IEC TR 62541-2:2020

IEC TR 62541-2 ®
Edition 3.0 2020-11
TECHNICAL
REPORT
colour
inside
OPC unified architecture –
Part 2: Security Model
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IEC TR 62541-2 ®
Edition 3.0 2020-11
TECHNICAL
REPORT
colour
inside
OPC unified architecture –
Part 2: Security Model
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 25.040.40; 35.100.01 ISBN 978-2-8322-9077-4

– 2 – IEC TR 62541-2:2020 © IEC 2020
CONTENTS
FOREWORD . 5
1 Scope . 7
2 Normative references . 7
3 Terms, definitions, and abbreviated terms . 8
3.1 Terms and definitions . 8
3.2 Abbreviated terms . 13
4 OPC UA security architecture . 13
4.1 OPC UA security environment . 13
4.2 Security objectives . 14
4.2.1 Overview . 14
4.2.2 Authentication. 15
4.2.3 Authorization . 15
4.2.4 Confidentiality . 15
4.2.5 Integrity . 15
4.2.6 Non-Repudiation . 15
4.2.7 Auditability . 15
4.2.8 Availability . 15
4.3 Security threats to OPC UA systems . 15
4.3.1 Overview . 15
4.3.2 Denial of Service . 16
4.3.3 Eavesdropping . 17
4.3.4 Message spoofing . 17
4.3.5 Message alteration . 17
4.3.6 Message replay . 17
4.3.7 Malformed Messages . 18
4.3.8 Server profiling . 18
4.3.9 Session hijacking . 18
4.3.10 Rogue Server . 18
4.3.11 Rogue Publisher . 18
4.3.12 Compromising user credentials . 19
4.3.13 Repudiation . 19
4.4 OPC UA relationship to site security . 19
4.5 OPC UA security architecture . 20
4.5.1 Overview . 20
4.5.2 Client / Server . 21
4.5.3 Publish-Subscribe . 22
4.6 SecurityPolicies . 23
4.7 Security Profiles . 24
4.8 Security Mode Settings . 24
4.9 User Authentication . 24
4.10 Application Authentication . 24
4.11 User Authorization . 25
4.12 Roles . 25
4.13 OPC UA security related Services . 25
4.14 Auditing . 26
4.14.1 General . 26

4.14.2 Single Client and Server . 27
4.14.3 Aggregating Server . 28
4.14.4 Aggregation through a non-auditing Server . 28
4.14.5 Aggregating Server with service distribution . 29
5 Security reconciliation . 30
5.1 Reconciliation of threats with OPC UA security mechanisms . 30
5.1.1 Overview . 30
5.1.2 Denial of Service . 31
5.1.3 Eavesdropping . 32
5.1.4 Message spoofing . 32
5.1.5 Message alteration . 33
5.1.6 Message replay . 33
5.1.7 Malformed Messages . 33
5.1.8 Server profiling . 33
5.1.9 Session hijacking . 33
5.1.10 Rogue Server or Publisher . 34
5.1.11 Compromising user credentials . 34
5.1.12 Repudiation . 34
5.2 Reconciliation of objectives with OPC UA security mechanisms . 34
5.2.1 Overview . 34
5.2.2 Application Authentication . 34
5.2.3 User Authentication . 35
5.2.4 Authorization . 35
5.2.5 Confidentiality . 35
5.2.6 Integrity . 35
5.2.7 Auditability . 35
5.2.8 Availability . 36
6 Implementation and deployment considerations . 36
6.1 Overview. 36
6.2 Appropriate timeouts . 36
6.3 Strict Message processing . 36
6.4 Random number generation . 37
6.5 Special and reserved packets . 37
6.6 Rate limiting and flow control . 37
6.7 Administrative access . 37
6.8 Cryptographic Keys . 38
6.9 Alarm related guidance . 38
6.10 Program access . 38
6.11 Audit event management . 39
6.12 OAuth2, JWT and User roles . 39
6.13 HTTPs, SSL/TLS & Websockets . 39
6.14 Reverse Connect . 39
7 Unsecured Services . 40
7.1 Overview. 40
7.2 Multicast Discovery . 40
7.3 Global Discovery Server Security . 40
7.3.1 Overview . 40
7.3.2 Rogue GDS . 40
7.3.3 Threats against a GDS . 41

– 4 – IEC TR 62541-2:2020 © IEC 2020
7.3.4 Certificate management threats . 41
8 Certificate management . 42
8.1.1 Overview . 42
8.1.2 Self-signed certificate management . 42
8.1.3 CA Signed Certificate management . 43
8.1.4 GDS Certificate Management . 44
Bibliography . 47

Figure 1 – OPC UA network example . 14
Figure 2 – OPC UA security architecture – Client / Server . 20
Figure 3 – OPC UA security architecture – Publisher-Subscriber . 21
Figure 4 – Role overview . 25
Figure 5 – Simple Servers . 27
Figure 6 – Aggregating Servers . 28
Figure 7 – Aggregation with a non-auditing Server . 29
Figure 8 – Aggregate Server with service distribution . 30
Figure 9 – Manual Certificate handling . 42
Figure 10 – CA Certificate handling . 43
Figure 11 – Certificate handling . 45

Table 1 – Security Reconciliation Threats Summary . 31

INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
OPC UNIFIED ARCHITECTURE –
Part 2: Security Model
FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
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The main task of IEC technical committees is to prepare International Standards. However, a
technical committee may propose the publication of a technical report when it has collected
data of a different kind from that which is normally published as an International Standard, for
example "state of the art".
IEC TR 62541-2, which is a technical report, has been prepared by subcommittee 65E: Devices
and integration in enterprise systems, of IEC technical committee 65: Industrial-process
measurement, control and automation.
This third edition cancels and replaces the second edition of IEC TR 62541-2, published in
2016. This edition constitutes a technical revision.
This edition includes the following significant technical changes with respect to the previous
edition:
a) protection-targets definition change;
b) threat type clarifications;
c) expanded best practices;
– 6 – IEC TR 62541-2:2020 © IEC 2020
d) added Websockets;
e) added Pub/Sub.
The text of this technical report is based on the following documents:
Enquiry draft Report on voting
65E/679/DTR 65E/703/RVDR
Full information on the voting for the approval of this technical report can be found in the report
on voting indicated in the above table.
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.
Throughout this document and the referenced other Parts of the series, certain document
conventions are used:
Italics are used to denote a defined term or definition that appears in the “Terms and definition”
clause in one of the parts of the series.
Italics are also used to denote the name of a service input or output parameter or the name of
a structure or element of a structure that are usually defined in tables.
The italicized terms and names are also often written in camel-case (the practice of writing
compound words or phrases in which the elements are joined without spaces, with each
element's initial letter capitalized within the compound). For example, the defined term is
AddressSpace instead of Address Space. This makes it easier to understand that there is a
single definition for AddressSpace, not separate definitions for Address and Space.
A list of all parts of the IEC 62541 series, published under the general title OPC Unified
Architecture, can be found on the IEC website.
The committee has decided that the contents of this publication will remain unchanged until the
stability date indicated on the IEC web site under "http://webstore.iec.ch" in the data related to
the specific publication. At this date, the publication will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
IMPORTANT – The 'colour inside' logo on the cover page of this publication indicates
that it contains colours which are considered to be useful for the correct understanding
of its contents. Users should therefore print this document using a colour printer.

OPC UNIFIED ARCHITECTURE –
Part 2: Security Model
1 Scope
This part of IEC 62541 describes the OPC Unified Architecture (OPC UA) security model. It
describes the security threats of the physical, hardware, and software environments in which
OPC UA is expected to run. It describes how OPC UA relies upon other standards for security.
It provides definition of common security terms that are used in this and other parts of the OPC
UA specification. It gives an overview of the security features that are specified in other parts
of the OPC UA specification. It references services, mappings, and Profiles that are specified
normatively in other parts of the OPC UA Specification. It provides suggestions or best practice
guidelines on implementing security. Any seeming ambiguity between this part and one of the
other normative parts does not remove or reduce the requirement specified in the other
normative part.
It is important to understand that there are many different aspects of security that have to be
addressed when developing applications. However, since OPC UA specifies a communication
protocol, the focus is on securing the data exchanged between applications. This does not mean
that an application developer can ignore the other aspects of security like protecting persistent
data against tampering. It is important that the developers look into all aspects of security and
decide how they can be addressed in the application.
This part is directed to readers who will develop OPC UA Client or Server applications or
implement the OPC UA services layer. It is also for end Users that wish to understand the
various security features and functionality provided by OPC UA. It also offers some suggestions
that can be applied when deploying systems. These suggestions are generic in nature since
the details would depend on the actual implementation of the OPC UA Applications and the
choices made for the site security.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies.
For undated references, the latest edition of the referenced document (including any
amendments) applies.
IEC TR 62541-1, OPC Unified Architecture – Part 1: Overview and Concepts
IEC 62541-4, OPC Unified Architecture – Part 4: Services
IEC 62541-5, OPC Unified Architecture – Part 5: Information Model
IEC 62541-6, OPC Unified Architecture – Part 6: Mappings
IEC 62541-7, OPC Unified Architecture – Part 7: Profiles
IEC 62541-12, OPC Unified Architecture – Part 12: Discovery and Global Services
IEC 62541-14, OPC Unified Architecture – Part 14: PubSub
IEC 62351 (all parts), Power systems management and associated information exchange

– 8 – IEC TR 62541-2:2020 © IEC 2020
3 Terms, definitions, and abbreviated terms
3.1 Terms and definitions
For the purposes of this document, the terms and definitions given in IEC TR 62541-1 and the
following apply.
ISO and IEC maintain terminological databases for use in standardization at the following
addresses:
• IEC Electropedia: available at http://www.electropedia.org/
• ISO Online browsing platform: available at http://www.iso.org/obp
3.1.1
Access Restriction
limit on the circumstances where an operation, such as a read, write or a call, can be performed
on a Node
Note 1 to entry: Operations can only be performed on a Node if the Client has the necessary Permissions and has
satisfied all of the Access Restrictions.
3.1.2
Access Token
digitally signed document that asserts that the subject is entitled to access a Resource
Note 1 to entry: The document includes the name of the subject and the Resource being accessed.
3.1.3
Application Instance
individual installation of a program running on one computer
Note 1 to entry: There can be several Application Instances of the same application running at the same time on
several computers or possibly the same computer.
3.1.4
Application Instance Certificate
Certificate of an individual Application Instance that has been installed in an individual host
Note 1 to entry: Different installations of one software product would have different Application Instance
Certificates. The use of an Application Instance Certificate for uses outside of what is described in the specification
could greatly reduce the security provided by the Application Instance Certificate and should be discouraged.
3.1.5
Asymmetric Cryptography
Cryptography method that uses a pair of keys, one that is designated the Private Key and kept
secret, the other called the Public Key that is generally made available
Note 1 to entry: Asymmetric Cryptography is also known as "public-key cryptography". In an Asymmetric Encryption
algorithm when an entity “A” requires Confidentiality for data sent to entity “B”, then entity “A” encrypts the data with
a Public Key provided by entity “B”. Only entity “B” has the matching Private Key that is needed to decrypt the data.
In an asymmetric Digital Signature algorithm when an entity “A” requires message Integrity or to provide
Authentication for data sent to entity “B”, entity A uses its Private Key to sign the data. To verify the signature, entity
B uses the matching Public Key that entity A has provided. In an asymmetric key agreement algorithm, entity A and
entity B each send their own Public Key to the other entity. Then each uses its own Private Key and the other's Public
Key to compute the new key value.’ according to IS Glossary.
3.1.6
Asymmetric Encryption
mechanism used by Asymmetric Cryptography for encrypting data with the Public Key of an
entity and for decrypting data with the associated Private Key

3.1.7
Asymmetric Signature
mechanism used by Asymmetric Cryptography for signing data with the Private Key of an entity
and for verifying the data’s signature with the associated Public Key
3.1.8
Auditability
security objective that assures that any actions or activities in a system can be recorded
3.1.9
Auditing
tracking of actions and activities in the system, including security related activities where Audit
records can be used to review and verify system operations
3.1.10
Authentication
security objective that assures that the identity of an entity such as a Client, Server, or user can
be verified
3.1.11
Authorization
ability to grant access to a system resource
Note 1 to entry: Authorization of access to resources should be based on the need-to-know principle. It is important
that access is restricted in a system.
3.1.12
AuthorizationService
Server which validates a request to access a Resource and can return an Access Token that
grants access to the Resource
Note 1 to entry: The AuthorizationService is also called STS (Security Token Service) in other standards.
3.1.13
Availability
security objective that assures that the system is running normally, that is, no services have
been compromised in such a way to become unavailable or severely degraded
3.1.14
Certificate Authority
entity that can issue Certificates, also known as a CA
Note 1 to entry: The Certificate certifies the ownership of a Public Key by the named subject of the Certificate. This
allows others (relying parties) to rely upon signatures or assertions made by the Private Key that corresponds to the
Public Key that is certified. In this model of trust relationships, a CA is a trusted third party that is trusted by both the
subject (owner) of the Certificate and the party relying upon the Certificate. CAs are characteristic of many Public
Key infrastructure (PKI) schemes
3.1.15
CertificateStore
persistent location where Certificates and Certificate revocation lists (CRLs) are stored
Note 1 to entry: It may be a disk resident file structure, or, on Windows platforms, it may be a Windows registry
location.
3.1.16
Claim
statement in an Access Token that asserts information about the subject which the Authorization
Service knows to be true
Note 1 to entry: Claims can include username, email, and Roles granted to the subject.

– 10 – IEC TR 62541-2:2020 © IEC 2020
3.1.17
Confidentiality
security objective that assures the protection of data from being read by unintended parties
3.1.18
Cryptography
transforming clear, meaningful information into an enciphered, unintelligible form using an
algorithm and a key
3.1.19
Cyber Security Management System
program designed by an organization to maintain the security of the entire organization’s assets
to an established level of Confidentiality, Integrity, and Availability, whether they are on the
business side or the industrial automation and control systems side of the organization
3.1.20
Digital Signature
value computed with a cryptographic algorithm and appended to data in such a way that any
recipient of the data can use the signature to verify the data’s origin and Integrity
3.1.21
Hash Function
algorithm such as SHA-1 for which it is computationally infeasible to find either a data object
that maps to a given hash result (the "one-way" property) or two data objects that map to the
same hash result (the "collision-free" property)
Note 1 to entry: See IS Glossary.
3.1.22
Hashed Message Authentication Code
MAC that has been generated using an iterative Hash Function
3.1.23
Integrity
security objective that assures that information has not been modified or destroyed in an
unauthorized manner
Note 1 to entry: See IS Glossary.
3.1.24
Identity Provider
Server which verifies credentials provided by a Security Principal and returns a token which can
be passed to an associated Authorization Service
3.1.25
Key Exchange Algorithm
protocol used for establishing a secure communication path between two entities in an
unsecured environment whereby both entities apply a specific algorithm to securely exchange
secret keys that are used for securing the communication between them
Note 1 to entry: A typical example of a Key Exchange Algorithm is the SSL Handshake Protocol specified in
SSL/TLS.
3.1.26
Message Authentication Code
short piece of data that results from an algorithm that uses a secret key (see Symmetric
Cryptography) to hash a Message whereby the receiver of the Message can check against
alteration of the Message by computing a MAC that should be identical using the same Message
and secret key
3.1.27
Message Signature
Digital Signature used to ensure the Integrity of Messages that are sent between two entities
Note 1 to entry: There are several ways to generate and verify Message Signatures; however, they can be
categorized as symmetric (See Entry 3.1.40 ) and asymmetric (See Entry 3.1.5) approaches.
3.1.28
Non-Repudiation
strong and substantial evidence of the identity of the signer of a Message and of Message
Integrity, sufficient to prevent a party from successfully denying the original submission or
delivery of the Message and the Integrity of its contents
3.1.29
Nonce
random number that is used once typically by algorithms that generate security keys
3.1.30
Permission
right to execute an operation, such as a read, write or a call, on a Node
3.1.31
Private Key
secret component of a pair of cryptographic keys used for Asymmetric Cryptography
Note 1 to entry: Public Key and Private Key are always generated as a pair, if either is updated the other shall also
be updated.
3.1.32
Public Key
publicly-disclosed component of a pair of cryptographic keys used for Asymmetric Cryptography
Note 1 to entry: See IS Glossary.
Note 2 to entry: Public Key and Private Key are always generated as a pair, if either is updated the other shall also
be updated.
3.1.33
Public Key Infrastructure
set of hardware, software, people, policies, and procedures needed to create, manage, store,
distribute, and revoke Certificates based on Asymmetric Cryptography
Note 1 to entry: The core PKI functions are to register users and issue their public-key Certificates, to revoke
Certificates when required, and to archive data needed to validate Certificates at a much later time. Key pairs for
data Confidentiality may be generated by a Certificate authority (CA); it is a good idea to require a Private Key owner
to generate their own key pair as it improves security because the Private Key would never be transmitted according
to IS Glossary. See PKI and X509 for more details on Public Key Infrastructures.
3.1.34
Resource
secured entity which an application needs to access
Note 1 to entry: A Resource is usually a Server.
3.1.35
Rivest-Shamir-Adleman
algorithm for Asymmetric Cryptography, invented in 1977 by Ron Rivest, Adi Shamir, and
Leonard Adleman
Note 1 to entry: See IS Glossary.

– 12 – IEC TR 62541-2:2020 © IEC 2020
3.1.36
Role
function assumed by a Client when it accesses a Server
Note 1 to entry: A Role may refer to a specific job function such as operator or engineer.
3.1.37
Scope
Claim representing a subset of a Resource
Note 1 to entry: A Scope may indicate a set Nodes managed by a Server.
3.1.38
Security Key Service
Server that accepts Access Tokens issued by the Authorization Service and returns security
keys that can be used to access the specified Resource
Note 1 to entry: The keys are typically used for cryptography operations such as encrypting or decrypting messages
sent on a PubSub stream.
3.1.39
Secure Channel
in OPC UA, communication path established between an OPC UA Client and Server that have
authenticated each other using certain OPC UA services and for which security parameters
have been negotiated and applied
3.1.40
Symmetric Cryptography
branch of cryptography involving algorithms that use the same key for two different steps of the
algorithm (such as encryption and decryption, or signature creation and signature verification)
Note 1 to entry: See IS Glossary.
3.1.41
Symmetric Encryption
mechanism used by Symmetric Cryptography for encrypting and decrypting data with a
cryptographic key shared by two entities
3.1.42
SecurityGroup
publisher and subscribers that utilize a shared security context
3.1.43
Symmetric Signature
mechanism used by Symmetric Cryptography for signing data with a cryptographic key shared
by two entities
Note 1 to entry: The signature is then validated by generating the signature for the data again and comparing these
two signatures. If they are the same, then the signature is valid, otherwise either the key or the data is different from
the two entities.
3.1.44
TrustList
list of Certificates that an OPC UA Application has been configured to trust
3.1.45
Transport Layer Security
standard protocol for creating Secure Channels over IP based networks

3.1.46
X.509 Certificate
Certificate in one of the formats defined by X.509 v1, 2, or 3
Note 1 to entry: An X.509 Certificate contains a sequence of data items and has a Digital Signature computed on
that sequence. OPC UA only uses V3.
3.2 Abbreviated terms
AES Advanced Encryption Standard
CA Certificate Authority
CRL Certificate Revocation List
CSMS Cyber Security Management System
DNS Domain Name System
DSA Digital Signature Algorithm
ECDH Elliptic Curve Diffie-Hellman
ECDSA Elliptic Curve Digital Signature Algorithm
HMAC Hash-based Message Authentication Code
JSON JavaScript Object Notation
JWT JSON Web Token
NIST National Institute of Standard and Technology
PKI Public Key Infrastructure
RSA Public key algorithm for signing or encryption, Rivest, Shamir, Adleman
SHA Secure Hash Algorithm (Multiple versions exist SHA1, SHA256,…)
SKS Security Key Server
SOAP Simple Object Access Protocol
SSL Secure Sockets Layer
TLS Transport Layer Security
UA Unified Architecture
UACP Unified Architecture Connection Protocol
UADP Unified Architecture Datagram Protocol
URI Uniform Resource Identifier
XML Extensible Mark-up Language
4 OPC UA security architecture
4.1 OPC UA security environment
OPC UA is a protocol used between components in the operation of an industrial facility at
multiple levels: from high-level enterprise management to low-level direct process control of a
device. The use of OPC UA for enterprise management involves dealings with customers and
suppliers. It may be an attractive target for industrial espionage or sabotage and may also be
exposed to threats through untargeted malware, such as worms, circulating on public networks.
Disruption of communications at the process control could result in financial losses, affect
employee and public safety or cause environmental damage.
OPC UA will be deployed in a diverse range of operational environments with varying
assumptions about threats and accessibility, and with a variety of security policies and
enforcement regimes. OPC UA, therefore, provides a flexible set of security mechanisms.
Figure 1 is a composite that shows a combination of such environments. Some OPC UA
Applications are on the same host and can be easily protected from external attack. Some OPC
UA Applications are on different hosts in the same operations network and might be protected

– 14 – IEC TR 62541-2:2020 © IEC 2020
by the security boundary protections that separate the operations network from external
connections. Some OPC UA Applications run in relatively open environments where users and
applications might be difficult to control. Other OPC UA Applications are embedded in control
systems that have no direct electronic connection to external systems.
Internet
OPC
OPC Client CA
S
Subscriber
Ted
Enterprise Network
S
S
CA
Attacker
Key S
Attacker OPC Server
Server
Eve
Theresa
OPC Client
OPC Client
Bob
Operations Network
OPC Client
OPC OPC Server OPC Server
Alice
Publisher
Subscriber
Subscriber
OPC Client OPC Client
Plant Floor Network
OPC Server
OPC OPC Server
Subscriber
Publisher
S = Security
Boundary
Protection
Figure 1 – OPC UA network example
OPC UA also supports multiple protocols and communication technologies that might require
different levels of security and different security infrastructure. For example, both Client –
Server and Publisher – Subscriber communication is shown in Figure 1
4.2 Security objectives
4.2.1 Overview
Fundamentally, information system security reduces the risk of damage from attacks. It does
this by identifying the threats to the system, identifying the system’s vulnerabilities to these
threats, and providing countermeasures. The countermeasures reduce vulnerabilities directly,
counteract threats, or recover from successful attacks.
Industrial automation system security is achieved by meeting a set of objectives. These
objectives have been refined through many years of experience in providing security for
information systems in general and they remain quite constant despite the ever-changing set of
threats to systems. They are described in 5.1 and 5.2 reconciles these objectives against the
OPC UA functions. Clause 6 offers additional best practice guidelines to Client and Server
developers or those that deploy OPC UA Applications.

4.2.2 Authentication
Entities such as clients, Servers, and users should prove their identities. Authentication can be
based on something the entity is, has, or knows.
4.2.3 Authorization
The access to read, write, or execute resources should be authorized for only those entities
that have a need for that access within the requirements of the system. Authorization can be as
coarse-grained as allowing or disallowing a Client to access a Server or it could be much finer
grained such as allowing specific actions on specific information items by specific users. The
granularity of a system depends in part on the functionality supported by the Server, but in
general Authorization should be given based on the need-to-know principle i.e. a user should
be granted access only to information they require for the function they are performing.
4.2.4 Confidentiality
Data is protected from passive attacks such as eavesdropping, whether the data is being
transmitted, in memory, or being stored. To provide Confidentiality, data encryption algorithms
using special secrets for securing data are used along with Authentication and Authorization
mechanisms for accessing that secret.
4.2.5 Integrity
Receivers receive the same information that the original sender sent, without
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