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

Macs Fan Control 1.3.1.0 - Monitor and c...
Macs Fan Control allows you to monitor and control almost any aspect of your computer's fans, with support for controlling fan speed, temperature sensors pane, menu-bar icon, and autostart with... Read more
NetShade 6.3.1 - Browse privately using...
NetShade is an anonymous proxy and VPN app+service for Mac. Unblock your Internet through NetShade's high-speed proxy and VPN servers spanning seven countries. NetShade masks your IP address as you... Read more
Dragon Dictate 4.0.7 - Premium voice-rec...
With Dragon Dictate speech recognition software, you can use your voice to create and edit text or interact with your favorite Mac applications. Far more than just speech-to-text, Dragon Dictate lets... Read more
Persecond 1.0.2 - Timelapse video made e...
Persecond is the easy, fun way to create a beautiful timelapse video. Import an image sequence from any camera, trim the length of your video, adjust the speed and playback direction, and you’re done... Read more
GIMP 2.8.14p2 - Powerful, free image edi...
GIMP is a multi-platform photo manipulation tool. GIMP is an acronym for GNU Image Manipulation Program. The GIMP is suitable for a variety of image manipulation tasks, including photo retouching,... Read more
Sandvox 2.10.2 - Easily build eye-catchi...
Sandvox is for Mac users who want to create a professional looking website quickly and easily. With Sandvox, you don't need to be a Web genius to build a stylish, feature-rich, standards-compliant... Read more
LibreOffice 5.0.1.2 - Free, open-source...
LibreOffice is an office suite (word processor, spreadsheet, presentations, drawing tool) compatible with other major office suites. The Document Foundation is coordinating development and... Read more
f.lux 36.1 - Adjusts the color of your d...
f.lux makes the color of your computer's display adapt to the time of day, warm at night and like sunlight during the day. Ever notice how people texting at night have that eerie blue glow? Or wake... Read more
VirtualBox 5.0.2 - x86 virtualization so...
VirtualBox is a family of powerful x86 virtualization products for enterprise as well as home use. Not only is VirtualBox an extremely feature rich, high performance product for enterprise customers... Read more
File Juicer 4.43 - Extract images, video...
File Juicer is a drag-and-drop can opener and data archaeologist. Its specialty is to find and extract images, video, audio, or text from files which are hard to open in other ways. In computer... Read more

ReBoard: Revolutionary Keyboard (Utilit...
ReBoard: Revolutionary Keyboard 1.0 Device: iOS Universal Category: Utilities Price: $1.99, Version: 1.0 (iTunes) Description: Do everything within the keyboard without switching apps! If you are in WhatsApp, how do you schedule a... | Read more »
Tiny Empire (Games)
Tiny Empire 1.1.3 Device: iOS Universal Category: Games Price: $2.99, Version: 1.1.3 (iTunes) Description: Launch cannonballs and blow tiny orcs into thousands of pieces in this intuitive fantasy-themed puzzle shooter! Embark on an... | Read more »
Astropad Mini (Productivity)
Astropad Mini 1.0 Device: iOS iPhone Category: Productivity Price: $4.99, Version: 1.0 (iTunes) Description: *** 50% off introductory price! ​*** Get the high-end experience of a Wacom tablet at a fraction of the price with Astropad... | Read more »
Emo Chorus (Music)
Emo Chorus 1.0.0 Device: iOS Universal Category: Music Price: $1.99, Version: 1.0.0 (iTunes) Description: Realistic Choir simulator ranging from simple Chorus emulation to full ensemble Choir with 128 members. ### introductory offer... | Read more »
Forest Spirit (Games)
Forest Spirit 1.0.5 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0.5 (iTunes) Description: | Read more »
Ski Safari 2 (Games)
Ski Safari 2 1.0 Device: iOS Universal Category: Games Price: $1.99, Version: 1.0 (iTunes) Description: The world's most fantastical, fun, family-friendly skiing game is back and better than ever! Play as Sven's sister Evana, share... | Read more »
Lara Croft GO (Games)
Lara Croft GO 1.0.47768 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0.47768 (iTunes) Description: Lara Croft GO is a turn based puzzle-adventure set in a long-forgotten world. Explore the ruins of an ancient... | Read more »
Whispering Willows (Games)
Whispering Willows 1.23 Device: iOS Universal Category: Games Price: $4.99, Version: 1.23 (iTunes) Description: **LAUNCH SALE 50% OFF** - Whispering Willows is on sale for 50% off ($4.99) until September 9th. | Read more »
Calvino Noir (Games)
Calvino Noir 1.1 Device: iOS iPhone Category: Games Price: $3.99, Version: 1.1 (iTunes) Description: The film noir stealth game. Calvino Noir is the exploratory, sneaking adventure through the 1930s European criminal underworld.... | Read more »
Angel Sword (Games)
Angel Sword 1.0 Device: iOS Universal Category: Games Price: $6.99, Version: 1.0 (iTunes) Description: Prepare to adventure in the most epic full scale multiplayer 3D RPG for mobile! Experience amazing detailed graphics in full HD.... | Read more »

Price Scanner via MacPrices.net

iPad Air 2 on sale for up to $100 off MSRP
Best Buy has iPad Air 2s on sale for up to $100 off MSRP on their online store for a limited time. Choose free shipping or free local store pickup (if available). Sale prices available for online... Read more
MacBook Airs on sale for $100 off MSRP
Best Buy has MacBook Airs on sale for $100 off MSRP on their online store. Choose free shipping or free local store pickup (if available). Sale prices for online orders only, in-store prices may vary... Read more
Big Grips Lift Handle For iPad Air and iPad A...
KEM Ventures, Inc. which pioneered the extra-large, super-protective iPad case market with the introduction of Big Grips Frame and Stand in 2011, is launching Big Grips Lift featuring a new super-... Read more
Samsung Launches Galaxy Tab S2, Its Most Powe...
Samsung Electronics America, Inc. has announced the U.S. release of the Galaxy Tab S2, its thinnest, lightest, ultra-fast tablet. Blending form and function, elegant design and multitasking power,... Read more
Tablet Screen Sizes Expanding as iPad Pro App...
Larger screen sizes are gaining favor as the tablet transforms into a productivity device, with shipments growing 185 percent year-over-year in 2015. According to a new Strategy Analytics’ Tablet... Read more
Today Only: Save US$50 on Adobe Elements 13;...
Keep the memories. lose the distractions. Summer’s winding down and it’s time to turn almost perfect shots into picture perfect memories with Elements 13. And get the power to edit both photos and... Read more
1.4GHz Mac mini on sale for $449, save $50
Best Buy has the 1.4GHz Mac mini on sale for $50 off MSRP on their online store. Choose free shipping or free local store pickup (if available). Price for online orders only, in-store price may vary... Read more
12-inch 1.1GHz Gold MacBook on sale for $1149...
B&H Photo has the 12″ 1.1GHz Gold Retina MacBook on sale for $1149.99 including free shipping plus NY sales tax only. Their price is $150 off MSRP, and it’s the lowest price available for this... Read more
27-inch 3.3GHz 5K iMac on sale for $1849, sav...
Best Buy has the 27″ 3.3GHz 5K iMac on sale for $1849.99. Their price is $150 off MSRP, and it’s the lowest price available for this model. Choose free shipping or free local store pickup (if... Read more
Worldwide Tablet Shipments Expected to Declin...
Does Apple badly need a touchscreen convertible/hybrid laptop MacBook? Yes, judging from a new market forecast from the International Data Corporation (IDC) Worldwide Quarterly Tablet Tracker, which... Read more

Jobs Board

*Apple* Retail - Multiple Positions (US) - A...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
*Apple* Retail Online Store: Customer Insigh...
**Job Summary** Apple Retail (Online Store) is seeking an experienced e-commerce analytics professional to join the Customer Insights Team. The Web e-Commerce Analyst Read more
*Apple* Music, Business Operations - Apple I...
Changing the world is all in a day039s work at Apple . If you love innovation, here039s your chance to make a career of it. You039ll work hard. But the job comes with Read more
WW *Apple* Retail Online Store: Customer In...
**Job Summary** The Apple Retail - Online Store is seeking an experienced web merchandising analytics professional to join the Customer Insights Team. The Web Read more
Senior Payments Security Manager - *Apple*...
**Job Summary** Apple , Inc. is looking for a highly motivated, innovative and hands-on senior payments security manager to join the Apple Pay security team. You will Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.