TweetFollow Us on Twitter

Enabling CGI Scripts, The Second

Volume Number: 19 (2003)
Issue Number: 10
Column Tag: Programming

Untangling the Web

Enabling CGI Scripts, The Second

by Kevin Hemenway

You've enabled CGI, but how do you know it's good?

In the last issue, we learned about CGI scripts: what they are, what they can do, how they're already enabled within Apache, and how to tweak that configuration to be more URL friendly. What we didn't do is teach you anything for the future: at most, we brought a wide-eyed wonder-boy to a patch of poison ivy, and backed away slowly. Will he rub it on his skinned knee? Pin it to little Susie's dress as a token of his affection? Roll around in it like catnip? Where is the inbred fear necessary for every child's survival?

Insert transitional one-liner here!

Dissection--Similarities

Before we can understand, be aware, and watch for the security ramifications of running CGI scripts from unknown and untrusted third parties, we need to see how they're coded, how poorly written ones can ruin our mornings, and how to look for some semblance of quality. The quickest way to get a general feel is with the two sample scripts already installed with Apache: /Library/WebServer/CGI-Executables/printenv and /Library/WebServer/CGI-Executables/printenv/test-cgi. If you looked at their source code last month, you may have noticed they're written in two different languages.

The smaller of the two scripts, test-cgi, starts with #!/bin/sh, whereas printenv instead uses #!/usr/bin/perl -T. These lines, specifically the #! prefix, are often called the "shebang", and tell us which interpreter will execute the programming instructions that follow. The interpreter located at /bin/sh, rarely seen in production CGI, indicates that the rest of the code is written in the shell scripting language. Any CGI script you deploy will need to have some sort of shebang--whether it's /bin/sh, /usr/bin/perl, /usr/bin/python or something else entirely, it's absolutely required. Not only is it necessary, it also has to be accurate: if your only Perl is /sw/bin/perl, then the shebang should point there instead. Shebangs can also contain command line arguments: in printenv, -T is passed directly to the /usr/bin/perl interpreter (where it means something we'll cover a bit later).

Another similar difference between our two scripts is the printing of something called a Content-type (Listing 1), which tell the requesting user-agent (your visitor's browser) what sort of data it's about to receive (an image to render, text to display, XML to parse, etc.). The Content-type will never actually be shown in your final output--it's hidden pixie dust for the browser's benefit only (if you're curious, Mozilla allows you to view the Content-type by getting the "Page Info" of the current URL). Without this crucial bit of contextual magic (and the two required newlines), Apache will fail your CGI scripts with an "Internal Server Error". This error is never a satisfying explanation--you'll need to check Apache's /var/log/httpd/error_log for the exact reasoning.

Listing 1: Printing the Content-type in Shell and Perl

From the sample CGI scripts printenv and test-cgi

# content type display from test-cgi
# note that echo spits out a newline,
# 2 echo's for the 2 required newlines.
echo Content-type: text/plain
echo
# and the similar entry from printenv
print "Content-type: text/html\n\n";

The values of our Content-types (text/plain and text/html) didn't just appear out of thin air--they're MIME types, and most any file you've ever worked with has one. You can find a large listing of MIME types, based on their common file extensions, by perusing the /etc/httpd/mime.types file. For example, the matching MIME types for JPEG, XHTML, Quicktime, and Microsoft Word files are:

image/jpeg                      jpeg jpg jpe
application/xhtml+xml           xhtml xht
video/quicktime                 qt mov
application/msword              doc

If you can't find the matching MIME type for the data you're interested in serving (either because it's not in the mime.types file or Google has spurned your search request), you can use the "some sort of data" MIME type of application/octet-stream. This has already been explicitly assigned to a number of files, including Apple disk images:

application/octet-stream   dms lha lzh exe class so dll dmg

Dissection--Why The Perl Script Is Arguably Stronger

All CGI scripts, regardless of what they're programmed in, can be run from the command line--whether they actually do anything useful is a case-by-case basis. This is a surprisingly useful bit of information: since troubleshooting and debugging happens best when unfrilled by complication, removing Apache from the process can prove helpful. Running your CGI scripts on the command line can preemptively weed out problems like missing Content-type's, file permission errors, invalid syntax problems, missing language extensions, and so forth.

Both the test-cgi and printenv scripts run "successfully" at the command line, although only the first gives any useful output (Figure 1). Compare this to the regular browser-based output we demonstrated in the last MacTech (or simply re-access http://127.0.0.1/cgi-bin/test-cgi). The first line is that dastardly Content-type and, as mentioned before, is normally processed by the browser and removed from the final display. Since we're running the script without the benefit of a web server or browser, the Content-type is viewable without extra effort. This becomes a handy barometer: if you run your CGI script from the command line and there's no Content-type, it'll never run correctly under Apache.


Figure 1: The slightly undefined test-cgi, when run in the Terminal

But wait... there's no Content-type if we try to run printenv (in fact, there's nothing at all), so why does it work when we access it by URL (http://127.0.0.1/cgi-bin/printenv)? In actuality, this is one of the "strengths" of the Perl version. If you check the source code, the next line after our required shebang (ignoring comments) is:

exit unless ($ENV{'REQUEST_METHOD'} eq "GET");

This terminates the script unless it was invoked via a GET request. Generically speaking, unless it is a POST, every request a web browser makes is a GET with or without key/value pairs. Since the shell isn't a web browser, no GET is issued and the script terminates. If we wanted to get fancy, we could fake the required method by running setenv REQUEST_METHOD GET && ./printenv (if you're using the tsch shell; REQUEST_METHOD=GET ./printenv if you prefer bash). As a result, we get a Terminal full of HTML listing the environment variables. We can redirect this mass of HTML to a file by adding > output.html to our previous command line; Figure 2 shows the generated file.


Figure 2: Shell output of our tricked printenv script

Figure 2 also gives us another reason why the Perl script is stronger: it doesn't pretend to know what the environment is going to look like. test-cgi, hard-coded to display the values of known variables (SERVER_SOFTWARE, SERVER_NAME, GATEWAY_INTERFACE, etc.), shows nothing but undefined values when run from the Terminal (Figure 1), where those specific entries don't normally exist.

Three Ways Perl CGI Scripts Can Be Improved

The bulk of the code within the printenv script caters to creating a pretty HTML page, something not important to the true purpose of generating a list of the current environment. To make our upcoming improvements more clearly, we'll base our changes on the Perl script shown in Listing 2, which does the exact same thing as printenv, only without the HTML. For all intents and purposes, this is a working CGI script: it's got the shebang pointing to the correct Perl interpreter, and it prints a plain-text Content-type before any other data.

Note that even though we're talking specifically about CGI scripts, the following improvements can, and should, be made in most any Perl script, especially those to be used in production environments. Security should never be a feature.

Listing 2: Printing the environment more simply

Our base.pl script could use some improvements.

#!/usr/bin/perl
print "Content-type: text/plain\n\n";
foreach $var (keys %ENV) {
   print "$var = $ENV{$var}\n";
}

Save this file as base.pl and run it from the command line; my output is in Figure 3. None of our upcoming improvements will change this display and, as you can see by comparing it to Figure 2, it's identical save for the loss of HTML (and the differences between Safari and the Terminal's interpretation of TERMCAP).


Figure 3: Our rewritten script's (base.pl) output

Our improvements to the script are quite minimal additions, but they ensure that user data has been properly checked for dangerous input, warnings have been enabled for common mistakes or typos that don't necessarily stop a script from running, and a stricter development environment has been used to encourage stronger coding and careful variable declaration. The revised script is shown in Listing 3.

Listing 3: Printing the environment more strongly

Our revised script is three times stronger than before.

#!/usr/bin/perl -wT
use strict;
print "Content-type: text/plain\n\n";
foreach my $var (keys %ENV) {
   print "$var = $ENV{$var}\n";
}
  • Use warnings: The first change, adding -w to the shebang, turns on Perl's warnings pragma, which spits a list of optional, non-fatal warnings to STDERR (which becomes Apache's error_log when run as a CGI). Technically, you don't have to address any of the messages since the script will continue on regardless, but they'll alert you to typos, uninitialized values, deprecated functions, and a slew of other mishaps that can eventually escalate into full-blown bugs. Typically, the messages are terse enough to be useful for seasoned Perl programmers, but you can increase their verbosity by adding use diagnostics; within the body of your code.

  • Use strict: Our third and fourth changes complement our warnings. Perl's strict pragma should be used in any script that is more than "casual", and ensures that every variable is pre-declared and localized, and that other "unsafe constructs" are detected and addressed. Unlike warnings, any error that triggers strict will stop your script from continuing further. You'll notice that we've localized our $var variable with the my() function. The first time you use strict, it'll feel like an unwieldy and overly doting mother, but scripts that compile cleanly benefit from an attention to detail that strengthens their quality immensely.

  • Use taint: Even though it is "strongly recommended", very few Perl or CGI scripts use taint mode, which is what the -T on the shebang enables. Under this mode, any outside data received by your code is considered highly dangerous, and will cause script errors until it has been checked for safety. These safety checks can be as simple as ensuring that a command line argument only contains alphanumerics, or that the process you're spawning isn't being handed potentially damaging shell metacharacters. While taint mode will force you to focus more strongly about the evils of the outside world and exactly what data you expect, programmers who misunderstand how to "untaint" data may inadvertently do so incorrectly, creating a false sense of security.

These programming additions aren't the ultimately panacea, but merely a placebo. Yes, your code will be stronger with them, but that doesn't mean crucial bugs won't creep in and ruin your day. Serious coders and sysadmins should take a look at the following sampling of Perl and CGI security links:

  • The Perl Security manpage, accessible by typing man perlsec in your Terminal, can also be read online at http://www.perldoc.com/perl5.6.1/pod/perlsec.html

  • "Avoiding security holes when developing an application", a six part series from LinuxFocus.org: http://www.linuxfocus.org/English/November2001/article203.shtml

  • SecureProgramming (http://www.secureprogramming.com/) offers a huge collection of links to over 50 articles, books, recipes to learn from and adapt, and more.

  • RFP's "Perl CGI problems", which appeared in an old issue of the seminal Phrack magazine, still remains relevant: http://www.wiretrip.net/rfp/txt/phrack55.txt

  • CERT's "How To Remove Meta-characters From User-Supplied Data In CGI Scripts", in both Perl and C: http://www.cert.org/tech_tips/cgi_metacharacters.html. Handy for when you're looking to untaint some data.

  • The "Securing Programming for Linux and Unix HOWTO", available from http://en.tldp.org/HOWTO/Secure-Programs-HOWTO/. Similar articles like "The Hack FAQ" (http://www.nmrc.org/pub/faq/hackfaq/index.html), and the "WWW Security FAQ" (http://www.w3.org/Security/Faq/www-security-faq.html) will also prove insightful.

    Choosing a CGI Script for Deployment

    The above programming suggestions are fine if you're solely looking at the code quality of a potential CGI script, but there are few more areas to investigate before you can consider a program worthy of being installed on your server:

    • Check the Bugtraq archives (http://securityfocus.com/archive/1). Anyone interested in security should be reading Bugtraq, where a large community of hackers, white hats, sysadmins, and professionals regularly post bugs, exploits, and warnings for insecure products. Occasionally, you'll also see new whitepapers concerning various aspects of security and programming. Before installing new scripts, comb the archives to see if any advisories have been posted. If so, ensure they've been fixed before using the code.

    • Googling for problems can prove illuminating, as you'll often find common tech support problems, heaps of praise or scorn for the code or author, and occasionally, other web hosts who offer the script for their own customer base.

    • Check the dates: When was the script last updated? Is it so long ago that no one will give a darn if you have a problem? Just because a script doesn't have any reported problems in Bugtraq doesn't mean that it isn't susceptible to relatively new exploits like cross-site scripting attacks (http://www.cgisecurity.com/articles/xss-faq.shtml). Code that has been updated recently has a better chance of good turnaround time for crucial fixes, updates, and support.

    • Got logfiles? Most CGI scripts don't have any logging capability, primarily because they only do one small thing (like email forms, add one to a number, display a calendar, etc.) Some complicated scripts, however, can benefit from logging, especially those with built-in user authentication ("who is using my site?") or flaw tracking ("a bug occurred at [time], and things turned awry [like this]"). Scripts can use their own logfiles or Perl's Sys::Syslog module to log directly to /var/log/system.log.

      Homework Malignments

      In our next column, we'll move on to configuring PHP, as well as explain the up- and downsides between forking processes (like CGI) and embedded modules (like mod_php). We'll explore the default configuration of PHP, the non-existent configuration file (php.ini) and, if we have time, how to install MySQL and do a few integration tests. For now, students may contact the teacher at morbus@disobey.com.

    • Besides -w, you can also enable Perl's warning pragma with use warnings; (similar to use strict;). Subtle differences exist between the two--research them and find out which satisfies your programming needs better.

    • Any Perl script with logging may eventually run up against a perceived "buffering" problem, the sordid details of which are explained in Mark Jason Dominus' "Suffering from Buffering?" (http://perl.plover.com/FAQs/Buffering.html).

    • If you're looking to brush up on your Perl knowledge, you can't go wrong with O'Reilly's Learning Perl, The Perl Cookbook (which just received an impressive Second Edition update), and the recent Learning Perl Objects, References, & Modules. You can read sample chapters from all the books at http://www.oreilly.com/.


      Kevin Hemenway, coauthor of Mac OS X Hacks and Spidering Hacks, is better known as Morbus Iff, the creator of disobey.com, which bills itself as "content for the discontented." Publisher and developer of more home cooking than you could ever imagine (like the popular open-sourced aggregator AmphetaDesk, the best-kept gaming secret Gamegrene.com, the ever ignorable Nonsense Network), he's twirling his hair and trying not to cheerlead. Contact him at morbus@disobey.com.

  •  

    Community Search:
    MacTech Search:

    Software Updates via MacUpdate

    Carbon Copy Cloner 4.1.16 - Easy-to-use...
    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
    Sketch 45 - Design app for UX/UI for iOS...
    Sketch is an innovative and fresh look at vector drawing. Its intentionally minimalist design is based upon a drawing space of unlimited size and layers, free of palettes, panels, menus, windows, and... Read more
    NeoFinder 7.1 - Catalog your external me...
    NeoFinder (formerly CDFinder) rapidly organizes your data, either on external or internal disks, or any other volumes. It catalogs all your data, so you stay in control of your data archive or disk... Read more
    TunnelBear 3.0.15 - Subscription-based p...
    TunnelBear is a subscription-based virtual private network (VPN) service and companion app, enabling you to browse the internet privately and securely. Features Browse privately - Secure your data... Read more
    Hopper Disassembler 4.2.5- - Binary disa...
    Hopper Disassembler is a binary disassembler, decompiler, and debugger for 32-bit and 64-bit executables. It will let you disassemble any binary you want, and provide you all the information about... Read more
    BetterTouchTool 2.261 - Customize Multi-...
    BetterTouchTool adds many new, fully customizable gestures to the Magic Mouse, Multi-Touch MacBook trackpad, and Magic Trackpad. These gestures are customizable: Magic Mouse: Pinch in / out (zoom... Read more
    Sketch 44.1 - Design app for UX/UI for i...
    Sketch is an innovative and fresh look at vector drawing. Its intentionally minimalist design is based upon a drawing space of unlimited size and layers, free of palettes, panels, menus, windows, and... Read more
    BetterTouchTool 2.260 - Customize Multi-...
    BetterTouchTool adds many new, fully customizable gestures to the Magic Mouse, Multi-Touch MacBook trackpad, and Magic Trackpad. These gestures are customizable: Magic Mouse: Pinch in / out (zoom... Read more
    Chromium 59.0.3071.115 - Fast and stable...
    Chromium is an open-source browser project that aims to build a safer, faster, and more stable way for all Internet users to experience the web. Version 59.0.3071.115: This update has no Flash plug... Read more
    SyncTwoFolders 2.2.3 - Syncs two user-sp...
    SyncTwoFolders simply synchronizes two folders. It supports synchronization across mounted network drives and it is a possibility to run a simulation showing in a log what will be done. Please visit... Read more

    Latest Forum Discussions

    See All

    Dive into epic summer adventure with Oce...
    Summer may be the best time to enjoy ocean adventures, and now you don’t even have to leave the comfort of your own home, thanks to the folks at Joycity, creators of Oceans & Empires. The old-timey naval MMO is getting a sizable new June Grand... | Read more »
    Missile Cards (Games)
    Missile Cards 1.0.9 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0.9 (iTunes) Description: "Missile Command meets Solitaire...only with more doomlasers, death, and explosions." | Read more »
    Collect mini assassins in 'Assassin...
    Assassin's Creed is traveling back in time to the Spanish Inquisition for its latest mobile entry, Assassin's Creed Rebellion. The game is giving the series a look that's a huge departure from its past design, recreating classic characters in a... | Read more »
    Animal Crossing is still coming to mobil...
    Animal Crossing is still coming to mobile in 2017, according to aWaypointinterview with Nintendo. Announced in 2016, the game was delayed without a defined release window. However, fans of Nintendo's fantasy slice of life game won't have to wait... | Read more »
    Ravenscroft 275 Piano (Music)
    Ravenscroft 275 Piano 1.0.0 Device: iOS Universal Category: Music Price: $35.99, Version: 1.0.0 (iTunes) Description: Experience the splendor of a Ravenscroft Grand with the most realistic sounding piano ever created for iOS. Launch... | Read more »
    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 »

    Price Scanner via MacPrices.net

    Will iPad Running iOS 11 Be The ‘Ute’ Of The...
    Steve Jobs’ analogy comparing iPads and PCs to cars and trucks respectively is seven years old but still stimulates discussion and debate. Appearing on an All Things D panel in 2010 shortly after the... Read more
    Free CarePassport App gives Patients control...
    Boston based CarePassport is on a mission to enable patients to take control of their medical records by allowing patients to aggregate, store, share and manage all their medical data including... Read more
    Western Digital Launches New My Passport Ultr...
    Western Digital Corporation has expanded its WD brand My Passport portable drive line with the redesigned My Passport Ultra drive. In addition to a new metallic look, the drive offers intuitive WD... Read more
    Clearance 2016 13-inch MacBook Pros available...
    B&H Photo has clearance 2016 13″ MacBook Pros in stock today for up to $210 off original MSRP. Shipping is free, and B&H charges NY & NJ sales tax only: - 13″ 2.9GHz/512GB Touch Bar... Read more
    Apple Releases iOS 11 Public Beta; How To Get...
    The official release of Apple’s latest mobile operating system iOS 11 is vaguely slated for the fall, but as of June 26, ordinary users can download an iOS 11 public beta. To download the iOS 11... Read more
    Extend Life of MacBook Pro Retina 2.0TB With...
    MacSales.com/Other World Computing has announced availability of the new OWC 2.0TB Aura Pro Solid State Drive for mid-2012 to early 2013 Apple MacBook Pro with Retina display. One of the fastest... Read more
    BBEdit SummerFest 2017 Discount Ends Friday,...
    You can get 20% off BBEdit for a limited time in Bare Bones Software’s http://www.eastgate.com/Tinderbox/Specials/SummerFest.html?mc_cid=f2101ca260&mc_eid=[UNIQID]SummerFest 2017 sale and... Read more
    Use Apple’s Education discount to save up to...
    Purchase a new Mac using Apple’s Education discount, and take up to $300 off MSRP. All teachers, students, and staff of any educational institution qualify for the discount. Shipping is free: - 15″ 2... Read more
    Clearance 27-inch 3.3GHz 5K iMac available fo...
    Amazon clearance 27″ 3.3GHz 5K iMacs (MK482LL/A) available for $1799.90 including free shipping. Their price is $500 off original MSRP, and it’s the lowest price available for this model from any... Read more
    13-inch 1.8GHz/256GB MacBook Air on sale for...
    B&H Photo has the updated 2017 13″ 1.8GHz/256GB MacBook Air (MQD42LL/A) in stock and on sale for $1129 including free shipping plus NY & NJ tax only. Their price is $70 off MSRP. 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.