TweetFollow Us on Twitter

File System Security

Volume Number: 19 (2003)
Issue Number: 11
Column Tag: Programming

Section 7

File System Security

by Rich Morin

How (not) to make OS X as secure as MS Windows...

Mac OS X inherits most of its notions of file system security from BSD. Each file system node (file, directory, ...) has sets of permission (i.e., mode) bits for its owner, its group, and everyone else. The node's owner is restricted by the first set of mode bits. Other members of the node's group are restricted by the next set. Everyone else is restricted by the final set.

Let's look at some of the top-level permissions on a Mac OS X (10.2.8) system, to see how this plays out in practice:

% ls -dl /bin /sbin
drwxr-xr-x  35 root  wheel ... /bin
drwxr-xr-x  60 root  wheel ... /sbin

The first set of mode bits (rwx) allows these directories' owner (root) full access permissions. S/he can read (e.g., look up entries), write (e.g., add or remove entries), and execute (e.g., access entries) in the directory.

The following sets (r-x, r-x) restrict other users from writing, but allow read and execute access. Note, by the way, that this does not prevent someone from writing into an existing file in one of these directories, if the permissions of the file allow this.

In summary, nobody but root is able to write (e.g., create files) in any of these directories. So, "normal" users (and the programs they may accidentally or unsuspectingly) aren't able to add, remove, or rename programs.

This is very much what we'd expect in a well-designed, BSD-based system. Allowing user errors, programming mistakes, or malware to modify the system's executable code is (as myriad Microsoft-specific viruses demonstrate) a serious design error.

Unfortunately, Apple doesn't follow BSD's example everywhere; some Mac OS X system directories are all too vulnerable to the aforementioned threats:

% ls -dl /App*s /Developer /Library
drwxrwxr-x 59 root admin ... /Applications
drwxrwxr-x 14 root admin ... /Developer
drwxrwxr-x 40 root admin ... /Library

In an effort to support "ease of use", Apple's engineers have made some critical directories far more open than they would be on a conventional Unix system. As a result, most users (and any programs they may run) can add, delete, or replace any node in these directories.

In a fine example of the "Law of Unintended Consequences", several plausible decisions work together to produce this undesirable result. Here's how it goes:

  • The first account created on a new system has "admin" privileges, by default, and few users bother to set up a separate administrative account. So, most users have admin privileges.

  • Any user who has admin privileges is put into the admin group.

  • The admin group has write permission for all three of these directories, so any member of the group can add, delete, or replace any node in these directories.

  • Any program run by a user has, by default, the same permissions as the user.

Here's a simple (and safe :-) experiment you can try. Note that the system prevents you from modifying /bin, but allows you to modify /Applications:

% groups
admin
% touch /Applications /bin
touch: /bin: Permission denied

In most cases, the system asks the user for authorization before taking any unusual or suspect action. Consider the password that sudo(8) requires and the authorization dialogs that come up on occasion (e.g., when installing software).

In this case, however, no warning is given. Any user with admin privileges is quite free to drag folders in and out of /Applications; no authorization dialog will come up. Apple is quite aware of this situation; in fact, their documentation suggests a possible workaround:

    Only admins can install software in the Applications folder. You may find that you want to set up a user account that doesn't have admin privileges and use that for day-to-day tasks. That way you won't absent-mindedly delete an application by accident.

    - http://www.apple.com/macosx/learning

I strongly suspect, however, that most machine owners will never see this advice. Even if they do, they may decide to ignore it. Logging in and out of accounts is a time-consuming hassle. Panther's "fast user switching" will improve this situation, but it will still break the user's concentration.

So, most users will run as admin, expecting the Finder (and other apps) to ask them before doing anything odd. Unfortunately, the Finder won't even be called into play if a rogue application is bent on rewriting parts of the file system (e.g., installing virus code).

In summary, Apple has opened up a major security hole that is not present in Mac OS X's forebears (Unix, FreeBSD, ...). Expecting application programmers (or worse, users!) to compensate for insecure directory permissions is simply bad design. The underlying system needs to be secure; exceptions can then be made on a carefully-controlled basis.

In this case, this means that the permissions need to be fixed. Administrative actions can then be performed using "privilege elevation", under the control of authorization dialogs, etc. Administrative users are quite used to being asked for this sort of authorization, so ease of use isn't being compromised.

Note: Because the root directory allows write permission to members of the admin group, you might think that it opens up a similar security hole. However, its permissions (drwxrwxr-t) include the use of the "sticky" bit (as indicated by a "t" in the last position). This allows admin users add items to the root directory, but prevents them from removing or renaming anything that they don't own. See sticky(8) for more details.

Third-party Apps

Third-party developers have some excuse for being unfamiliar with permissions issues (classic Mac OS wasn't real big on security :-), but by now, they should have learned the basics. So, it's disturbing to find many vendors leaving their application packages wide-open to writing by any user (or program) on the system.. Try:

% cd /Applications
% ls -ld * | grep rwxrwxrwx
drwxrwxrwx  ...  Alarm Clock S.E..app
drwxrwxrwx  ...  AutoSync.app
drwxrwxrwx  ...  Classic Toggler folder
drwxrwxrwx  ...  Cocoa Browser.app
drwxrwxrwx  ...  GraphicConverter US
drwxrwxrwx  ...  Multiple Launcher X.app
drwxrwxrwx  ...  OmniDictionary.app
drwxrwxrwx  ...  OmniGraffle.app
drwxrwxrwx  ...  PTHPasteboard
drwxrwxrwx  ...  RBrowser.app
drwxrwxrwx  ...  ShuX.app
drwxrwxrwx  ...  SliMP3 Server.app
drwxrwxrwx  ...  Text Wielder
drwxrwxrwx  ...  Tri-BACKUP Folder
drwxrwxrwx  ...  VLC.app
-rwxrwxrwx  ...  iCab

Let's say that Susie downloads a nifty-looking program and runs it (e.g., from her Downloads directory). Gee, it didn't do anything. That's no fun; let's try something else... Meanwhile, the "nifty-looking program" has infected any vulnerable third-party apps. When Susie's mom runs one of these (using an admin account), the infection can spread to the rest of the system.

Can anything be done?

Some folks at Apple are very concerned by these (and other) security holes, but they (clearly) aren't in control of Apple's overall security policy. As a result, OSX is ripe for the kind of bad publicity that MS Windows has recently received.

As a developer, you have a responsibility to set appropriate permissions on your package directories. Do that, and your app won't be part of the problem. Then, file bug reports with Apple, asking them to tighten up their own security holes and give developers automated feedback and assistance in closing holes in third-party applications.

Several possibilities spring to mind. If Interface Builder can draw helpful blue lines to indicate that a widget is too near the edge of a window, why can't Xcode (or whatever tool is used for package creation) tell the developer when a package's permissions are "too near the edge"?

For that matter, why can't the installation software check for this sort of thing? And, while Disk Utility is looking for weird permissions and ownerships, why can't it look for wide-open package directories? Like that...

More generally, when you talk to Apple, tell them that you don't want security concerns to be completely overridden by "ease of use" considerations. Both are critical; a proper solution won't ignore either one.


Rich Morin has been using computers since 1970, Unix since 1983, and Mac-based Unix since 1986 (when he helped Apple create A/UX 1.0). When he isn't writing this column, Rich runs Prime Time Freeware (www.ptf.com), a publisher of books and CD-ROMs for the Free and Open Source software community. Feel free to write to Rich at rdm@ptf.com.

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

OS X Yosemite 10.10.2 - Apple's lat...
OS X Yosemite is Apple's newest operating system for Mac. An elegant design that feels entirely fresh, yet inherently familiar. The apps you use every day, enhanced with new features. And a... Read more
Apple iOS 8.1.3 - The latest version of...
The latest version of iOS can be downloaded through iTunes. Apple iOS 8 comes with big updates to apps you use every day, like Messages and Photos. A whole new way to share content with your family.... Read more
SpamSieve 2.9.19 - Robust spam filter fo...
SpamSieve is a robust spam filter for major email clients that uses powerful Bayesian spam filtering. SpamSieve understands what your spam looks like in order to block it all, but also learns what... Read more
RapidWeaver 6.0.5 - Create template-base...
RapidWeaver is a next-generation Web design application to help you easily create professional-looking Web sites in minutes. No knowledge of complex code is required, RapidWeaver will take care of... Read more
Duet 1.2.2 - Use your iPad as an externa...
Duet is the first app that allows you to use your iDevice as an extra display for your Mac using the Lightning or 30-pin cable. Note: This app requires a $14.99 iOS companion app. Version 1.2.2:... Read more
Alfred 2.6 - Quick launcher for apps and...
Alfred is an award-winning productivity application for OS X. Alfred saves you time when you search for files online or on your Mac. Be more productive with hotkeys, keywords, and file actions at... Read more
1Password 5.1 - Powerful password manage...
1Password is a password manager that uniquely brings you both security and convenience. It is the only program that provides anti-phishing protection and goes beyond password management by adding Web... Read more
GarageSale 6.9.2 - Create outstanding eB...
GarageSale is a slick, full-featured client application for the eBay online auction system. Create and manage your auctions with ease. With GarageSale, you can create, edit, track, and manage... Read more
calibre 2.17 - Complete e-library manage...
Calibre is a complete e-book library manager. Organize your collection, convert your books to multiple formats, and sync with all of your devices. Let Calibre be your multi-tasking digital librarian... Read more
OmniGraffle Pro 6.1.2 - Create diagrams,...
OmniGraffle Pro helps you draw beautiful diagrams, family trees, flow charts, org charts, layouts, and (mathematically speaking) any other directed or non-directed graphs. We've had people use... Read more

New WOTA: U-Boat Compass ‘Wets’ Your App...
New WOTA: U-Boat Compass ‘Wets’ Your Appetite for WOTA: Wolves of the Atlantic. Posted by Jessica Fisher on January 27th, 2015 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »
Warship Solitaire Review
Warship Solitaire Review By Nadia Oxford on January 27th, 2015 Our Rating: :: A WAR OF NUMBERSUniversal App - Designed for iPhone and iPad Warship Solitaire might lack frills and looks, but it offers some thought provoking... | Read more »
Keep Track of Your Hectic Work Schedule...
Keep Track of Your Hectic Work Schedule With Shifts Posted by Jessica Fisher on January 27th, 2015 [ permalink ] iPhone App - Designed for the iPhone, compatible with the iPad | Read more »
All Star Quarterback Review
All Star Quarterback Review By Campbell Bird on January 27th, 2015 Our Rating: :: PLAY YOUR PARTUniversal App - Designed for iPhone and iPad Train your way to the Super Bowl in this free-to-play sports management game.   | Read more »
Sentinel 4: Dark Star Adds New Campaign...
Sentinel 4: Dark Star Adds New Campaign and More Posted by Ellis Spice on January 27th, 2015 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »
Wheel & Deal Review
Wheel & Deal Review By Tre Lawrence on January 27th, 2015 Our Rating: :: SIMPLE FUNUniversal App - Designed for iPhone and iPad A simple arcade shooter that has a retro feel.   | Read more »
Join the NAVY in a Big New Update for WW...
Join the NAVY in a Big New Update for WW2: Sandbox. Strategy & Tactics Posted by Jessica Fisher on January 27th, 2015 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »
Earthworm Alchemy Review
Earthworm Alchemy Review By Campbell Bird on January 27th, 2015 Our Rating: :: FEED THE WORMUniversal App - Designed for iPhone and iPad Feed this funny looking worm and dodge bombs in this charming, free-to-play arcade game.   | Read more »
Graphic Novel App netwars – The Butterfl...
Graphic Novel App netwars – The Butterfly Attack has Received Some New Episodes Posted by Jessica Fisher on January 27th, 2015 [ permalink ] | Read more »
DataVault Password Manager Review
DataVault Password Manager Review By Jennifer Allen on January 27th, 2015 Our Rating: :: SECURE DATA STORAGEUniversal App - Designed for iPhone and iPad Store your passwords and other sensitive information safely, thanks to... | Read more »

Price Scanner via MacPrices.net

Apple Releases New Tranche Of OS Updates For...
Apple has released incremental bugfix and security updates for its most recent Mac and iOS versions. OS X Yosemite 10.10.2 Update Apple says this update includes the following improvements: Resolves... Read more
Samsung To Be Main Supplier Of A9 Chips For i...
Korea’s Maeil Business Newspaper reports that Samsung has beat out its chipmaking rivals, Taiwan’s Qualcomm and TSMC, as supplier of chipsets for the next generations of both its own Galaxy S6 and... Read more
Save up to $340 on iMacs with Apple Certified...
The Apple Store has Apple Certified Refurbished iMacs available for up to $340 off the cost of new models. Apple’s one-year warranty is standard, and shipping is free. These are the best prices on... Read more
AppleCare Protection Plans on sale for up to...
B&H Photo has 3-Year AppleCare Warranties on sale for up to $105 off MSRP including free shipping plus NY sales tax only: - Mac Laptops 15″ and Above: $244 $105 off MSRP - Mac Laptops 13″ and... Read more
Apple refurbished Time Capsules available sta...
The Apple Store has certified refurbished Time Capsules available for up to $60 off MSRP. Apple’s one-year warranty is included with each Time Capsule, and shipping is free: - 2TB Time Capsule: $255... Read more
3TB AirPort Time Capsule on sale for $349, $5...
B&H Photo has 3TB AirPort Time Capsules on sale for $349 including free shipping plus NY sales tax only. Their price is $50 off MSRP, and it’s the lowest price available for this model (except... Read more
16GB iPad Air 2 on sale for $446, save $53
Walmart has 16GB WiFi iPad Air 2s on sale for $53 off MSRP on their online store. Choose free shipping or free local store pickup (if available). Sale prices available for online orders only, in-... Read more
Save up to $300 on a new Mac, $30 on an iPad,...
Purchase a new Mac or iPad at The Apple Store for Education and take up to $300 off MSRP. All teachers, students, and staff of any educational institution qualify for the discount. Shipping is free,... Read more
MasterCard Brings Contactless Payment, Apple...
MasterCard, the official card of the Waste Management Phoenix Open, and the PGA TOUR are bringing the latest payment technology to the spectator experience with the introduction of contactless... Read more
Petitioning Dropbox For Mac OS X 10.4 and 10....
Last week Dropbox announced to its users that app support for Mac OS X 10.4 and 10.5 will end May 18 — disappointing news for those of us who are still getting useful service out of older PPC Macs... Read more

Jobs Board

*Apple* Solutions Consultant - Retail Sales...
**Job Summary** As an Apple Solutions Consultant (ASC) you are the link between our customers and our products. Your role is to drive the Apple business in a retail Read more
*Apple* Solutions Consultant - Retail Sales...
As an ambassador of the Apple brand, the ASC is accountable for driving sales performance by: Connecting with customers. Discovering customers' needs and values. Showing Read more
*Apple* Solutions Consultant- Retail Sales -...
**Job Summary** The ASC is an Apple employee who serves as an Apple brand ambassador and influencer in a Resellers store. The ASCs role is to grow Apple product Read more
Detailer *Apple* Ford Body Shop / Collision...
Apple Automotive is one of the fastest growing dealer…and it shows. Consider making the switch to the Apple Automotive Group today! At Apple Automotive , Read more
*Apple* Acura/Subaru Service Technicians - A...
Apple Automotive is one of the fastest growing dealer…and it shows. Consider making the switch to the Apple Automotive Group today! At Apple Automotive , Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.