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Security Patrol: Where Does *PUBLIC Get All That Authority?

IBM i (OS/400, i5/OS)
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Q: I recently went back to work for a former employer after a four-year absence. When I worked here in the '90s, everything was very secure, but now it seems like *PUBLIC has authority to everything. The system administrator I replaced just didn't care about security. Now I'm noticing that every new system moved into production gives *PUBLIC *ALL authority. How can I stop this?

A: The situation you describe is more the rule than the exception. As I review security for OS/400 shops, I find it is common that *PUBLIC has *CHANGE authority to large numbers of application objects. If *PUBLIC has too much authority in your shop, you should review your system settings to reign in this excessive level of public authority.

First, let's review how *PUBLIC receives authority to new objects. With every Create command (CRTPF, CRTJOBD, CRTCLPGM--all those commands that begin with the prefix CRT), there is a *PUBLIC Authority (AUT) parameter.

CRTPF FILE(COMPANY1/CUSTOMER) SRCFILE(PRODSRC/QDDSSRC) MBR(CUSTOMER) AUT(*LIBCRTAUT)


This parameter tells each Create command what authority *PUBLIC should have to these newly created objects. The default value for all of these commands is something called Library Create Authority (*LIBCRTAUT), which I'll explain later. However, the person who creates the new object can specify that *PUBLIC should have *ALL, *USE, *CHANGE, or even *EXCLUDE authority. Regardless of which authority level is chosen, *PUBLIC will be granted that authority as soon as the object is created. If the person who creates the object neglects to choose a specific authority for *PUBLIC, all of the Create commands will revert to the default value of *LIBCRTAUT, which means "Go ask the library description what authority *PUBLIC will have to this new object."

Every library description has a a default *PUBLIC authority parameter, called Create Authority (CRTAUT). This value is used to determine what authority *PUBLIC will get to new objects that were created in this library when the AUT parameter on the CRT command was left to "*LIBCRTAUT". You can run the Display Library (DSPLIB) command for any library, and the library header description will show you the Create Authority for that library. The valid values include *ALL, *USE, *CHANGE, and *EXCLUDE. The other valid value, and the one most often used, is *SYSVAL. This value tells new objects to look to the system value QCRTAUT to determine what authority *PUBLIC should have to those new objects. Most libraries on your system are probably set to this default *SYSVAL value. You can change this library value by issuing the Change Library (CHGLIB) command:

CHGLIB LIB(COMPANY1) CRTAUT(*EXCLUDE)  


Unfortunately for security-conscious OS/400 shops, the factory-shipped default value for system value QCRTAUT is *CHANGE. This means that, on most iSeries machines, when a new object is created, the Create command defers to the library description and the library description defers to the system value, which is set at the factory as *CHANGE. So on most systems, *PUBLIC ends up with *CHANGE authority to all new objects. If you were to look at the system value for QCRTAUT for the system in question here, your system would likely show a value of *ALL. This means that *PUBLIC receives *ALL authority to all newly created objects. This is much too much authority. In the highly networked world we live in, you don't want to give *PUBLIC authority to anything you wouldn't want posted on the Internet. Giving *PUBLIC *ALL authority (which includes object delete rights) is positively scary. In a perfect world, you'd change this system value to *EXCLUDE as shown below. However, not many of us get to live in a perfect world, so read further for a real-world solution.

CHGSYSVAL SYSVAL(QCRTAUT ) VALUE(*EXCLUDE)


Setting things straight again is really quite easy once you understand how they got this way. Ideally, you already have a software-change process in place that manages the authority of new objects. All of the good third-party software-change-management packages do this for you, which is just one of the reasons I strongly advocate change-management software. This way, every time a new system is moved into production, the automated turnover process can determine *PUBLIC's authority based on a template that you have set up.

Whether or not you have a change-management process set up, you should review your production libraries to determine what authority *PUBLIC needs to these new objects. If all of your users already belong to group profiles, or if you are using a change-management tool to manage individual objects' *PUBLIC settings, you can safely set *PUBLIC's authority to *EXCLUDE and authorize the group profiles directly to the objects.

However, if your users are still using the *PUBLIC entry to receive their authority to production objects, and you don't foresee changing to group profiles soon, you are left in a much more compromised position. But you shouldn't abandon all hope. You can raise your security bar a little by setting the CRTAUT flag on the library to a lower setting for each production library. A good rule of thumb is that users need no more than *USE authority to programs and usually no more than *CHANGE authority to data, so the CRTAUT value on the library could be set to *USE for libraries containing programs and *CHANGE to libraries containing files--at least until you can implement a more comprehensive solution. Use the Change Library (CHGLIB) command to modify this library description value to a more appropriate setting.

After you've modified all of your production library descriptions, contemplate changing the system value for QCRTAUT. Since this system value was set to *CHANGE at the factory, someone in your organization must have changed it to *ALL. Many times, a value of *USE is recommended, but I have seen this value changed to *EXCLUDE with success.

There are some things you should understand about the QCRTAUT system value and its relationship to the all-important library QSYS. When new objects are created in library QSYS, QSYS uses the same method as any other library to determine *PUBLIC's default authority. Luckily, there are not many new objects created in QSYS during the course of regular iSeries operations. The most common object types created in QSYS are User Profiles (*USRPRF), Authorization Lists (*AUTL), and Libraries (*LIB), as well as communications descriptions (*LIND, *CTLD, and *DEVD). User profiles and authorization lists are not normally affected by the library's Create Authority settings, because the Create commands for these objects already default to AUT(*EXCLUDE).

New libraries could pose some difficulty if the default authority were set to *EXCLUDE. Users would have to be authorized to newly created libraries, which is not necessarily a bad thing. Communications descriptions have the potential to be more troublesome, however.

To use a communications object, the user must have *CHANGE authority to that communications object. (This may seem counterintuitive, but that is how OS/400 communications authority works.) All communications description objects must exist in library QSYS, so if you were to change the CRTAUT parameter of library QSYS to something less than *CHANGE, you would have to authorize your users to all new device descriptions so they could actually use them. One logical way to accomplish this would be to grant *PUBLIC *CHANGE authority to all existing device descriptions and then change the CRTAUT parameter on QSYS to *EXCLUDE. Until you start using Telnet connections, this security method should serve you well. But those of us using Telnet will discover a little-known secret about the Telnet server and the devices that connect to it.

Because Telnet devices are very dynamic, and every client that connects to them could be using a different logical device type, the OS/400 Telnet server actually deletes and recreates QPADEVnnnn virtual devices (which are the default device names created by OS/400 each time a Telnet session starts) every time a new Telnet session is initiated. In this way, Telnet prepares the QPADEVnnnn device to accept the connection device type of the new user, regardless of what device type the new user presents to the Telnet server. If user Anne connected to the iSeries using Telnet, the Telnet server would create device description QPADEV0005, for example, using a 3487C device type to provide Anne with the proper emulation environment. When Anne signed off of that device and made it available for reuse, there is a high probability that the next device that attempted to connect to device QPADEV0005 would have a different connection device type. If user Marie attempted to connect to the QPADEV0005 device description with a 3780-2 device type, the Telnet server would actually delete the existing QPADEV0005 device description and recreate it as a 3780 device.

A problem arises when you change library QSYS's CRTAUT value to *EXCLUDE. The Telnet server runs under the authority of user QTCP. When a new device is created by the Telnet server, that device is owned by QTCP and *PUBLIC is *EXCLUDED from using it. This means that when user Marie connects to the Telnet server, the Telnet server will create a device for her but neglect to authorize Marie to use the device.

There are two ways that I know of to get out of this configuration authorization cul-de-sac. The first is to set the CRTAUT value on library QSYS to *CHANGE and then change the system value to something more restrictive, like *EXCLUDE. This provides strong security for every library except QSYS. The risk here (albeit a small one) is that *PUBLIC would have access to some new object in QSYS.

The other method is to create an authorization list, attach it to all of your Telnet virtual devices, and then give your user community *CHANGE authority to that authorization list. This gives your users direct, specific authority to Telnet virtual devices. What's nice about this method is that when the Telnet server deletes and recreates a device, the server is smart enough to reattach your authorization list to the new device. This method allows you to set both the system value QCRTAUT and the QSYS library's CRTAUT to *EXCLUDE without fear of locking users out of needed objects.

To recap the fixes that you should put into place on any production iSeries 400 machine, take into account the following:

  • Use a change-management process as the lowest level of control over *PUBLIC's default authority.
  • Change the default value for the CRTAUT parameter on your individual production libraries to the least-permissive setting possible. *EXCLUDE is ideal, but be sure that the settings fit the needs of that library.
  • After setting your production libraries and QSYS to their proper levels, set the system value QCRTAUT to *EXCLUDE.


These steps will tighten *PUBLIC's authority and make your iSeries a more secure box.

John Earl is chief technology officer for the PowerTech Group in Kent, Washington. This month, John's hobby is trying to count all of the people in the world who qualify as members of *PUBLIC. If you have a security question or problem, you can send John an email at This email address is being protected from spambots. You need JavaScript enabled to view it. or post it online in the Security Forum on MCPressOnline at www.mcpressonline.com.


John Earl

John Earl is the founder and Director of Technology and Security for  The PowerTech Group.  a Seattle-area software company that specializes in System i security. He has over 25 years experience with IBM midrange systems and security, has published numerous articles and columns for industry magazines, and served as a Subject Matter Expert (SME) for Security for COMMON. A highly regarded speaker on OS/400 and i5/OS security, Mr. Earl has presented several hundred of iSeries security sessions at industry conferences and user groups all over the world. He is a three-time winner of COMMON's Speaker Excellence award and has also served on the board of directors of COMMON U.S.

 

He can be reached at 253.872.7788 or at john.earl@powertech.com.

 

 

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