Home > Articles

This chapter is from the book

This chapter is from the book

31.2 Requirements and Policy

The problem of sharing a password arises when a system implements administrative roles as separate user accounts.

  • EXAMPLE: Linux systems implement the administrator role as the account root (and several other accounts that have more limited functionality).1 All individuals who share access to the account know the account’s password. If the password is changed, all must be notified. All these individuals must remember to notify the other individuals should they change the password.

An alternative to using passwords is to constrain access on the basis of identity and other attributes. With this scheme, a user would execute a special program that would check the user’s identity and any ancillary conditions. If all these conditions were satisfied, the user would be given access to the role account.

31.2.1 Requirements

The first requirement comes directly from the description of the alternative scheme above. The system administrators choose to constrain access through known paths (locations) and at times of day when the user is expected to access the role account.

  • Requirement 31.1. Access to a role account is based on user, location, and time of request.

Users often tailor their environments to fit their needs. This is also true of role accounts. For example, a role account may use special programs kept in a subdirectory of the role account’s home directory. This new directory must be on the role account’s search path, and would typically be set in the startup file executed when the user logged in. A question is whether the user’s environment should be discarded and replaced by the role account’s environment, or whether the two environments should be merged. The requirement chosen for this program is as follows.

  • Requirement 31.2. The settings of the role account’s environment shall replace the corresponding settings of the user’s environment, but the remainder of the user’s environment shall be preserved.

The set of role accounts chosen for access using this scheme is critical. If unrestricted access is given (essentially, a full command interpreter), then any user in the role that maintains the access control information can change that information and acquire unrestricted access to the system. Presumably, if the access control information is kept accessible only to root, then the users who can alter the information—all of whom have access to root—are trusted. Thus:

  • Requirement 31.3. Only root can alter the access control information for access to a role account.

In most cases, a user assuming a particular role will perform specific actions while in that role. For example, someone who enters the role of oper may perform backups but may not use other commands. This restricts the danger of commands interacting with the system to produce undesirable effects (such as security violations) and follows from the principle of least privilege.2 This form of access is called “restricted access.”

  • Requirement 31.4. The mechanism shall allow both restricted access and unrestricted access to a role account. For unrestricted access, the user shall have access to a standard command interpreter. For restricted access, the user shall be able to execute only a specified set of commands.

Requirement 31.4 implicitly requires that access to the role account be granted to authorized users meeting the conditions in Requirement 31.1. Finally, the role account itself must be protected from unauthorized changes.

  • Requirement 31.5. Access to the files, directories, and objects owned by any account administered by use of this mechanism shall be restricted to those authorized to use the role account, to users trusted to install system programs, and to root.

We next check that these requirements are complete with respect to the threats of concern.

31.2.2 Threats

The threats against this mechanism fall into distinct classes. We enumerate the classes and discuss the requirements that handle each threat. Group 1: Unauthorized Users Accessing Role Accounts

There are four threats that involve attackers trying to acquire access to role accounts using this mechanism.

  • Threat 31.1. An unauthorized user may obtain access to a role account as though she were an authorized user.

  • Threat 31.2. An authorized user may use a nonsecure channel to obtain access to a role account, thereby revealing her authentication information to unauthorized individuals.

  • Threat 31.3. An unauthorized user may alter the access control information to grant access to the role account.

  • Threat 31.4. An authorized user may execute a Trojan horse (or other form of malicious logic),3 giving an unauthorized user access to the role account.

Requirements 31.1 and 31.5 handle Threat 31.1 by restricting the set of users who can access a role account and protecting the access control data. Requirement 31.1 also handles Threat 31.2 by restricting the locations from which the user can request access. For example, if the set of locations contains only those on trusted or confidential networks, a passive wiretapper cannot discover the authorized user’s password or hijack a session begun by an authorized user. Similarly, if an authorized user connects from an untrusted system, Requirement 31.1 allows the system administrator to configure the mechanism so that the user’s request is rejected.

The access control information that Requirement 31.1 specifies can be changed. Requirement 31.3 acknowledges this but restricts changes to trusted users (defined as those with access to the root account). This answers Threat 31.3.

Threat 31.4 is more complex. This threat arises from an untrusted user, without authorization, planting a Trojan horse at some location at which an authorized user might execute it. If the attacker can write into a directory in the role account’s search path, this attack is feasible. Requirement 31.2 states that the role account’s search path may be selected from two other search paths: the default search path for the role account, and the user’s search path altered to include those components of the role account’s search path that are not present. This leads to Requirement 31.5 which states that, regardless of how the search path is derived, the final search path may contain only directories (and may access only programs) that trusted users or the role account itself can manipulate. In this case, the attacker cannot place a Trojan horse where someone using the role account may execute it.

Finally, if a user is authorized to use the role account but is a novice and may change the search path, Requirement 31.4 allows the administrators to restrict the set of commands that the user may execute in that role. Group 2: Authorized Users Accessing Role Accounts

Because access is allowed here, the threats relate to an authorized user changing access permissions or executing unauthorized commands.

  • Threat 31.5. An authorized user may obtain access to a role account and perform unauthorized commands.

  • Threat 31.6. An authorized user may execute a command that performs functions that the user is not authorized to perform.

  • Threat 31.7. An authorized user may change the restrictions on the user’s ability to obtain access to the account.

The difference between Threats 31.5 and 31.6 is subtle but important. In the former, the user deliberately executes commands that violate the site security policy. In the latter, the user executes authorized commands that perform covert, unauthorized actions as well as overt, authorized actions—the classic Trojan horse. Threat 31.6 differs from Threat 31.4 because the action may not give access to authorized users; it may simply damage or destroy the system.

Requirement 31.4 handles Threat 31.5. If the user accessing the role account should execute only a specific set of commands, then the access controls must be configured to restrict the user’s access to executing only those commands.

Requirements 31.2 and 31.5 handle Threat 31.6 by preventing the introduction of a Trojan horse, as discussed in the preceding section.

Requirement 31.3 answers Threat 31.7. Because all users who have access to root are trusted by definition, the only way for an authorized user to change the restrictions on obtaining access to the role account is to implant a backdoor (which is equivalent to a Trojan horse) or to modify the access control information. But the requirement holds that only trusted users can do that, so the authorized user cannot change the information unless he is trusted—in which case, by definition, the threat is handled. Summary

Because the requirements handle the threats, and because all requirements are used, the set of requirements is both necessary and sufficient. We now proceed with the design.

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.


Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.


If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information

Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.


This site is not directed to children under the age of 13.


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure

Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact

Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice

We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020