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Security Patrol: Security Questions and Answers

IBM i (OS/400, i5/OS)
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Q: Our auditors have requested that we change the system value for Create default public authority (QCRTAUT) to *EXCLUDE rather than the IBM default of *CHANGE. The *PUBLIC authority will now be *EXCLUDE for new objects. Will this change modify the *PUBLIC authority for existing objects? Are there any side effects of making this change?

A:The system value for QCRTAUT is used as the *PUBLIC authority for new objects. Changing the QCRTAUT value will have no effect on the public authority of existing objects.

The *PUBLIC authority will be taken from the system value for QCRTAUT and assigned to new objects during the object creation under the following conditions:

1. The authority (AUT) parameter of any of the Create (CRTxxx) commands specifies *LIBCRTAUT. This indicates to use the create authority for the library in which the object is created.

2. The create authority (CRTAUT) value of the library in which the object is created is set to *SYSVAL. This indicates that the *PUBLIC authority should come from the system value for QCRTAUT.

If you change the system value for QCRTAUT to *EXCLUDE, new objects have a *PUBLIC authority that does not allow access. Only the owner of the new object and users with *ALLOBJ authority can access the new object. Other users must be granted access to the object explicitly.

There is one side effect: when installations change the system value for QCRTAUT to *USE or *EXCLUDE, they often discover that users cannot sign on to new devices. This is because IBM assigns the *PUBLIC authority for device descriptions from the QCRTAUT system value. A user must have *CHANGE access to the device description to sign on to the AS/400.

To eliminate this side effect, specify CRTAUT(*CHANGE) for library QSYS using the following command:

 CHGLIB LIB(QSYS) CRTAUT(*CHANGE) 

The CRTAUT value of *CHANGE will cause the public's (*PUBLIC) object authority for new objects created in library QSYS to be *CHANGE. New device descriptions will have *CHANGE authority so users will have no problem signing on.

Q: I have heard that the Integrated File System (IFS) added in V3R1 allows Client Access/400 users to view the names of AS/400 objects. Allowing PC users to view objects in AS/400 libraries represents a major exposure. Is there some method to prevent PC users from getting to AS/400 objects through the IFS?

A: What you were told is correct. The combination of Client Access/400 and IFS does allow another method for users to access AS/400 in V3R1. Two Client Access clients (Client Access for Windows 3.1 and Optimized for OS/2) give a PC user the ability to use PC interfaces to view AS/400 libraries and the objects in the libraries. I will describe the nature of the exposure and a method that can be used to restrict PC users from the QSYS.LIB portion.

IFS support provides not only an integrated file management system for AS/400 objects but also stream I/O and storage management similar to those of personal computers and UNIX operating systems. When the Windows File Manager is used to view a PC drive assigned to the AS/400, the PC user gets a list of all the IFS file systems, such as the root file system (/), the LAN server file system (QLANSrv), the document library file system (QDLS), the UNIX file system (QOpenSys), and the library file system (QSYS.LIB).

By expanding the QSYS.LIB folder from the file manager, a user can display objects he is authorized to in library QSYS. (This operation takes a long time and is not recommended unless you want to wait for the display to come back. A PTF is planned to take care of this performance problem.) The PC user can then select individual subdirectories to view the objects in other AS/400 libraries. Windows is limited by the DOS naming conventions and only lists files whose names are eight characters or less.

Many AS/400 installations restrict user access to the command line as a form of security. In these installations, if the user can access the command line, the user can access production objects. Often the number of objects the user is authorized to is large because of the *PUBLIC and group profile authorities. Many shops protect their objects from access through a command line by specifying the limit capabilities value of *YES on user profiles. Access to QSYS.LIB through the IFS system from a PC is similar to the command line access just described, except the limit capabilities user profile attributes do nothing to prevent object access.

IBM is creating a program temporary fix (PTF) SF23879 for V3R1 which allows an installation to restrict PC users from accessing QSYS.LIB (AS/400 objects) through the IFS. (As of this writing the PTF is not yet available, but should be by the time you read this.)

This PTF will create an authorization list called QPWFSERVER. The authorization list should be distributed with *PUBLIC authority of *USE, which does not restrict any users. You would most likely want to change the authority on the authorization list to limit PC users from the QSYS.LIB file system.

To restrict PC users from the QSYS.LIB file system, change the *PUBLIC authority of the QPWFSERVER authorization list to *EXCLUDE using the Edit Authorization List (EDTAUTL) command. Then grant *USE access to users who need access to the QSYS.LIB file system. Users who have *ALLOBJ authority are not restricted by this special authorization list.

PTF SF23879 does not solve the issue of performance impact should an authorized user attempt to list the contents of the QSYS.LIB file system. IBM is working on another PTF (member unknown) that will improve the performance by limiting the number of objects returned when the PC user requests a generic list of all objects in the QSYS.LIB directory. This PTF will not return the object programs that begin with a Q or any of the commands. The user-created objects, such as user profiles, user files, and device, line, and controller descriptions, will be returned.

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