400 MB of Top PC Shareware/400 MB Of Top PC Shareware.iso/misc/acc_sec.txt

Text File | 1993-07-01 | 26KB | 524 lines

File Info for SECURE.TXT

Date:  2/11/93
Title: Additional Microsoft Access Security document

The purpose of this document is to explain the Microsoft Access security model.  The
hope is that if the user understands the intricacies of security, and is conscientious,
he will be able to safely and successfully use Microsoft Access security in his
database.

===================================================================
DOWNLOADED FROM COMPUSERVE BY [e-mail] on 6/28/93
===================================================================

Microsoft Access Security


Purpose
-------

The purpose of this document is to explain the Microsoft Access security
model.  The hope is that if the user understands the intricacies of
security, and is conscientious, he will be able to safely and successfully
use Microsoft Access security in his databases.

Examples of scenarios that have caused difficulty for people in the past
have been set apart under the heading of Gotcha.  These are intended to
give practical examples of common misunderstandings, and to explain why
Microsoft Access security works the way it does.


Overview
--------

Microsoft Access security consists of 2 parts which are stored in different
places.  Information regarding the permissions that users and groups have
on the objects in a database is stored in the database itself.  This way
the permission information travels with the .mdb file in which the objects
exist.  All other security information is stored in system.mda.  This
includes:  which Users & Groups exist;  which Groups each User belongs to;
and User logon passwords.  In Microsoft Access parlance, a system.mda
defines a "Workgroup".


Logging On
----------

Each User and Group has associated with it a Security ID (SID).  The SID is
a binary string that uniquely identifies the User or Group.  When a user
logs on, Microsoft Access looks in the MSysAccounts table of the system.mda
specified in the user's msaccess.ini for a user of the same name (case-
insensitive).  If a user with the same name is found, it then validates the
password (case-sensitive).  If the password matches, the SID of the user is
retrieved and saved in an internal structure.  The password is only used to
validate the user when he logs on.  Other than validating the identity of
the user when he logs on, it has no effect on security.

By default, Microsoft Access first attempts to logon as the user Admin,
with a blank password.  If this logon fails, the user is presented with the
Logon dialog.  If a user name and password were specified on the command
line (using the /USER and /PWD flags), Microsoft Access first tries to
logon using that user name and password.  If this logon fails, the user is
presented with the Logon dialog.  Once logged on, the user's SID is
retrieved.  This SID is used for all subsequent operations within Microsoft
Access.


Users and Groups
----------------

Each user can be a member of 1 or more groups.  Users & Groups share the
same namespace.  This means that you can't have both a group and user with
the same name.

Microsoft Access defines 3 default groups:  Admins, Users, and Guests.
Whenever a user is added to the system.mda, they are automatically given
membership in the group Users, and they cannot be removed from the Users
group.  There is one exception to this rule---the user Guest CAN be removed
from the Users group.  The Admins group must always have at least one
member.

Microsoft Access defines 2 users:  Admin and Guest.  The user Admin is a
member of the Admins and Users groups.  The user Guest is a member of the
Guests group (only).

The pre-defined groups (Admins, Users, and Guests) cannot be deleted.  The
user Guest cannot be deleted, though the user Admin CAN (and should be to
preserve security) if there is at least one other member of the Admins
group.

Each user and group has a SID associated with it.  The SIDs of the Users
and Guests groups, and of the Admin and Guest users, are the same in all
Microsoft Access installations.  The SID of the Admins group, however, is
unique across all Microsoft Access installations.  The SID of the Admins
group is determined when Microsoft Access is installed.  This prevents
someone who is in the Admins group of one system.mda from being able to
have the permissions of any other Admins group.

Gotcha:  If you delete the user Admin, and then create another user
named Admin, the SID of the new Admin will NOT be the same as the SID
of the Admin that exists in an unsecure system.  This is because the
Admin user that you create will have a different PIN that the Admin
user that exists in a freshly-installed system.mda.

The SID of the Admins group is a function of the Setup disk serial number
and the User and Company names given (case-insensitive).  Therefore, it is
critical that the disks used to install Microsoft Access be kept in a safe
place, and that the User and Company names be recorded.  In the event that
the system.mda has to be recreated, the same installation disks must be
used.

The SIDs of users and groups that you create are a function of the user or
group name (case-sensitive) and Personal Identification Number (PIN) that
you specify.  For this reason it is critical that the user and group names
and their PINs be backed up.  If there is ever a need to recreate the
system.mda, you will need the name and PIN of each user and group that was
in the system.mda.  Note that by using different PINs, it possible for
users and groups in different system.mda's to have the same name, but they
will actually be different accounts as far as Microsoft Access security is
concerned because they have different SIDs.

Gotcha:  When you setup Microsoft Access, the User and Company names
are written to Disk #1.  If you then give anyone else your disks to
setup from, Microsoft Access doesn't prompt for the User and Company
name.  The Admins group in the system.mda that is created will have
the same SID as the Admins group in your system.mda.  This means that
anyone who uses that new system.mda will have all the permissions that
you do, in any databases that you have permissions.  While this
functionality wasn't intended as a form of copy protection, it is a
good incentive to abide by the License Agreement.


Permissions
-----------

There are 2 types of permissions:  Explicit and Implicit.  Explicit
permissions are those given directly to a user.  When they are granted, no
other users are affected.  Implicit permissions are those granted to a
group.  When they are granted, all users who are members of the group get
the permissions of the group.  Implicit permissions belong to the group,
not the users.  If a user is removed from the group, he no longer has the
permissions of the group.  If the permissions of the group are changed, all
users in the group are affected.

When a user attempts to do something, he will get the least restrictive
permissions of:  1) all the groups in which he is a member (his Implicit
permissions), and 2) those given directly to him (his Explicit
permissions.)

In the Microsoft Access Permissions dialog, only the Explicit permissions
are displayed.  This is a limitation of v.1.0.  To view implicit
permissions, view the permissions of each group that the user is a member
of.

The Admins group of the system.mda in use when the database was created
will always have permission to change permissions on all objects in the
database.  This permission cannot be taken away by anyone!!!  This
permission remains even when all permissions have been revoked from the
Admins group, and isn't displayed in the Permissions dialog.  It should
therefore be obvious that it is very important to know and keep track of
which system.mda was in use when the database was created.  To stress this
point, italics will be used throughout this document where it comes up.

The Users group initially has Full permissions on all objects created in
the database.  The Guests group has Read Definition and/or Read Data
permissions on them, as appropriate for the object type.  Basically, Guests
can read all objects and data, but can't change anything.

Permissions on an object can be changed by anyone having permissions to do
so.  The following users have this permission:  members of the Admins group
of the system.mda in use when the database was created;  the creator of the
database (it's owner);  the creator of the object (it's owner); and anyone
who has Full permissions on the object, either explicitly or implicitly.

Gotcha:  Jane gives Joe a database that she created.  Joe is a member
of the Users group, and no other groups.  He is using a different
system.mda (the SID of the Admins group of his system.mda is different
from the SID of the Admins group in Jane's system.mda).  Jane's
database is "unsecure", meaning that the Users group has Full
permission on all objects.  Joe revokes Full permissions from the
Users group on a Table1.  From that point forward, Joe can no longer
change permissions on that Table1.  If he also revoked Read Data
permissions, he has prevented himself from being able to read Table1,
because he no longer has permission to do so.  For both these cases,
he just revoked his own permissions on the Table1.  Because Joe isn't
a member of the Admins group of the system.mda in use when the
database was created, and because he didn't create Table1, he's
completely locked out of Table1 until Jane (using her own system.mda)
gives him permissions again.

All users always have permission to open any database and create objects in
it.  Microsoft Access relies on operating system security to prevent a user
from being able to open a database.

Gotcha:  Even if a user doesn't have an account in your system.mda, he
can use any other system.mda in which he has an account to logon to
Microsoft Access.  Then he can open any database that he can get to
through the operating system.  Once the database is open, he can see
which objects exist, and create new objects.  There is no method to
prevent creation of objects.

It is possible for a user to have permissions on an object in a database,
and not be able to see those permissions in the Permissions dialog.  This
will happen if the user was assigned permissions in the database and then
was deleted from the system.mda.  The permissions given to the user are
stored in the .mdb, and aren't removed when the user is removed.  The
following steps will show these permissions:

1. Set the Show System Objects option to Yes
2. Give yourself Read Data permissions on the MSysACEs table
     If you don't have permissions to do this, you're not a member of the
     Admins group of the system.mda in use when the database was created,
     and you didn't create the database.
3. Attach to or import the MSysAccounts table from the system.mda
     currently in use
     If you don't have permissions to do this, you're not a member of the
     Admins group of the system.mda currently in use.
4. Run this query:
     SELECT DISTINCTROW MSysObjects.Name AS Object, MSysAccounts.Name As
     UserGroup FROM MSysACEs, MSysAccounts, MSysObjects, MSysACEs LEFT JOIN
     MSysAccounts ON MSysACEs.SID = MSysAccounts.SID, MSysACEs INNER JOIN
     MSysObjects ON MSysACEs.ObjectId = MSysObjects.Id;

This will show the names of all users and groups who have permissions on
all objects in the database.  If the UserGroup field is blank, it means
that the user or group doesn't exist in your system.mda, yet still has some
kind of permissions on that object.  Disclaimer:  The query above works
fine in Microsoft Access 1.0.  However, the Microsoft Access system tables
(MSys*) are officially "undocumented", and are subject to change in future
versions.


Ownership
---------

Ownership is a very important part of Microsoft Access' security model.
The SID of the user who creates a database is considered the "owner" of the
database.  He will always have permission to give himself Full Permission
on any object in the database, regardless of who creates the object, or
what permissions are on the object.  He will therefore also always have
permissions to give anyone else permissions on any object in the database.

The SID of the user who creates an object is considered the "owner" of the
object.  He will always have permission to change permissions on the
object, even if the database owner has revoked his permissions on the
object.

The Admins group of the system.mda in use when the database was created
will always have permissions to change permissions on any object in that
database.

The only way to transfer ownership of a database or object is to recreate
it.  For databases, this means doing a File New Database and importing all
the objects from the old database.  Table relationships will have to be
recreated after importing, as they're not preserved when importing or
exporting.

For objects, ownership can be transferred by importing or exporting the
object, or by copying and pasting it.  These operations essentially create
a brand new object, one that is completely unsecured.  Note that for both
of these situations, the person doing the importing/exporting or
copy/pasting must have permission to read the objects in the first place.

Gotcha:  As mentioned above, the Admin and Guest users have the same
SID in all system.mda's.  It is therefore critical to security that
the Admin and Guest users NOT create (are the owner of) any objects or
databases.  If they do, anyone who has Microsoft Access and can get to
the database .mdb file can open it and have Full permission on the
objects created by Admin or Guest, and all objects in the database if
either of these users created the database.


The "Hole"
----------
By default, the Users and Guests groups have Read Data permissions on the
MSysObjects table.  For various technical reasons, this is necessary to
allow the user to do File Load From Query in the Filter window.

The astute reader will see that MSysObjects contains a field named "Owner",
and he will figure out that it contains the SID of the owner of each
object.  The astute reader will also realize that the SID field in the
MSysAccounts table of system.mda contains his own SID.  Because everyone
has Read Data permissions on MSysObjects, it would be possible to read the
SID of the owner of any given object, and replace his SID in MSysAccounts
with it.  The next time he logged on, he would have the SID of the owner of
the object, and all rights associated with it.  He could change permissions
on the objects, and therefore do anything he wanted to with it.

SIDs are binary fields.  When MSysObjects is browsed, the binary fields are
truncated at the first null terminator (ASCII 0).  Additionally, any
characters whose ASCII values are below 32 will display as garbage.  All of
this essentially prevents someone from being able to edit or copy and paste
this value directly.  However, it would be a relatively simple matter to
change your own SID using Microsoft Access Basic code or a query.

Fortunately, there is a way to patch this security hole:

1.  Make a backup of you database!!!
2.  Open your database
3.  Set the Show System Objects option to Yes
4.  Select Permissions from the Security menu
5.  Give the Users group Read Definition permissions on MSysObjects
6.  Revoke Read Definition permissions on MSysObjects from the Users group
7.  Repeat steps 5 & 6 for the Guests group

This will prevent someone from reading the SID from the Owner field and
writing it into his own MSysAccounts table.  The downside is that when
people who aren't in the Admins group (of the system.mda in use when the
database was created) attempt to use the "Load From Query" functionality
while in MyApp.mdb, they will get an error that says "No permission for
'MSysObjects'".  The only people who will be able to use the "trick" above
are the person who created the database, and anyone who is a member of the
Admins group of the system.mda in use when the database was created.  It is
not recommended that you repeat steps 5 & 6 for them.

Needless to say, this hole will be plugged in a future version of Microsoft
Access.  Disclaimer:  Again, the Microsoft Access system tables (MSys*) are
officially "undocumented", and are subject to change in future versions.
Don't count on any information regarding them to be applicable to anything
but Access version 1.0.


The "Run with Owner's Permissions" Query Property
-------------------------------------------------

Microsoft Access queries have an property called "Run with Owner's
Permissions" (RWOP).  This is in the Query Properties dialog.  If this
property is checked, the permissions of the owner of the query are the only
permissions that are considered when attempting to access tables or queries
that the query is dependent on.

Gotcha:  Joe has Read Definition permissions on the Salary table, but
not Read Data.  Joe creates Query1, which is based on the Salary
table.  Joe has left RWOP turned on.  Joe is unable to execute the
query, because he doesn't have Read Data permissions on the Salary
table.  This much is intuitive.  Jane created the Salary table, and
has Full permission on it.  Jane tries to execute Query1.  She cannot,
because the RWOP option of Query1 is turned on, and the owner of the
query (Joe) doesn't have Read Data permissions on the Salary table.
While this may not be entirely intuitive, it is correct functionality.

Only the owner (creator) of a query may save it if the RWOP property is
turned on.  Even the database owner or members of the Admins group of the
system.mda in use when the database was created can't save a query created
by another user if the RWOP property is turned on.  It would be a security
breach to allow them to.  Note though, that anyone with Modify Definition
permissions on the query can uncheck RWOP and successfully save the query.
This may lead to other complications if other people were using the query
and depending on RWOP being there.

This functionality extends to the QueryDef object as well.  Only the owner
of a query can change the query using the "Set Q.SQL = MySQLString$" syntax
if MySQLString$ ends with "WITH OWNER ACCESS OPTION;".
Common Scenarios


Securing an Existing Database
-----------------------------

You've written an application using Microsoft Access---call it MyApp.mdb.
You've created all the objects while you were logged on as Admin (the
default.)  Now you want to "secure" the database.  Here are the steps to
follow to do so:

1.  Make a backup of MyApp.mdb!!!
2.  Choose Change Password from the security menu and give a password to
        the user Admin.
3.  Create a new user and add him to the Admins group.  For this
        example, the user is Joe.
4.  Exit Microsoft Access and restart it, giving Joe as the user when
        the Logon dialog is given.
5.  Create a new database---call it MyApp2.mdb
        This will establish Joe as the database owner.
6.  Import all the objects from MyApp.mdb into MyApp2.mdb
        This will establish Joe as owner of all the objects.
7.  If you had any table relationships, recreate them in MyApp2.mdb
8.  Revoke Full permissions from the Users group on all objects, leaving
        them with whatever other permissions you like.  You may also want to
        revoke permissions from the Guests group.
9.  If desired, create any of your own groups and users, and give them
        appropriate permissions on all the objects.
10.  Rename MyApp.mdb to MyApp.old and rename MyApp2.mdb to MyApp.mdb.

Your database is now secure.  The only people who can get into the objects
in MyApp.mdb are those you gave permissions to in steps 8 & 9, and anyone
who is a member of the Admins group of your system.mda.


Unsecuring a Secured Database
-----------------------------

You've followed the steps under Securing an Existing Database, above.  Joe
is the owner of the database and all objects in it.  The Admins group of
Joe's system.mda has permissions to change permissions on any object in the
database.  The goal here is to make the user Admin the owner of the
database and all objects in it, and to give the Users group Full
permissions on all objects.  Remember, the SIDs of the user Admin and the
Users group are constant in all system.mda's, so this will effectively give
everyone Full permissions on all objects in the database, and make them the
database owner as well.

1.  Make a backup of MyApp.mdb!!!
2.  Boot Microsoft Access as a member of the Admins group of the
        system.mda in use when MyApp.mdb was created.
3.  Give the Users group Full permissions on all objects in the
        database.
4.  Edit your msaccess.ini file so that SystemDB points to a
        system.mda in which no users exist.
5.  Exit Microsoft Access and restart it logging on as the user Admin.
        Note that this Admin MUST be the same Admin that exists in the
        "clean" system.mda that is created when you setup Microsoft Access
        so that the SID is correct.
6.  Create a new database (call it MyApp2.mdb)
7.  Import all the objects from MyApp.mdb into MyApp2.mdb

Your database is now completely unsecured.  Anyone who can to the
MyApp2.mdb file have Full permissions on all the objects in it.


Protecting Your Code
--------------------

You've written a bunch of Microsoft Access Basic code that you want to
distribute, perhaps as a Library Database, but you don't want anyone to be
able to read your code.  The modules are in MyApp.mdb, and were all created
while you were logged on as Admin.  Assuming that you're starting from a
completely unsecure configuration, here are the steps to secure the code:

1.  Make a backup of MyApp.mdb!!!
2.  Choose Change Password from the security menu and give a password to
        the user Admin.
3.  Create a new user and add him to the Admins group.  For this
        example, the user is Joe.
4.  Exit Microsoft Access and restart it, giving Joe as the user when
        the Logon dialog is given.
5.  Open MyApp.mdb
6.  For each module you want secured, do the following:
        a.  Open the module in design mode
        b.  File Save Text to save the module to a text file
        c.  File Close
        d.  Delete the module
        e.  Create a new module
        f.  File Load Text to bring the text file from step b back in
        g.  File Save, giving the original module name
                This will establish Joe as owner of  the modules.
7.  Revoke all permissions from the Users and Guests groups on all the
        modules.  All users will still be able to execute the code---there's
        no way to prevent someone from being able to execute code.

Your modules are now secured so that nobody except Joe and members of the
Admins group of the system.mda in use when MyApp.mdb was created can read
the modules.


Moving a Database to Another System.mda
---------------------------------------

You are Joe, and have written MyApp.mdb and want to give it to Jane who is
using a different system.mda.  You want Jane to have complete
administrative authority over your database.  The problem is this:  The
Admins group of Jane's system.mda doesn't have any permissions on anything
in MyApp.mdb.

There are 2 variations of this scenario:

Variation #1:  MyApp.mdb is completely unsecured (the Users group has
Full permissions on everything).

The situation here is as follows...  The user Admin created MyApp.mdb
and all the objects in it, and is therefore the owner of everything.
The Admins group of Joe's system.mda has permissions to change
permissions on all the objects in MyApp.mdb.  The Users group (which
has the same SID in all Microsoft Access installations) has Full
permissions on all objects in MyApp.mdb.  Jane's system.mda is
unsecure (the Admin password is blank, and there are no user accounts
or groups.)

To give MyApp.mdb to Jane, and have her get complete administrative
authority over it, follow these steps:

1.  Make a backup of MyApp.mdb!!!
2.  Boot Microsoft Access using Jane's system.mda.
3.  Create a new database---call it MyApp2.mdb
        This will establish Admin as the database owner.
4.  Import all the objects from MyApp.mdb into MyApp2.mdb
        This will establish the user Admin as owner of all the objects, and
        give the Admins group of Jane's system.mda permission to change
        permissions on all objects created in MyApp2.mdb.
5.  If you had any table relationships, recreate them in MyApp2.mdb
6.  Rename MyApp.mdb to MyApp.old and rename MyApp2.mdb to MyApp.mdb.

Variation #2:  MyApp.mdb is secured using the steps under Securing an
Existing Database.

To give MyApp.mdb to Jane, and have the Admins group of her system.mda
get complete administrative authority over it, follow these steps:

1.  Follow the instructions under Unsecuring a Secured Database, above
2.  Follow the instructions under Variation #1, above.

For all intents and purposes, Jane is now the "owner" of MyApp.mdb and all
objects in it, and Joe is just another user.  Under both these scenarios,
you can include steps 1-4 and 8-10 of Securing an Existing Database, above,
as necessary to make the database secured under Jane's system.mda.


Miscellaneous Tidbits
---------------------

The User() function in Microsoft Access Basic returns the name of the
current user.  This is not the same as the SID.  As mentioned under Users
and Groups, above, it is possible for 2 users in different system.mda's to
have the same name but different SIDs.

Gotcha:  This can cause a problem if 2 people that are using different
system.mda's have the same name but different PINs (and therefore
different SIDs).  For example, if you are using the User() function to
log activity, and both are using your database, the activity logging
could be inaccurate.  These are actually 2 different accounts, even
though their user names are the same.