Network Working Group                                         A. Barbir
Internet-Draft                                           Nortel Networks
Expires: July 26, 2003                                        O. Batuner
                                                  Independent consultant
                                                             B. Srinivas
                                                              M. Hofmann
                                           Bell Labs/Lucent Technologies
                                                                H. Orman
                                               Purple Streak Development
                                                        January 25, 2003

                  Security Threats and Risks for OPES

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
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   The list of current Internet-Drafts can be accessed at http://

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   This Internet-Draft will expire on July 26, 2003.

Copyright Notice

   Copyright (C) The Internet Society (2003).  All Rights Reserved.


   The document investigates the security threats associated with OPES.
   The effects of security threats on the underlying architecture are
   discussed.  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 . . . . . . . . . . . . . . . . . . . . . . .   3
   2.     OPES Data Flow Threats . . . . . . . . . . . . . . . . . .   5
   2.1    OPES Flow Network Level Threats  . . . . . . . . . . . . .   6
   2.1.1  Connection-Flow Denial-of-Service (DoS)  . . . . . . . . .   6
   2.1.2  Threats to network robustness  . . . . . . . . . . . . . .   7
   2.2    OPES Flow Application Level Threats  . . . . . . . . . . .   7
   2.2.1  Unauthorized OPES entities . . . . . . . . . . . . . . . .   7
   2.2.2  Unauthorized actions of legitimate OPES entities . . . . .   7
   2.2.3  Unwanted content transformations . . . . . . . . . . . . .   8
   2.2.4  Corrupted content  . . . . . . . . . . . . . . . . . . . .   8
   2.2.5  Threats to message structure integrity . . . . . . . . . .   8
   2.2.6  Granularity of protection  . . . . . . . . . . . . . . . .   9
   2.2.7  Risks of hop-by-hop protection . . . . . . . . . . . . . .   9
   2.2.8  Threats to integrity of complex data . . . . . . . . . . .  10
   2.2.9  Denial of Service (DoS)  . . . . . . . . . . . . . . . . .  10
   2.2.10 Tracing and notification information . . . . . . . . . . .  10
   2.2.11 Unauthenticated Communication in OPES Flow . . . . . . . .  10
   3.     Threats to out-of-band data  . . . . . . . . . . . . . . .  11
   3.1    Threats that endanger the OPES data flow . . . . . . . . .  11
   3.2    Inaccurate Accounting Information  . . . . . . . . . . . .  11
   3.3    OPES service request repudiation . . . . . . . . . . . . .  12
   3.4    Inconsistent privacy policy  . . . . . . . . . . . . . . .  12
   3.5    Exposure of privacy preferences  . . . . . . . . . . . . .  12
   3.6    Exposure of security settings  . . . . . . . . . . . . . .  12
   3.7    Improper enforcement of privacy and security policy  . . .  13
   3.8    DOS Attacks  . . . . . . . . . . . . . . . . . . . . . . .  13
   4.     Security Considerations  . . . . . . . . . . . . . . . . .  14
          Normative References . . . . . . . . . . . . . . . . . . .  15
          Informative References . . . . . . . . . . . . . . . . . .  16
          Authors' Addresses . . . . . . . . . . . . . . . . . . . .  16
   A.     Acknowledgements . . . . . . . . . . . . . . . . . . . . .  18
          Intellectual Property and Copyright Statements . . . . . .  19

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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].

   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 limited trust
   relationship with another OPES administrative domain for certain

   OPES service provide 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

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   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.

   The document is organized as follows: Section 2 discusses threats to
   OPES data flow on network and application level, section 3 discusses
   threats to other parts of the system and section 4 discusses security

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2. OPES Data Flow Threats

   Threats to 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 possible modify all passing data.  An OPES processor
   meets this definition.  This calls for  special attention to
   protection measures at all levels of the system.

   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 OPES data flow

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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 limit the ability of service
   providers of positioning the OPES processor behind firewalls and may
   exposes the OPES processor,  including remote callout servers, to
   network level attacks.  For example, the use of TCP/IP as 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 covers possible DOS attack on OPES processor,
   remote callout server or data consumer application and network

2.1.1 Connection-Flow Denial-of-Service (DoS)

   OPES processors, or callout servers or data consumer applications can
   be vulnerable to DOS attacks.  DOS attacks can be of various types.
   One example of DOS attacks 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,

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   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 does violate 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 due to
   inappropriate implementations such as programming and 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.

   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

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   was set, it looks at least impractical if not impossible to request
   such permission from the end user.  The authorization is basically
   given for the class of 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
   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.

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).

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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, or 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

   OPES services cannot be applied to data protected with end-to-end
   encryption methods because, by definition, the decryption key cannot
   be shared with OPES processors.  This means that if the endpoint
   policies permit OPES services, the data must either be transmitted
   without confidentiality protections or else with an alternative to
   end-to-end encryption: hop-by-hop encryption.  In the latter case,
   all the parties in the OPES processing path must understand the
   encryption requirement and negotiate encrypted connections with their
   OPES partners.

   Hop-by-hop protection is less effective than end-to-end protection,
   because any processor in the path which shares a cryptographic key,
   can violate the confidentiality or integrity of the data without

   If a pair of processors in the delivery path use weak cryptography or
   manage keys poorly, there is a danger of data leakage.  For this
   reason, different cryptographic keys should be used for each leg of
   the data stream.

   Even if the data is not confidential, one might desire some checks on
   data integrity, to avoid modifications by unauthorized parties.  The
   comments above apply to the use of end-to-end integrity, if it is
   based on shared-key cryptography.  Again, it should be possible to
   use hop-by-hop data integrity to protect data as it moves between
   protection domains.

   Currently there is no method to signal hop-by-hop encryption or

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   integrity requirements.  Either this must be added to the application
   protocol, or OPES must define its own signaling protocol, or all OPES
   traffic MUST ALWAYS be encrypted.

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
   (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.

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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
   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 content provider,
   service provider or as a basis  for third party service.  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

   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

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   for the services performed.

   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 accounting system (by distortion
   of data or issuing false configuration commands) may result in
   incorrect resource management and DoS by artificial resource

3.3 OPES service request repudiation

   An entity (producer or consumer) makes an authorized request and
   claim, later, 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 the behalf of all
   parties.  This action  SHOULD have a request, and SHOULD have an
   action, and SHOULD have a means of repudiation of the request, and
   SHOULD have a means to specify 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 regulation
   which he is not even aware of.

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.

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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 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.

   There is also the possibility of security and privacy leaks due to
   the accidental misconfiguration or, due to missunderstanding 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 ability to withstand such attacks is of
   paramount importance.

3.8 DOS Attacks

   DOS attacks can be of various types.  One type of DOS attacks can
   take 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.

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4. Security Considerations

   This document discusses multiple security and privacy issues related
   to the OPES services.

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Normative References

   [1]  A. Barbir et. al, "An Architecture for Open Pluggable Edge
        Services (OPES)", Internet-Draft:
        internet-drafts/draft-ietf-opes-architecture-04.txt, Aug 2002.

   [2]  A. Barbir et. al, "OPES Use Cases and Deployment Scenarios",
        draft-ietf-opes-scenarios-01, July 2002.

   [3]  A. Barbir et. al, "Requirements for Policy, Authorization  and
        Enforcement of OPES Services", Internet-Draft: http://  drafts/
        draft-ietf-opes-authorization-00.txt , October  2002.

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Informative References

   [4]  Floyd, S. and L. Daigle, "IAB Architectural and Policy
        Considerations for Open Pluggable Edge Services", RFC 3238,
        January 2002.

   [5]  Fielding, R., Gettys, J., Mogul, J., Nielsen, H., Masinter, L.,
        Leach, P. and T. Berners-Lee, "Hypertext Transfer Protocol --
        HTTP/1.1", RFC 2616, June 1999.

   [6]  Cooper, I., "Internet Web Replication and Caching Taxonomy", RFC
        3040, January 2001.

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


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   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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Appendix A. Acknowledgements


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Intellectual Property Statement

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Internet-Draft    Security Threats and Risks for OPES       January 2003



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