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.

 
AAPL
$119.00
Apple Inc.
+1.40
MSFT
$47.75
Microsoft Corpora
+0.28
GOOG
$540.37
Google Inc.
-0.71

MacTech Search:
Community Search:

Software Updates via MacUpdate

HoudahSpot 3.9.6 - Advanced file search...
HoudahSpot is a powerful file search tool built upon MacOS X Spotlight. Spotlight unleashed Create detailed queries to locate the exact file you need Narrow down searches. Zero in on files Save... Read more
RapidWeaver 6.0.3 - 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
iPhoto Library Manager 4.1.10 - Manage m...
iPhoto Library Manager lets you organize your photos into multiple iPhoto libraries. Separate your high school and college photos from your latest summer vacation pictures. Or keep some photo... Read more
iExplorer 3.5.1.9 - View and transfer al...
iExplorer is an iPhone browser for Mac lets you view the files on your iOS device. By using a drag and drop interface, you can quickly copy files and folders between your Mac and your iPhone or... Read more
MacUpdate Desktop 6.0.3 - Discover and i...
MacUpdate Desktop 6 brings seamless 1-click installs and version updates to your Mac. With a free MacUpdate account and MacUpdate Desktop 6, Mac users can now install almost any Mac app on macupdate.... Read more
SteerMouse 4.2.2 - Powerful third-party...
SteerMouse is an advanced driver for USB and Bluetooth mice. It also supports Apple Mighty Mouse very well. SteerMouse can assign various functions to buttons that Apple's software does not allow,... Read more
iMazing 1.1 - Complete iOS device manage...
iMazing (was DiskAid) is the ultimate iOS device manager with capabilities far beyond what iTunes offers. With iMazing and your iOS device (iPhone, iPad, or iPod), you can: Copy music to and from... Read more
PopChar X 7.0 - Floating window shows av...
PopChar X helps you get the most out of your font collection. With its crystal-clear interface, PopChar X provides a frustration-free way to access any font's special characters. Expanded... Read more
OneNote 15.4 - Free digital notebook fro...
OneNote is your very own digital notebook. With OneNote, you can capture that flash of genius, that moment of inspiration, or that list of errands that's too important to forget. Whether you're at... Read more
Carbon Copy Cloner 4.0.3 - Easy-to-use b...
Carbon Copy Cloner backups are better than ordinary backups. Suppose the unthinkable happens while you're under deadline to finish a project: your Mac is unresponsive and all you hear is an ominous,... Read more

Latest Forum Discussions

See All

Raby (Games)
Raby 1.0.3 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0.3 (iTunes) Description: ***WARNING - Raby runs on: iPhone 5, iPhone 5C, iPhone 5S, iPhone 6, iPhone 6 Plus, iPad Mini Retina, iPad Mini 3, iPad 4, iPad Air,... | Read more »
Oddworld: Stranger's Wrath (Games)
Oddworld: Stranger's Wrath 1.0 Device: iOS Universal Category: Games Price: $5.99, Version: 1.0 (iTunes) Description: ** PLEASE NOTE: Oddworld Stranger's Wrath requires at least an iPhone 4S, iPad 2, iPad Mini or iPod Touch 5th gen... | Read more »
Bounce On Back (Games)
Bounce On Back 1.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0 (iTunes) Description: | Read more »
Make Way for Fat Chicken, from the Maker...
Make Way for Fat Chicken, from the Makers of Scrap Squad Posted by Jessica Fisher on November 26th, 2014 [ permalink ] Relevant Games has announced they will be releasing their reverse tower defense game, | Read more »
Tripnary Review
Tripnary Review By Jennifer Allen on November 26th, 2014 Our Rating: :: TRAVEL BUCKET LISTiPhone App - Designed for the iPhone, compatible with the iPad Want to create a travel bucket list? Tripnary is a fun way to do exactly that... | Read more »
Ossian Studios’ RPG, The Shadow Sun, is...
Ossian Studios’ RPG, The Shadow Sun, is Now Available for $4.99 Posted by Jessica Fisher on November 26th, 2014 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »
Mmmm, Tasty – Having the Angry Birds for...
The very first Angry Birds debuted on iOS back in 2009. When you sit back and tally up the number of Angry Birds games out there and the impact they’ve had on pop culture as a whole, you just need to ask yourself: “How would the birds taste... | Read more »
Rescue Quest Review
Rescue Quest Review By Jennifer Allen on November 26th, 2014 Our Rating: :: PATH BASED MATCH-3Universal App - Designed for iPhone and iPad Guide a wizard to safety by matching gems. Rescue Quest might not be an entirely original... | Read more »
You Can Play the Final Chapter of Lone W...
You Can Play the Final Chapter of Lone Wolf: Dawn Over V’taag Right Now Posted by Jessica Fisher on November 26th, 2014 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »
Swords of Anima (Games)
Swords of Anima 1.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0 (iTunes) Description: A new tactical turn-based RPG experience. Command the Savior Rex Squad in an epic journey of courage and deception. Can you... | Read more »

Price Scanner via MacPrices.net

2014 1.4GHz Mac mini on sale for $449, save $...
 B&H Photo has the new 1.4GHz Mac mini on sale for $449.99 including free shipping plus NY tax only. Their price is $50 off MSRP, and it’s the lowest price available for this new model. Adorama... Read more
Early Black Friday pricing on 27-inch 5K iMac...
 B&H Photo continues to offer Black Friday sale prices on the 27″ 3.5GHz 5K iMac, in stock today and on sale for $2299 including free shipping plus NY sales tax only. Their price is $200 off MSRP... Read more
Early Black Friday sale prices on iPad Air 2,...
 MacMall is discounting iPad Air 2s by up to $75 off MSRP as part of their Black Friday sale. Shipping is free: - 16GB iPad Air WiFi: $459 $40 off - 64GB iPad Air WiFi: $559 $40 off - 128GB iPad Air... Read more
Early Black Friday MacBook Air sale prices, $...
 MacMall has posted early Black Friday MacBook Air sale prices. Save $101 on all models for a limited time: - 11″ 1.4GHz/128GB MacBook Air: $798 - 11″ 1.4GHz/256GB MacBook Air: $998 - 13″ 1.4GHz/... Read more
Why iPhone 6 Tablet/Laptop Cannibalization Is...
247wallst.com blogger Douglas A. McIntyre noted last week that according to research posted on the Applovin blog site the iPhone 6 is outselling the iPhone 6 Plus by a wide margin . Hardly a surprise... Read more
Worldwide Tablet Growth Expected to Slow to 7...
The global tablet market is expected to record massive deceleration in 2014 with year-over-year growth slowing to 7.2%, down from 52.5% in 2013, according to a new forecast from International Data... Read more
Touchscreen Glove Company Announces New Produ...
Surrey, United Kingdom based TouchAbility specializes in design and manufacture of a wide variety of products compatible with touchscreen devices including smartphones, tablets and computers. Their... Read more
OtterBox Alpha Glass Screen Protectors for iP...
To complement the bigger, sharper displays on the latest Apple devices, OtterBox has introduced Alpha Glass screen protectors to the iPhone 6 and iPhone 6 Plus. The fortified glass screen protectors... Read more
Early Black Friday Mac Pro sale, 6-Core 3.5GH...
 B&H Photo has the 6-Core 3.5GHz Mac Pro on sale today for $3499 including free shipping plus NY sales tax. Their price is $500 off MSRP, and it’s the lowest price available for this model from... Read more
Early Black Friday sale price: 15-inch 2.2GHz...
 B&H Photo has the 2014 15″ 2.2GHz Retina MacBook Pro on sale today for $1699.99. Shipping is free, and B&H charges NY sales tax only. Their price is $300 off MSRP, equalling Best Buy’s price... Read more

Jobs Board

*Apple* Solutions Consultant (ASC) - Apple (...
**Job Summary** The ASC is an Apple employee who serves as an Apple brand ambassador and influencer in a Reseller's store. The ASC's role is to grow Apple Read more
Senior Event Manager, *Apple* Retail Market...
…This senior level position is responsible for leading and imagining the Apple Retail Team's global event strategy. Delivering an overarching brand story; in-store, Read more
*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* Solutions Consultant (ASC) - Apple (...
**Job Summary** The ASC is an Apple employee who serves as an Apple brand ambassador and influencer in a Reseller's store. The ASC's role is to grow Apple Read more
*Apple* Solutions Consultant (ASC) - Apple (...
**Job Summary** The ASC is an Apple employee who serves as an Apple brand ambassador and influencer in a Reseller's store. The ASC's role is to grow Apple Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.