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many programs by placing this code in a dll all the applications on the system can use it without using more memory this contrasts with static libraries which are functionally similar but copy the code directly into the application in this case every application grows by the size of all the libraries it uses and this can be quite large for modern programs the problem arises when the version of the dll on the computer is different from the version that was used when the program was being created dlls have no built in mechanism for backward compatibility and even minor changes to the dll can render its internal structure so different from previous versions that attempting to use them will generally cause the application to crash static libraries avoid this problem because the version that was used to build the application is included inside it so even if a newer version exists elsewhere on the system this does not affect the application a key reason for the version incompatibility is the structure of the dll file the file contains a directory of the individual methods procedures routines etc contained within the dll and the types of data they take and return even minor changes to the dll code can cause this directory to be re arranged in which case an application that calls a particular method believing it to be the 4th item in the directory might end up calling an entirely different and incompatible routine which would normally cause the application to crash there are several problems commonly encountered with dlls especially after numerous applications have been installed and uninstalled on a system the difficulties include conflicts between dll versions difficulty in obtaining required dlls and having many unnecessary dll copies solutions to these problems were known even while microsoft was writing the dll system citation needed these have been incorporated into the net replacement assemblies incompatible versions edit a particular version of a library can be compatible with some programs that use it and incompatible with others windows has been particularly vulnerable to this because of its emphasis on dynamic linking of c libraries and object linking and embedding ole objects c classes export many methods and a single change to the class such as a new virtual method can make it incompatible with programs that were built against an earlier version object linking and embedding has very strict rules to prevent this interfaces are required to be stable and memory managers are not shared this is insufficient however because the semantics of a class can change a bug fix for one application may result in the removal of a feature from another before windows 2000 windows was vulnerable to this because the com class table was shared across all users and processes only one com object in one dll exe could be declared as having a specific global com class id on a system if any program needed to create an instance of that class it got whatever was the current centrally registered implementation as a result an installation of a program that installed a new version of a common object might inadvertently break other programs that were previously installed dll stomping edit a common and troublesome problem occurs when a newly installed program overwrites a working system dll with an earlier incompatible version early examples of this were the ctl3d dll and ctl3dv2 dll libraries for windows 3 1 microsoft created libraries that third party publishers would distribute with their software but each distributing the version they developed with rather than the most recent version 2 dll stomping occurs because microsoft in the past distributed runtime dlls as shared system components 3 originally c windows and c windows system as a way of efficiently sharing code in a shared memory os with limited ram and disk space consequently third party developers also distributed these in such a manner application installers are typically executed in a privileged security context that has access to install dlls into the system directories and to edit the system registry to register new dlls as com objects a poorly written or misconfigured installer can therefore downgrade a system library on legacy versions of windows on which windows file protection or windows resource protection does not roll back the change on windows vista and later only the trusted installer account can make changes to core operating system libraries windows applications were permitted to include os updates in their own installation programs that is many microsoft dlls are redistributable meaning that the applications can include them if they need the services of the particular libraries before windows installer windows installers historically were commercial products many people attempted to write their own installers overlooking or mishandling versioning problems in the process 4 some development environments did not automatically add a version resource in their compiled libraries so many developers overlooked this aspect checking file dates overwriting existing files or skipping the copy operation if the dll was already installed were the only options available instead of correct versioning citation needed sometimes the os itself removed or replaced dlls with older or obsolete versions for example windows 2000 would install black and white printer dlls on top of color aware dlls if a black and white printer was installed after the color printer 5 incorrect com registration edit in com and other parts of windows prior to the introduction of side by side registry free assemblies 6 the registry was used for determining which underlying dll to use if a different version of a module was registered this dll would be loaded instead of the expected one this scenario could be caused by conflicting installations that register different versions of the same libraries in which case the last installation would prevail shared in memory modules edit 16 bit versions of windows and windows on windows load only one instance of any given dll all applications reference the same in memory copy until no applications are using it and it is unloaded from memory for 32 bit and 64 bit versions of windows inter process sharing occurs only where different executables load a module from exactly the same directory the code but not the stack is shared between processes through a process called memory mapping thus even when the desired dll is located in a directory where it can be expected to be found such as in the system directory or the application directory neither of these instances will be used if another application has started with an incompatible version from a third directory this issue can manifest itself as a 16 bit application error that occurs only when applications are started in a specific order lack of serviceability edit in direct conflict with the dll stomping problem if updates to a dll do not affect all applications that use it then it becomes much harder to service the dll that is to eliminate problems that exist in the current versions of the dll security fixes are a particularly compelling and painful case instead of fixing just the latest version of the dll the implementor must ideally make their fixes and test them for compatibility on every released version of the dll causes edit dll incompatibility has been caused by memory constraints combined with lack of separation of process memory space in 16 bit versions of windows lack of enforced standard versioning naming and file system location schemata for dlls lack of an enforced standard method for software installation and removing package management lack of centralized authoritative support for dll application binary interface management and safeguards allowing incompatible dlls with the same file name and internal version numbers to be released oversimplified management tools preventing the identification of changed or problematic dlls by users and administrators developers breaking backward compatibility of functions in shared modules microsoft releasing out of band updates to operating system runtime components inability of earlier versions of windows to run side by side conflicting versions of the same library reliance on the current directory or path environment variable both of which vary over time and from system to system to find dependent dlls instead of loading them from an explicitly configured directory developers re using the classids from sample applications for the com interfaces of their applications rather than generating their own new guids dll hell was a very common phenomenon on pre windows nt versions of microsoft operating systems the primary cause being that the 16 bit operating systems did not restrict processes to their own memory space thereby not allowing them to load their own version of a shared module that they were compatible with application installers were expected to be good citizens and verify dll version information before overwriting the existing system dlls standard tools to simplify application deployment which always involves shipping the dependent operating system dlls were provided by microsoft and other 3rd party tools vendors microsoft even required application vendors to use a standard installer and have their installation program certified to work correctly before being granted use of the microsoft logo the good citizen installer approach did not mitigate the problem as the rise in popularity of the internet provided more opportunities to obtain non conforming applications use by malware edit windows searches several locations for ambiguous dlls i e ones not fully qualified malware can exploit this behavior in several ways collectively known as dll search order hijacking one method is dll preloading or a binary planting attack it places dll files with the same name in a location that is searched earlier such as the current working directory when the vulnerable program tries to load the dll the malicious version is executed possibly at high privilege levels if the program runs at that level 7 another method is relative path dll hijacking which moves the vulnerable program to a location together with the malicious dll the dll is loaded because the application s directory is searched early according to crowdstrike this method is the most common 8 dll sideloading delivers both the legitimate program and malicious library it may avoid detection because the execution seems as running a reputable program 9 other methods include phantom dll hijacking where a malicious dll file is created against references to a non existent library and changing registry values to abuse dll redirection which changes the dll search order 7 dll hijacking was used by state sponsored groups including lazarus group and tropic trooper 9 solutions edit various forms of dll hell have been solved or mitigated over the years static linking edit a simple solution to dll hell in an application is to statically link all the libraries i e to include the library version required in the program instead of picking up a system library with a specified name 10 this is common in c c applications where instead of having to worry about which version of mfc42 dll is installed the application is compiled to be statically linked against the same libraries this eliminates the dlls entirely and is possible in standalone applications using only libraries that offer a static option as microsoft foundation class library does however the main purpose of dlls runtime library sharing between programs to reduce memory overhead is sacrificed duplicating library code in several programs creates software bloat and complicates the deployment of security fixes or newer versions of dependent software windows file protection edit the dll overwriting problem referred to as dll stomping by microsoft was somewhat reduced with windows file protection wfp 11 which was introduced in windows 2000 12 this prevents unauthorized applications from overwriting system dlls unless they use the specific windows apis that permit this there may still be a risk that updates from microsoft are incompatible with existing applications but this risk is typically reduced in current versions of windows through the use of side by side assemblies third party applications cannot stomp on os files unless they bundle legitimate windows updates with their installer or if they disable the windows file protection service during installation and on windows vista or later also take ownership of system files and grant themselves access the sfc utility could revert these changes at any time running conflicting dlls simultaneously edit the solutions here consist of having different copies of the same dlls for each application both on disk and in memory an easy manual solution to conflicts was placing the different versions of the problem dll into the applications folders rather than a common system wide folder this works in general as long as the application is 32 bit or 64 bit and that the dll does not use shared memory in the case of 16 bit applications the two applications cannot be executed simultaneously on a 16 bit platform or in the same 16 bit virtual machine under a 32 bit operating system ole prevented this before windows 98 se 2000 because earlier versions of windows had a single registry of com objects for all applications windows 98 se 2000 introduced a solution called side by side assembly 13 which loads separate copies of dlls for each application that requires them and thus allows applications that require conflicting dlls to run simultaneously this approach eliminates conflicts by allowing applications to load unique versions of a module into their address space while preserving the primary benefit of sharing dlls between applications i e reducing memory use by using memory mapping techniques to share common code between different processes that do still use the same module yet dlls using shared data between multiple processes cannot take this approach 14 one negative side effect is that orphaned instances of dlls may not be updated during automated processes portable applications edit depending on the application architecture and runtime environment portable applications may be an effective way to reduce some dll problems since every program bundles its own private copies of any dlls it requires 12 the mechanism relies on applications not fully qualifying the paths to dependent dlls when loading them and the operating system searching the executable directory before any shared location 15 however this technique can also be exploited by malware 16 and the increased flexibility may also come at the expense of security if the private dlls are not kept up to date with security patches in the same way that the shared ones are application virtualization can also allow applications to run in a bubble which avoids installing dll files directly into the operating system ot...