The following practices, some of which are illustrated in “Sample .profile”, help provide layered defenses:

  1. Setting the gtm_noceenable environment variable to a value to specify that <CTRL-C> should be ignored by the application, at least until it sets up a <CTRL-C> handler. As part of its startup, the application process might execute:


    to set up a handler such as:

    DONE: QUIT ; or HALT or ZHALT, as appropriate

  2. Providing a value to the gtm_etrap environment variable, as illustrated “Sample .profile”. This overrides GT.M's default value of "B" for $ZTRAP, which puts the application into direct mode. Of course, in a development environment, going to direct mode may be the correct behavior, in which case there is no need to set gtm_etrap.

  3. Providing a value to the gtm_zinterrupt environment to override the default of "IF $ZJOBEXAM()" which causes the process to create a text file of its state in response to a MUPIP INTRPT (or SIGUSR1 signal). Such a text file may contain confidential information that the process is actively computing. Note that a user can only send INTRPT signals as permitted by the configuration of system security for the user. If your application uses INTRPT signals, review the code they invoke carefully to ensure processes respond appropriately to the signal. If any response produces an output file, be sure they have write access to the destination; restrict read access to such files appropriately. The “Sample .profile” example does not illustrate an alternative value for gtm_interrupt.

  4. Setting the SHELL environment variable to /bin/false disables the ZSYSTEM command, which if executed without an argument takes the user to a shell prompt. While a correctly coded application might not have a ZSYSTEM without an argument, setting SHELL to a value such as /bin/false, as illustrated above, protects an added layer of defense against a possible application bug. Of course, if an application uses the ZSYSTEM command, then an executable SHELL is required. If your application uses ZSYSTEM to run a command, consider whether a PIPE device might provide a better alternative.

  5. Setting the PATH environment explicitly to only those directories that contain executable files that the GT.M process will need to execute, with a ZSYSTEM command or a PIPE device.

  6. Because some text editors include functionality to run a shell in an edit buffer, setting the EDITOR variable to an editor which does not have such functionality is a way to block shell access in the event the application uses the ZEDIT command to edit a text file. Note that if an application allows users to edit text files, they can also edit GT.M program source files, and application configuration should ensure that such program files cannot be accessed by the $ZROUTINES of the process unless that is the desired behavior.

loading table of contents...