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Security Threats and Risks for Open Pluggable Edge Services (OPES)


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Network Working Group                                          A. Barbir
Request for Comments: 3837                               Nortel Networks
Category: Informational                                       O. Batuner
                                                  Independent consultant
                                                             B. Srinivas
                                                              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).


   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

   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

   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

   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

   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

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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

   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

   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

   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

   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

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

   Phone: +1 613 763 5229

   Oskar Batuner
   Independent consultant


   Bindignavile Srinivas
   5 Wayside Road
   Burlington, MA  01803


   Markus Hofmann
   Bell Labs/Lucent Technologies
   Room 4F-513
   101 Crawfords Corner Road
   Holmdel, NJ  07733

   Phone: +1 732 332 5983

   Hilarie Orman
   Purple Streak Development


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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

Intellectual Property

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   Intellectual Property Rights or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; nor does it represent that it has
   made any independent effort to identify any such rights.  Information
   on the procedures with respect to rights in RFC documents can be
   found in BCP 78 and BCP 79.

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   The IETF invites any interested party to bring to its attention any
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   this standard.  Please address the information to the IETF at ietf-


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