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

calibre 3.1.1 - Complete e-book library...
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
Sparkle Pro 2.2 - $79.99
Sparkle Pro will change your mind if you thought building websites wasn't for you. Sparkle is the intuitive site builder that lets you create sites for your online portfolio, team or band pages, or... Read more
Spotify 1.0.57.474. - Stream music, crea...
Spotify is a streaming music service that gives you on-demand access to millions of songs. Whether you like driving rock, silky R&B, or grandiose classical music, Spotify's massive catalogue puts... Read more
beaTunes 5.0.1 - Organize your music col...
beaTunes is a full-featured music player and organizational tool for music collections. How well organized is your music library? Are your artists always spelled the same way? Any R.E.M. vs REM?... Read more
LibreOffice 5.3.4.2 - 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
VOX 2.8.26 - Music player that supports...
VOX just sounds better! The beauty is in its simplicity, yet behind the minimal exterior lies a powerful music player with a ton of features and support for all audio formats you should ever need.... Read more
iFFmpeg 6.4.0 - Convert multimedia files...
iFFmpeg is a comprehensive media tool to convert movie, audio and media files between formats. The FFmpeg command line instructions can be very hard to master/understand, so iFFmpeg does all the hard... Read more
Beamer 3.3 - Stream any movie file from...
Beamer streams to your Apple TV.... Plays any movie file - Just like the popular desktop movie players, Beamer accepts all common formats, codecs and resolutions. AVI, MKV, MOV, MP4, WMV, FLV. To... Read more
jAlbum Pro 14.0 - Organize your digital...
jAlbum Pro has all the features you love in jAlbum, but comes with a commercial license. You can create gorgeous custom photo galleries for the Web without writing a line of code! Beginner-friendly... Read more
Apple Remote Desktop Client 3.9.3 - Clie...
Apple Remote Desktop Client is the best way to manage the Mac computers on your network. Distribute software, provide real-time online help to end users, create detailed software and hardware reports... Read more

Latest Forum Discussions

See All

This War of Mine gets a new ending and m...
This War of Mine just got a big new update, featuring free DLC that adds a new ending to the game, among other exciting changes. The update is celebrating the game's two-year release anniversary. Apart from the new ending, which will be quite... | Read more »
Summon eight new heroes in Fire Emblem H...
Nintendo keeps coming at us with Fire Emblem Heroes updates, and it doesn't look like that trend is stopping anytime soon. The folks behind the game have just announced the new War of the Clerics Voting Gauntlet, expected to start next Tuesday. [... | Read more »
The best deals on the App Store this wee...
iOS publishers are pulling out all the stops this week -- there's a huge number of seriously great games at discounted prices this week. Let's not waste any time and get right down to business. [Read more] | Read more »
The House of da Vinci (Games)
The House of da Vinci 1.0.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0.0 (iTunes) Description: Enter The House of Da Vinci, a new must-try 3D puzzle adventure game. Solve mechanical puzzles, discover hidden... | Read more »
Solve the disappearance of history’s gre...
Blue Brain Games invites you to indulge in an immersive hands-on 3D puzzle adventure in similar vein to The Room series, with its debut release The House of Da Vinci. Set during the historic period of the Italian Renaissance (when Leonardo himself... | Read more »
Age of Rivals (Games)
Age of Rivals 3.3 Device: iOS Universal Category: Games Price: $.99, Version: 3.3 (iTunes) Description: Deep civilization-building strategy in a fast-paced card game! | Read more »
Panthera Frontier (Games)
Panthera Frontier 1.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0 (iTunes) Description: | Read more »
Angry Birds Evolution beginner's gu...
Angry Birds changes things up a fair bit in its latest iteration, Angry Birds Evolution. The familiar sling-shot physics mechanics are still there, but the game now features team-based gameplay, RPG elements, and a new top-down view. With all of... | Read more »
Sega Forever is for the retro game fans
Sega is launching a new retro games service titled Sega Forever, in a move that's sure to delight games enthusiasts with a bit of nostalgia. Sega's releasing five classic games for free. The titles include Sonic the Hedgehog, Phantasy Star II,... | Read more »
The Little Acre (Games)
The Little Acre 1.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0 (iTunes) Description: | Read more »

Price Scanner via MacPrices.net

ABBYY TextGrabber 6 for iOS Implements Instan...
ABBYY has announced the release of TextGrabber 6.0.0, an important feature update to the company’s productivity app developed for iOS and Android devices. TextGrabber 6.0 now offers Real-Time... Read more
vPhone, First Smartphone That Can’t Be Lost,...
Austin, Texas based Hypori has introduced the vPhone, a virtual smartphone that affords every business user the benefits of separate work and personal phones, conveniently delivered on a single... Read more
Save this weekend with 2016 refurbished MacBo...
Apple has dropped prices on Certified Refurbished 2016 15″ and 13″ MacBook Pros by as much as $590 off original MSRP. An Apple one-year warranty is included with each model, and shipping is free: -... Read more
New 27-inch 3.4GHz iMac on sale for $1699, sa...
MacMall has the new 2017 27″ 3.4GHz iMac (MNE92LL/A) in stock and on sale for $1699 including free shipping. Their price is $100 off MSRP. Read more
Clearance 2016 MacBook Pros available for up...
B&H Photo has clearance 2016 13″ and 15″ MacBook Pros in stock today and on sale for up to $400 off original MSRP. Shipping is free, and B&H charges NY & NJ sales tax only: - 15″ 2.7GHz... Read more
Apple Ranks 9th In comScore Top 50 U.S. Digit...
comScore, Inc. has released its monthly ranking of U.S. online activity at the top digital media properties for May 2017 based on data from comScore Media Metrix Multi-Platform. * Entity has... Read more
10.5-inch iPad Pros available for up to $20 o...
B&H Photo has the new 2017 10.5″ iPad Pros available for up to $20 off MSRP including free shipping plus NY & NJ sales tax only: - 64GB iPad Pro WiFi: $649 - 256GB iPad Pro WiFi: $749 - 512GB... Read more
Three Off-The-Beaten-Track iOS Apps That Dese...
One of the great things about using iPads and iPhones is the vast selection of apps available for most anything you want or need to do. The three outlined in this article have been in my core app... Read more
Apple No. 1 Spot In Gartner Top 100 Vendors i...
Gartner, Inc. has unveiled the top global 100 vendors in IT in 2016 based on their revenue across IT (excluding communication services) and component market segments. In the Gartner Global Top 100:... Read more
Clearance iMacs available for up to $300 off...
B&H Photo has clearance 21″ and 27″ Apple iMacs available starting at $949, each including free shipping plus NY & NJ sales tax only: - 27″ 3.3GHz iMac 5K: $1999 $300 off original MSRP - 27″... Read more

Jobs Board

*Apple* News Product Marketing Mgr., Publish...
…organizational consensus on strategy and vision for publisher tools, authoring, and Apple News Format.Carries this strategy and vision across the organization to Read more
*Apple* Retail - Multiple Positions - Apple,...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
Security Data Analyst - *Apple* Information...
…data sources need to be collected to allow Information Security to better protect Apple employees and customers from a wide range of threats.Act as the subject matter Read more
Lead *Apple* Solutions Consultant - Apple I...
…integrity, and trust.Success Metrics/Key Performance Indicators:Quantitative* Year over Year growth in Apple Product and Beyond the Box sales in the assigned Point of Read more
*Apple* Solutions Consultant till v%u00E5r...
…ethics, integrity, and trust.Success Metrics/Key Performance Indicators:QuantitativeYear over Year growth in Apple Product and Beyond the Box sales in the assigned Point Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.