OPES Entities
Although one party authorization is mandated by the OPES
architecture, such authorization occurs out-of-band. Discovering the
presence of an OPES entity and verifying authorization requires
special actions and may present a problem.
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Adding notification and authorization information to the data
messages (by using base protocol extensions) may help, especially if
the data consumer's software is aware of such extensions.
2.2.2. Unauthorized Actions of Legitimate OPES Entities
According to the OPES architecture, the authorization is not tightly
coupled with specific rules and procedures triggered by the rules.
Even if a requirement to approve each particular rule and procedure
was set, it looks at least impractical, if not impossible, to request
such permission from the end user. Authorization granularity extends
to transformation classes, but not to individual rules or
transformations. The actual rules and triggered procedures may
(maliciously or due to a programming error) perform actions that they
are not authorized for.
2.2.3. Unwanted Content Transformations
An authorized OPES service may perform actions that do not adhere to
the expectations of the party that gave the authorization for the
service. Examples may include ad flooding by a local ad insertion
service or use of inappropriate policy by a content filtering
service.
On the other hand, an OPES entity acting on behalf of one party may
perform transformations that another party deems inappropriate.
Examples may include replacing ads initially inserted by the content
provider or applying filtering transformations that change the
meaning of the text.
2.2.4. Corrupted Content
The OPES system may deliver outdated or otherwise distorted
information due to programming problems or as a result of malicious
attacks. For example, a compromised server, instead of performing an
OPES service, may inject bogus content. Such an action may be an act
of cyber-vandalism (including virus injection) or intentional
distribution of misleading information (such as manipulations with
financial data).
A compromised OPES server or malicious entity in the data flow may
introduce changes specifically intended to cause improper actions in
the OPES server or callout server. These changes may be in the
message body, headers, or both. This type of threat is discussed in
more detail below.
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2.2.5. Threats to Message Structure Integrity
An OPES server may add, remove, or delete certain headers in a
request and/or response message (for example, to implement additional
privacy protection or assist in content filtering). Such changes may
violate end-to-end integrity requirements or defeat services that use
information provided in such headers (for example, some local
filtering services or reference-based services).
2.2.6. Granularity of Protection
OPES services have implicit permission to modify content. However,
the permissions generally apply only to portions of the content, for
example, URL's between particular HTML tags, text in headlines, or
URL's matching particular patterns. In order to express such
policies, one must be able to refer to portions of messages and to
detect modifications to message parts.
Because there is currently very little support for policies that are
expressed in terms of message parts, it will be difficult to
attribute any particular modification to a particular OPES processor,
or to automatically detect policy violations.
A fine-grained policy language should be devised, and it could be
enforced using digital signatures. This would avoid the problems
inherent in hop-by-hop data integrity measures (see next section).
2.2.7. Risks of Hop-by-Hop Protection
Generally, OPES services cannot be applied to data protected with
end-to-end encryption methods because the decryption key cannot be
shared with OPES processors without compromising the intended
confidentiality of the data. This means that if the endpoint
policies permit OPES services, the data must either be transmitted
without confidentiality protections or an alternative model to end-
to-end encryption must be developed, one in which the confidentiality
is guaranteed hop-by-hop. Extending the end-to-end encryption model
is out of scope of this work.
OPES services that modify data are incompatible with end-to-end
integrity protection methods, and this work will not attempt to
define hop-by-hop integrity protection methods.
2.2.8. Threats to Integrity of Complex Data
The OPES system may violate data integrity by applying inconsistent
transformations to interrelated data objects or references within the
data object. Problems may range from a broken reference structure
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(modified/missing targets, references to wrong locations or missing
documents) to deliberate replacement/deletion/insertion of links that
violate intentions of the content provider.
2.2.9. Denial of Service (DoS)
The data consumer application may not be able to access data if the
OPES system fails for any reason.
A malicious or malfunctioning node may be able to block all traffic.
The data traffic destined for the OPES processor (or callout server)
may not be able to use the services of the OPES device. The DoS may
be achieved by preventing the data traffic from reaching the
processor or the callout server.
2.2.10. Tracing and Notification Information
Inadequate or vulnerable implementation of the tracing and
notification mechanisms may defeat safeguards built into the OPES
architecture.
Tracing and notification facilities may become a target of malicious
attack. Such an attack may create problems in discovering and
stopping other attacks.
The absence of a standard for tracing and notification information
may present an additional problem. This information is produced and
consumed by the independent entities (OPES servers/user agents/
content provider facilities). This calls for a set of standards
related to each base protocol in use.
2.2.11. Unauthenticated Communication in OPES Flow
There are risks and threats that could arise from unauthenticated
communication between the OPES server and callout servers. Lack of
use of strong authentication between OPES processors and callout
servers may open security holes whereby DoS and other types of
attacks (see sections [2 and 3]) can be performed.
3. Threats to Out-of-Band Data
The OPES architecture separates a data flow from a control
information flow (loading rulesets, trust establishment, tracing,
policy propagation, etc.). There are certain requirements set for
the latter, but no specific mechanism is prescribed. This gives more
flexibility for implementations, but creates more burden for
implementers and potential customers to ensure that each specific
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implementation meets all requirements for data security, entity
authentication, and action authorization.
In addition to performing correct actions on the OPES data flow, any
OPES implementation has to provide an adequate mechanism to satisfy
requirements for out-of-band data and signaling information
integrity.
Whatever the specific mechanism may be, it inevitably becomes subject
to multiple security threats and possible attacks. The way the
threats and attacks may be realized depends on implementation
specifics but the resulting harm generally falls into two categories:
threats to OPES data flow and threats to data integrity.
The specific threats are:
3.1. Threats that Endanger the OPES Data Flow
Any weakness in the implementation of a security, authentication, or
authorization mechanism may open the door to the attacks described in
section 2.
An OPES system implementation should address all these threats and
prove its robustness and ability to withstand malicious attacks or
networking and programming problems.
3.2. Inaccurate Accounting Information
Collecting and reporting accurate accounting data may be vital when
OPES servers are used to extend a business model of a content
provider, service provider, or as a basis for third party service.
The ability to collect and process accounting data is an important
part of OPES' system functionality. This functionality may be
challenged by distortion or destruction of base accounting data
(usually logs), processed accounting data, accounting parameters, and
reporting configuration.
As a result a data consumer may be inappropriately charged for
viewing content that was not successfully delivered, or a content
provider or independent OPES services provider may not be compensated
for the services performed.
The OPES system may use accounting information to distribute
resources between different consumers or limit resource usage by a
specific consumer. In this case an attack on the accounting system
(by distortion of data or issuing false configuration commands) may
result in incorrect resource management and DoS by artificial
resource starvation.
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3.3. OPES Service Request Repudiation
An entity (producer or consumer) might make an authorized request and
later claim that it did not make that request. As a result, an OPES
entity may be held liable for unauthorized changes to the data flow,
or will be unable to receive compensation for a service.
There should be a clear request that this service is required and
there should be a clear course of action on behalf of all parties.
This action should have a request, an action, a non-repudiable means
of verifying the request, and a means of specifying the effect of the
action.
3.4. Inconsistent Privacy Policy
The OPES entities may have privacy policies that are not consistent
with the data consumer application or content provider application.
Privacy related problems may be further complicated if OPES entities,
content providers, and end users belong to different jurisdictions
with different requirements and different levels of legal protection.
As a result, the end user may not be aware that he or she does not
have the expected legal protection. The content provider may be
exposed to legal risks due to a failure to comply with regulations
of which he is not even aware.
3.5. Exposure of Privacy Preferences
The OPES system may inadvertently or maliciously expose end user
privacy settings and requirements.
3.6. Exposure of Security Settings
There are risks that the OPES system may expose end user security
settings when handling the request and responses. The user data must
be handled as sensitive system information and protected against
accidental and deliberate disclosure.
3.7. Improper Enforcement of Privacy and Security Policy
OPES entities are part of the content distribution system and as such
take on certain obligations to support security and privacy policies
mandated by the content producer and/or end user. However there is a
danger that these policies are not properly implemented and enforced.
The data consumer application may not be aware that its protections
are no longer in effect.
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There is also the possibility of security and privacy leaks due to
the accidental misconfiguration or, due to misunderstanding what
rules are in effect for a particular user or request.
Privacy and security related parts of the systems can be targeted by
malicious attacks and the ability to withstand such attacks is of
paramount importance.
3.8. DoS Attacks
DoS attacks can be of various types. One type of DoS attack takes
effect by overloading the client. For example, an intruder can
direct an OPES processor to issue numerous responses to a client.
There is also additional DoS risk from a rule misconfiguration that
would have the OPES processor ignore a data consumer application.
4. Security Considerations
This document discusses multiple security and privacy issues related
to the OPES services.
5. References
5.1. Normative References
[1] Barbir, A., Penno, R., Chen, R., Hofmann, M., and H. Orman, "An
Architecture for Open Pluggable Edge Services (OPES)", RFC 3835,
August 2004.
[2] Barbir, A., Burger, E., Chen, R., McHenry, S., Orman, H., and R.
Penno, "OPES Use Cases and Deployment Scenarios", RFC 3752,
April 2004.
[3] Barbir, A., Batuner, O., Beck, A., Chan, T., and H. Orman,
"Policy, Authorization, and Enforcement Requirements of Open
Pluggable Edge Services (OPES)", RFC 3838, August 2004.
5.2. Informative References
[4] Floyd, S. and L. Daigle, "IAB Architectural and Policy
Considerations for Open Pluggable Edge Services", RFC 3238,
January 2002.
6. Acknowledgements
Many thanks to T. Chan (Nokia) and A. Beck (Lucent).
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7. Authors' Addresses
Abbie Barbir
Nortel Networks
3500 Carling Avenue
Nepean, Ontario K2H 8E9
Canada
Phone: +1 613 763 5229
EMail: abbieb@nortelnetworks.com
Oskar Batuner
Independent consultant
EMail: batuner@attbi.com
Bindignavile Srinivas
Nokia
5 Wayside Road
Burlington, MA 01803
USA
EMail: bindignavile.srinivas@nokia.com
Markus Hofmann
Bell Labs/Lucent Technologies
Room 4F-513
101 Crawfords Corner Road
Holmdel, NJ 07733
US
Phone: +1 732 332 5983
EMail: hofmann@bell-labs.com
Hilarie Orman
Purple Streak Development
EMail: ho@alum.mit.edu
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8. Full Copyright Statement
Copyright (C) The Internet Society (2004). This document is subject
to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights.
This document and the information contained herein are provided on an
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OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
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INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
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RFC, FYI, BCP