Monday, July 18, 2011

Dynamic Linked Libraries (DLL) vs Static Libraries

We no longer use DLL's with the Platypus Billing System, except where absolutely necessary.  In some cases, with high level languages (such as Visual Basic 6 and Visual FoxPro) and 3rd party libraries (such as OpenSSL) written in C/C++, we have no other choice.  Plus there are ActiveX/COM libraries (such as MSXML, Mailbee, DBI, and Crystal Reports), which cannot be linked to statically.  But, in many cases, it can be avoided.

Without getting into an argument over which is better or worse, when the stability of a product is on the line, having DLL's creates another point of failure.  For that reason alone, it was more important for us to statically link our C/C++ code where possible.  Sure, the binaries may be larger and updates basically meant a re-install; but it has been well worth these minor difficulties. 

Since the switch to Visual C++ 2005 and static linking back in 2009, the number of C++ dependency issues we have encountered are still in the single digits - and that is only because of ActiveX/COM.  Just to relay the point, here a few of the specific cases I have encountered over the past few years.

Case #1: PHP vs Pidgin

Both PHP and Pidgin include a spell check library - Aspell - in the form of aspell-15.dll.  Since the web pages for our product are written in PHP, I - of course - need PHP installed on my dev machine.  Also, I have Pidgin installed for chatting with technical support - or anyone else at work when a face-to-face confab is not required.

Now, normally these two products are not in conflict and everything works swimmingly.  But, one day, I decided to grab one of the newer - more stable, secure, and compiled in VC 2008 - PHP editions from the PHP for Windows.  Everything worked fine at first.  Then, as happens, I needed to reboot.  Afterwards, Pidgin crashed every time I tried to start it up.

After yanking my hair out using Dependency Walker and Process Monitor, I finally figured out that it was because of Aspell.  I renamed aspell-15.dll in the PHP folder and everything started working again.  Because PHP was in the system path, Pidgin was loading the PHP version of the dll instead of the one in the Pidgin folder.  It shouldn't have done this, and I could find no logical reasoning for it, but that is what was happening.

Regardless, I didn't have the time to look into it further.  I knew the cause and could bypass it.  Spell check is nice, but completely unnecessary for IM.  So, I uninstalled Pidgin, and reinstalled it without the spell check feature.  Problem solved - or, at least, dealt with.

Case #2: PHP vs OpenSSL

With our product, we include a COM DLL (tu_app.dll) for interacting with the Tucows Email Service.  This COM library was written in Visual C++ 6.0 and was linked to some severely old versions of the OpenSSL libraries.  Again, because I decided to go mucking about with my installation of PHP, I broke yet another thing on my dev machine.

I was performing some fixes for our integration with Tucows Email and had to do some unit tests.  Every time I tried to load the COM object, the program would crash spectacularly.  After some more hair pulling, I traced it down to the OpenSSL libraries.  I replaced the DLL's installed by PHP with the one included in our installation set and it started working again.

Problem solved?  No, definitely not.  While crashng my IM client is one thing, the possibility that someone could install a special version of PHP on the same machine as our product - which is normally the case - is another.  Only the older versions of the OpenSSL libraries would work with our COM library.

Those OpenSSL libraries were ancient and would not pass any scrutiny when it came to PA-DSS.  Plus, having our product crash because we required using outdated and insecure versions of the OpenSSL libraries was completely unacceptable.  So, we ported the code from Visual C++ 6.0 to Visual C++ 2005 and statically linked to OpenSSL.  Now, the problem was solved.

Case #3: ATL Vulnerability

When this problem first came out, I was working on a separate major rewrite/port of our C++ code - specifically a Windows service for hosting our API - from Visual C++ 6.0 to Visual C++ 2005.  I had everything working.  It was beautiful and simple code, it compiled without warnings, it had no memory leaks, and it passed every test I threw at it.

Next, came compatibility testing.  After making an installation set for our product, I started testing on all the operating systems we supported - Windows 2000 up to Windows Vista/2008.  Upon start up on Windows Vista and 2008, the service immediately crashed.  It worked fine on Windows 2000 and XP.

I checked the Eventlog and found a side-by-side dependency error.  Considering this was my first venture into something newer than VC6, I wasn't fully competent with Application Manifests at the time.  So, I had no idea what this error really meant.

I checked the installation set to make sure it included the Visual C++ runtime - and it did.  I checked the installation log (and Add/Remove Programs) to make sure it installed - and it did.  After even more hair pulling, I found out about the ATL update.

The worst part was, no installation set for the Visual C++ runtime existed - which included the ATL fix.  There is now, but there wasn't at the time (or I just suck at using a search engine).  So, I could try to install the runtime files manually, but that involved a huge amount of effort and testing on all those OS's.  Especially, for something that had to be done that night.  I needed to finish my testing so we could release the next day (and possibly grab some sleep that night).  Plus, I had no idea what DLL's to install or where to install them or how to deal with WinSxS from a NSIS installation set.

So, my only option was to switch to static linking.  No more dependencies.  No unnecessary points of failure.  Or more simply, no more DLL Hell.  Finally, problem solved and a few hours sleep before the release.

Case #4: DLL Preloading Vulnerability

This is a generic definition of case #1.  A DLL from an unexpected location is loaded instead of the intended one.  While, case #1 wasn't officially an attack, it did crash a program and caused me a couple hours of unneeded stress.

Now, in cases like this, there are officially two ways to deal with it.  First, you can mitigate the attack surface by using SetDLLDirectory.  This limits the possibility of an attack, but doesn't eliminate it as I found out.  The second way is to do away with the problem altogether by static linking.  I am a firm believer that elimination is far better than mitigation - especially considering it requires no actual code change and reduces the amount of installation set testing required.

Sunday, July 17, 2011

Application Manifests & Visual Basic 6

Embedding Application Manifests in Visual Basic 6 binaries is really easy.  Microsoft even wrote a command line utility just for this purpose.  Well, not for VB6 but for binaries in general.  The Manifest Tool (mt.exe), which is included in both Visual Studio and the Windows SDK is extremely simple to use.  The best part is that it handles any necessary padding and can update just about any binary with no fuss.

Here's an example command line using the naming convention of Visual Studio 2005, where the manifest filename contains the program name with ".intermediate.manifest" appended. 

mt.exe -nologo -manifest "program.exe.intermediate.manifest" -outputresource:"program.exe;#1

And now for a story...

When we were first confronted with the need for manifests - specifically for triggering UAC prompts in our configuration tools written in VB6 - I performed my due diligence.  I researched the topic thoroughly, I took examples of the manifests provided by Microsoft, and I tested on each and every Windows OS we supported - Windows 2000 all the way up to Windows Vista/2008.

The one thing I couldn't find was a simple way to embed the manifest in those executables, that could be easily automated.  The articles I read covered GUI tools like XN Resource Editor and Resource Hacker, writing my own C/++ program using UpdateResource, a long winding route using the Resource Compiler (rc.exe) or finally just leaving the manifest as a separate file.

Even though manifests had been around since Windows XP, there wasn't a single article I could find that even mentioned the Manifest Tool. Even in the Microsoft articles I have found, there is never any mention of VB6 and the Manifest Tool together.  Of course, VB6 was considered legacy by the time Application Manifests came out; so, while frustrating, I can't really blame them.  I can blame my search engine skills, but that's no fun.

Anyway, all but the last option were complicated, convoluted, or required too much effort.  We, of course, finally settled on that last option - using external manifests - out of necessity to get something out the door.  It wasn't until we started migrating code from Visual C++ 6.0 over to Visual Studio 2005 that I noticed the mt.exe command line in the build log, which was over a year and half later.  Now, along with code signing, through signtool.exe the Manifest Tool is included in much of our automated build process, and I am much happier for discovering it.

Saturday, July 09, 2011

Taxes Are Hard (Texas Tax Edition) - Part 2

Taxes are hard; and as Texas has proved so far, Texas taxes are extraordinarily difficult.  Even after all the specifics laid out in part 1 of this post, a great deal of information is left before I can even begin talking about what it actually means.

State Tax
Regardless of whether something is sold by a company in Texas or is sold to a customer in Texas, the state tax of 6.25% always applies.  This is perhaps the simplest feature of Texas taxes.  If it weren't for the $25 internet access exemption or the 20% web development/information service exemption, Texas state taxes would be easy. 

Local Sales Tax
Beyond the state tax, the next type of tax that must be calculated is the local sales tax.  This tax is based on the location of the seller's place of business.

Local Use Tax
Next, after both the state tax and the local sales tax are calculated comes the local use tax.  This tax is based on the location of the customer or where the customer receives the goods and services.

Further Complications of Local Taxes
Both the local sales and local use taxes are further broken down into four different locale types: city, county, special purpose districts and transit.  So, combined, this creates nine - count them nine - different types of taxes that go into the calculation.

Next, after all that breakdown, city tax rates are different for each city, county tax rates are different for each county and so on.  All combined, the local tax rate - for both local sales and local use taxes - cannot exceed 2%.  This limiting factor of 2% works on a priority basis, adding each subsequent type to the total until the 2% is reached.  If adding one of the local tax rates exceeds the 2% limit, only the amount necessary to reach the 2% limit is used.  The order of the local tax rates is as follows.
  1. local city sales tax
  2. local county sales tax
  3. local special purpose district sales tax
  4. local transit sales tax
  5. local city use tax
  6. local county use tax
  7. local special purpose district use tax
  8. local transit use tax
In addition, city taxes do not apply if outside of the city limits.  So, a company located outside of the city limits, the local city sales tax will not apply; and if a customer is located outside of the city limits, the local city use tax will not apply.

Also, while the terms "sales" and "use" imply a different set of rules or percentages, they actually don't. Local taxes rates are the same for both sales and use. Plus, when reporting local taxes, they are done based on the different locale types: city, county, special purpose district, and transit.  Beyond the initial calculation, the terms sales and use are not applied (at least, to my knowledge).

Finally, along those same lines, duplicates are ignored.  Local sales taxes for the seller are calculated; then, local use taxes for the customer are calculated - ignoring any the local use tax for duplicates.  For example, if both the company and the customer are located within the same county, the local county sales tax will apply but the local county use tax will not; or more apply put, the local city tax is only calculated once.

The information provided in this article is just a summary of the Texas local tax calculations.  The Window on State Government web site provides an article - and is the basis for this post - which covers may different scenarios with specific examples for each in the February 2009 Local Sales and Use Tax Bulletin - Guidelines for Collecting Local Sales and Use Tax.

Taxes Are Hard (Texas Tax Edition) - Part 1

Taxes are hard, and along the lines of "don't mess with Texas", internet taxes in Texas go above and beyond the norm.  The basics of Texas internet taxes are as follows.

Note: Because of the complications of Texas taxes, this article is broken down into several manageable posts. The first two, of which, specifically cover a summary of rules for Texas taxes.

Internet Access Service
Web Development and Information Services
Seems simple, doesn't it?  Now for the semantics.
Continued in Part 2.

    Friday, July 08, 2011

    Application Manifests, UAC & Windows Vista - Part 2

    Way back in the days of Windows 2000, new guidelines were introduced to improve the overall usability and security of the OS.  Although, in order to maintain compatibility, none of these guidelines were actually enforced at the time.  Then, when Windows Vista was released, the game changed.  Those guidelines were no longer optional, but there was a backup plan.

    New Guidelines for Windows 2000
    1. Only binaries or read only files should be stored in Program Files.
    2. Any documents or user created files should be stored in My Documents.
    3. Temporary files should be created in the user (or system) Temp folder.
    4. Anything else should be written to Application Data.
    Now, in a perfect world, everyone would have read those guidelines and started following them.  After all, there is no shortage of documentation on how to implement them, but things rarely go according to plan.  Even we were not immune to this thinking.  I guess it could be summed up as "Well, if Microsoft doesn't care if we do it, then why should we change?"  And, you can't exactly cry foul just because you didn't read the rules, can you?

    With the introduction of UAC, it was no longer possible to write to Program Files without administrative privileges, but somehow older programs still worked.  This is where the the requestedExecutionLevel flag in Application Manifests comes into play.  If the flag is missing from the manifest, the OS does its fancy footwork of guessing whether the program should be elevated.  If the program is not elevated, then the program is run in compatibility mode.  Any attempts to write to a system folder - such as Program Files or System32 - or write to a system registry hive - such as HKLM - will result in virtualization.

    While you may think your program is writing to HKLM, it's not.  Not really.  It may look like it to you.  Even to the program itself, it will appear that way, because it is actually reading from a virtualized section of the registry.  This means that in your own little world, everything is working as you expected, but it is not directly affecting the OS.  So, any changes you make to your little world don't affect anyone else who logs into the OS.  This is one of the key points that makes UAC really work.  Without this, Vista would truly be the nightmare that you hear in those Mac commercials.

    Still, even with virtualization in place, some wacky things can occur.  for example, there's a specific case I found with the SDelete utility provided by SysInternals, which runs in compatibility/virtualization mode.  If the program is run as unpriviledged and is passed a file located in Program Files, it simultaneously finds the file and cannot find the file.  It actually finds the file in Program Files, and then attempts to open a file of the same name in the Virtual Store.  So, after all that, it reports a success saying the file was correctly wiped and deleted, but the file in Program Files is never touched.

    User Account Control (UAC)

    Application Manifests, UAC & Windows Vista - Part 1

    Support for Manifest Resources continued with Windows Vista.  In addition to the new Shell Common Controls library and Side-by-side assemblies introduced in Windows XP, support for High-DPI Applications and User Account Control (UAC) was added to the Manifest Resource.  The most scrutinized feature of the two, of course, is User Account Control, which provided additional security to the operating system by specifying within the program what type of permission was required for using the program.

    With Windows XP and lower, an administrator was always logged in as an administrator and a restricted user was always logged in as a restricted user.  This meant that any program executed by an administrator would always be executed with full administrative rights.  While any program executed by a restricted user would never have administrative privileges.

    With the introduction of UAC, if a program required administrative privileges, such as writing to HKLM in the registry or writing to a system folder, the OS would notify the user of the required privileges before executing potentially damaging code - even when logged in as an administrative user.  The OS determined this through the requestedExecutionLevel flag in the application manifest.  If this flag was missing, the OS would do some fancy footwork and make a guess.

    The key point here is that by embedding the required privilege within the program, a user - even a user with administrative privileges - did not have to work in a completely unprotected environment.  This helps restrict access to much of the operating system when performing day-to-day tasks; and when malicious software is introduced on the machine, the restricted access given to that software should help mitigate any damage.

    Vista Styles
    User Account Control (UAC)

    Application Manifests & Windows XP

    Starting with Windows XP, Microsoft created extensive changes to the Windows Manager and Shell Common Controls. These changes included a whole new set of controls that could be used with programs to provide a fresh look and to provide additional Theme support. Older programs would continue to use the older Shell Common Controls library (comctl32.dll), while newer programs would use the new Shell Common Controls library (uxctrl.dll). By having two separate libraries instead of updating the existing Shell Common Controls library, compatibility issues with older programs were completely bypassed.[1].

    To determine whether programs would use the new library was accomplished through a new PE Resource type called a Manifest. This Manifest Resource - in its original form - contained a list of DLL's and other resources that would be used with the executable. This allowed easier control over dependencies by specifying which versions of a DLL would be used with the program - specifically which Shell Common Controls library. So, newer programs, which included a specific Manifest Resource, would use the uxctrl.dll; and older ones, which did not have a Manifest Resource, would continue to use comctl32.dll.

    This created the concept of Side-by-side Assemblies - a new feature in Windows XP - where multiple versions of a DLL could be installed on a machine, and depending on the Manifest Resource within the binary only a specific version of that DLL would be used by the program. For several reasons Microsoft has shifted away from supporting side-by-side assemblies - at least in C/C++ runtime libraries - in favor of including the version of the appended to the file name.[2]

    XP Styles
    Side-By-Side Assemblies