• The Old New Thing

    Dubious security vulnerability: Luring somebody into your lair

    • 21 Comments

    A security report was received that went something like this:

    The XYZ application does not load its DLLs securely. Create a directory, say, C:\Vulnerable, and copy XYZ.EXE and a rogue copy of ABC.DLL in that directory. When C:\Vulnerable\XYZ.EXE is run, the XYZ program will load the rogue DLL instead of the official copy in the System32 directory. This is a security flaw in the XYZ program.

    Recall that the directory is the application bundle, The fact that the XYZ.EXE program loads ABC.DLL from the application directory rather than the System32 directory is not surprising because the ABC.DLL has been placed inside the XYZ.EXE program's trusted circle.

    But what is the security flaw, exactly?

    Let's identify the attacker, the victim, and the attack scenario.

    The attacker is the person who created the directory with the copy of XYZ.EXE and the rogue ABC.DLL.

    The victim is whatever poor sap runs the XYZ.EXE program from the custom directory instead of from its normal location.

    The attack scenario is

    • Attacker creates a directory, say, C:\Vulnerable.
    • copy C:\Windows\System32\XYZ.EXE C:\Vulnerable\XYZ.EXE
    • copy rogue.dll C:\Vulnerable\ABC.DLL
    • Convince a victim to run C:\Vulnerable\XYZ.EXE.

    When the victim runs C:\Vulnerable\XYZ.EXE, the rogue DLL gets loaded, and the victim is pwned.

    But the victim was already pwned even before getting to that point! Because the victim ran C:\Vulnerable\XYZ.EXE.

    A much simpler attack is to do this:

    • Attacker creates a directory, say, C:\Vulnerable.
    • copy pwned.exe C:\Vulnerable\XYZ.EXE
    • Convince a victim to run C:\Vulnerable\XYZ.EXE.

    The rogue ABC.DLL is immaterial. All it does is crank up the degree of difficulty without changing the fundamental issue: If you can trick a user into running a program you control, then the user is pwned.

    This is another case of if I can run an arbitrary program, then I can do arbitrary things, also known as MS07-052: Code execution results in code execution.

    Note that the real copy of XYZ.EXE in the System32 directory is unaffected. The attack doesn't affect users which run the real copy. And since C:\Vulnerable isn't on the default PATH, the only way to get somebody to run the rogue copy is to trick them into running the wrong copy.

    It's like saying that there's a security flaw in Anna Kournikova because people can create things that look like Anna Kournikova and trick victims into running it.

  • The Old New Thing

    The tadpole operators explained

    • 24 Comments

    Last time,¹ I introduced the tadpole operators. As you have probably figured out by now, it was a joke. There are no new tadpole operators.

    But the sample code works. What's going on?

    The tadpole operators are pseudo-operators, like the goes to operator or the sproing operator: They take advantage of existing language features, and come with a creative story.

    The tadpole operators exploit two's complement arithmetic and overflow.² The __ENABLE_EXPERIMENTAL_TADPOLE_OPERATORS is just a red herring.

    Start with the identity for two's complement negation

    -x = ~x + 1
    

    then move the -x to the right hand side and the ~x to the left hand side:

    -~x = x + 1
    

    If that was too fast for you, we can do it a different way: start with the identity for two's complement negation

    -x = ~x + 1
    

    subtract 1 from both sides

    -x - 1 = ~x
    

    and finally, negate both sides

    x + 1 = -~x
    

    To get the decrement tadpole operator, start with

    -x = ~x + 1
    

    and substitute x = -y:

    -(-y) = ~-y + 1
    

    subtract 1 from both sides and simplify -(-y) to y.

    y - 1 = ~-y
    

    Update: Justin Olbrantz (Quantam) and Ben Voigt provide a simpler derivation, starting with the identity for two's complement negation.

    -x = ~x + 1
    Rearrange terms ~x = -x - 1
    Let x = ~y Let x = -y
    -~y = ~(~y) + 1 ~-y = -(-y) - 1
    -~y = y + 1 ~-y = y - 1

    ¹Why didn't I post it on April 1st? Well, for one thing, April 1st is overcrowded. Second, it would have interfered with the run-up to the //build conference. And third, yesterday was a holiday in the United States, and I tend to schedule lighter fare on holidays.

    ²This means that they don't work on a machine that does not use two's complement, or one which checks overflow. Still, maybe they'll be useful if you're entering the IOCCC or some other contest which values minimal code size or obfuscation (or both).

  • The Old New Thing

    New C++ experimental feature: The tadpole operators

    • 115 Comments

    How often have you had to write code like this:

    x = (y + 1) % 10;
    x = (y + 1) * (z - 1);
    x = (wcslen(s) + 1) * sizeof(wchar_t);
    

    Since the + and - operators have such low precedence, you end up having to parenthesize them a lot, which can lead to heavily nested code that is hard to read.

    Visual Studio 2015 RC contains a pair of experimental operators, nicknamed tadpole operators. They let you add and subtract one from an integer value without needing parentheses.

    x = -~y % 10;
    x = -~y * ~-z;
    x = -~wcslen(s) * sizeof(wchar_t);
    

    They're called tadpole operators because they look like a tadpole swimming toward or away from the value. The tilde is the tadpole's head and the hyphen is the tail.

    Syntax Meaning Mnemonic
    -~y y + 1 Tadpole swimming toward a value makes it bigger
    ~-y y - 1 Tadpole swimming away from a value makes it smaller

    To enable the experimental tadpole operators, add this line to the top of your C++ file

    #define __ENABLE_EXPERIMENTAL_TADPOLE_OPERATORS
    

    For example, here's a simple program that illustrates the tadpole operators.

    #define __ENABLE_EXPERIMENTAL_TADPOLE_OPERATORS 
    #include <ios>
    #include <iostream>
    #include <istream>
     
    int __cdecl main(int, char**)
    {
       int n = 3;
       std::cout << "3 + 1 = " << -~n << std::endl;
       std::cout << "(3 - 1) * (3 + 1) " << ~-n * -~n << std::endl;
       return 0;
    }
    

    Remember that these operators are still experimental. They are not officially part of C++, but you can play with them and give your feedback here learn more about them here.

  • The Old New Thing

    So you decided to call SHFileOperation from a service, at least remember to disable copy hooks

    • 9 Comments

    I noted some time ago that it is highly inadvisable to call SHFile­Operation from a service, and then I thought about it some more and concluded, it's flat-out wrong, at least in the case where you call it while impersonating.

    Now, I'm sure that my opinion won't dissuade many of you, but if you decide to do it anyway, at least disable shell copy hooks by passing the FOFX_NO­COPY­HOOKS flag to IFile­Operation::Set­Operation­Flags. (We've met this flag before.)

    By default, shell copy hooks are active during shell file operations, and this creates a number of problems when called from a service.

    First of all, those copy hooks are unlikely to be designed to handle being run in a service. (When you write your own shell extension, do you make sure it also works when run in a service?) They're probably going to try to log the file activity, possibly to a back-end service, or maybe check with a back-end service whether this file copy should be allowed, and if not, they're probably going to display a dialog box saying "The file cannot be moved/copied/deleted because I'm a mean person who hates you due to restrictions imposed by your administrator." (Or worse, they may secretly copy the file to an undisclosed location before allowing the operation through.)

    Second of all, you probably don't want those copy hooks intefering with your file operation, whatever it is. Your service is trying to clean up files, or it's moving files around for its own internal purposes, and if a copy hook showed up and blocked the operation, your service is now in a weird inconsistent state.

    Note that I still consider SHFile­Operation inadvisable from a service. I'm just trying to stop you from digging your hole any deeper than it already is.

  • The Old New Thing

    If you can set enforcement for a rule, you can set up lack of enforcement

    • 9 Comments

    One of the things you can do with an internal tool I've been calling Program Q is run a program any time somebody wants to add or modify a record. The program has wide latitude in what it can do. It can inspect the record being added/modified, maybe record side information in another table, and one of the things it can decide to do is to reject the operation.

    We have set up a validator in our main table to ensure that the widget being added or modified is priced within the approver's limit. But sometimes, there is an urgent widget request and we want to be able to bypass the validation temporarily. Is there a way to disable the validator just for a specific record, or to disable it for all records temporarily?

    If you can set up a program to validate a record, you can also set up a program to not validate a record.

    Suppose your current validator for adding a widget goes like this:

    if (record.approver.limit < record.price) {
     record.Reject("Price exceeds approver's limit");
     return;
    }
    ... other tests go here ...
    

    And say you want to be able to allow emergency requests to go through even though, say, all approvers are unavailable. Because, maybe, the widget is on fire.

    You could decide that a widget whose description begins with the word EMERGENCY is exempt from all validation, but it generates email to a special mailing list.

    if (record.description.beginsWith("EMERGENCY"))
    {
     // emergency override: send email
     // and bypass the rest of validation
     generateNotificationEmail(record);
     return;
    }
    if (record.approver.limit < record.price) {
     record.Reject("Price exceeds approver's limit");
     return;
    }
    ... other tests go here ...
    

    Of course, the EMERGENCY rule was completely arbitrary. You can come up with whatever rules you like. The point is: If you wrote the rules, you can also write the rules so that they have exceptions.

  • The Old New Thing

    When you inadvertently become a collector of something you really aren't all that into

    • 25 Comments

    As I was heading home at the end of the day, I ran into one of my colleagues who was also going home, and he was carrying a Star Wars-themed metal lunchbox similar to this one. For those who didn't grow up in the United States, these metal lunchboxes are the type of things elementary school children use to carry their lunch to school.

    I remarked, "Nice lunchbox."

    My colleague explained, "Yeah, I sort of ended up as the lunchbox guy. It started when somebody gave me a lunchbox as a semi-humorous gift, and I kept it on my shelf. Then other people saw that I had a metal lunchbox and concluded, 'Oh, he must collect metal lunchboxes,' and they started giving me metal lunchboxes. And before I knew it, I became an unwitting collector of metal lunchboxes."

    The same thing happened to a different colleague of mine. As his first birthday after he got married approached, his new in-laws asked his wife, "What does Bob like?"

    His wife shrugged. "I dunno. He kind of likes Coca-Cola?"

    That year, he got a vintage Coca-Cola serving tray. The next year, he got a Coca-Cola clock. And then Coca-Cola drinking glasses. And so on.

    Eventually, he had to ask his wife to tell her family, "Okay, you can stop now. Bob doesn't like Coca-Cola that much."

  • The Old New Thing

    It rather involved being on the other side of this airtight hatchway: Code injection via QueueUserAPC

    • 11 Comments

    A security vulnerability report arrived that took the following form:

    The Queue­User­APC function can be used to effect an elevation of privilege, as follows:

    1. Identify a process you wish to attack.
    2. Obtain access to a thread with THREAD_SET_­CONTEXT access.
    3. Make some educated guesses as to what DLLs are loaded in that process. Start with kernel32.dll, since you're going to need it in step 5.
    4. From the attacking process, scan the memory of those DLLs looking for a backslash, followed by something that can pass for a path and file name. Such strings are relatively abundant because there are a lot of registry paths hard-coded into those binaries. Suppose you found the string \Windows NT\Current­Version\App­Compat­Flags. Even though ASLR randomizes DLL placement, the placement is consistent among all processes, so an address calculated in one process is highliy likely to be valid in all processes.
    5. Create a DLL called C:\Windows NT\Current­Version\App­Compat­Flags.dll. Put your payload in this DLL.
    6. From the attacking thread, call Queue­User­APC with the address of Load­LibraryW as the function pointer, the victim thread as the thread handle, and a pointer to the fixed string identified in part 4 as the last parameter.
    7. The next time the victim thread processes APCs, it will pass \Windows NT\Current­Version\App­Compat­Flags to the Load­LibraryW function, which will load the payload DLL, thereby effecting code injection and consequent elevation of privilege.

    Note that this attack fails if the victim thread never waits alertably, which is true of most threads.

    If you have been paying attention, the alarm bells probably went off at step 2. If you have THREAD_SET_­CONTEXT access to a thread, then you pwn that thread. There's no need to use Queue­User­APC to make the thread do your bidding. You already have enough to make the thread dance to your music. In other words, you are already on the other side of the airtight hatchway.

    Here's how: Look for a code sequence that goes

        push someregister
        call LoadLibraryW
    

    Use the ­Set­Thread­Context function to set the pushed register equal to the address of the string you found in step 4, and set the instruction pointer to the code fragment. The thread will then resume execution at the specified instruction pointer: It pushes the address of the string, and then it calls Load­LibraryW. Bingo, your DLL loads, and you didn't even have to wait for the thread to wait alertably.

    On non-x86 platforms, this is even easier: Since all other platforms use register-based calling conventions, you merely have to load the address of the string into the "first parameter" register (rcx for x64) and set the instruction pointer to the beginning of Load­LibaryW.

    By default, THREAD_SET_­CONTEXT access is granted only to the user, and never to lower integrity levels. In other words, a low IL process cannot get THREAD_SET_­CONTEXT access to a medium or high integrity thread, and a medium IL process cannot get access to a high integrity thread. This means that, by default, you can only get THREAD_SET_­CONTEXT access to threads that have equivalent permissions to what you already have, so there is no elevation.

  • The Old New Thing

    Determining programmatically whether a file was built with LAA, ASLR, DEP, or OS-assisted /GS

    • 25 Comments

    Today's Little Program parses a module to determine whether or not it was built with the following flags:

    Remember, Little Programs do little error checking. In particular, this Little Program does no range checking, so a malformed binary can result in wild pointers.

    #include <windows.h>
    #include <imagehlp.h>
    #include <stdio.h> // horrors! mixing stdio and C++!
    #include <stddef.h>
    
    class MappedImage
    {
    public:
     bool MapImage(const char* fileName);
     void ProcessResults();
     ~MappedImage();
    
    private:
     WORD GetCharacteristics();
    
     template<typename T>
     WORD GetDllCharacteristics();
    
     template<typename T>
     bool HasSecurityCookie();
    
    private:
     HANDLE file_ = INVALID_HANDLE_VALUE;
     HANDLE mapping_ = nullptr;
     void *imageBase_ = nullptr;
     IMAGE_NT_HEADERS* headers_ = nullptr;
     int bitness_ = 0;
    };
    
    bool MappedImage::MapImage(const char* fileName)
    {
     file_ = CreateFile(fileName, GENERIC_READ,
        FILE_SHARE_READ,
        NULL,
        OPEN_EXISTING,
        0,
        NULL);
     if (file_ == INVALID_HANDLE_VALUE) return false;
    
     mapping_ = CreateFileMapping(file_, NULL, PAGE_READONLY,
                                  0, 0, NULL);
     if (!mapping_) return false;
    
     imageBase_ = MapViewOfFile(mapping_, FILE_MAP_READ, 0, 0, 0);
     if (!imageBase_) return false;
    
     headers_ = ImageNtHeader(imageBase_);
     if (!headers_) return false;
     if (headers_->Signature != IMAGE_NT_SIGNATURE) return false;
    
     switch (headers_->OptionalHeader.Magic) {
     case IMAGE_NT_OPTIONAL_HDR32_MAGIC: bitness_ = 32; break;
     case IMAGE_NT_OPTIONAL_HDR64_MAGIC: bitness_ = 64; break;
     default: return false;
     }
    
     return true;
    }
    
    MappedImage::~MappedImage()
    {
     if (imageBase_) UnmapViewOfFile(imageBase_);
     if (mapping_) CloseHandle(mapping_);
     if (file_ != INVALID_HANDLE_VALUE) CloseHandle(file_);
    }
    
    WORD MappedImage::GetCharacteristics()
    {
     return headers_->FileHeader.Characteristics;
    }
    
    template<typename T>
    WORD MappedImage::GetDllCharacteristics()
    {
      return reinterpret_cast<T*>(headers_)->
        OptionalHeader.DllCharacteristics;
    }
    
    template<typename T>
    bool MappedImage::HasSecurityCookie()
    {
     ULONG size;
     T *data = static_cast<T*>(ImageDirectoryEntryToDataEx(
        imageBase_, TRUE, IMAGE_DIRECTORY_ENTRY_LOAD_CONFIG,
        &size, NULL));
     if (!data) return false;
     ULONG minSize = offsetof(T, SecurityCookie) +
                     sizeof(data->SecurityCookie);
     if (size < minSize) return false;
     if (data->Size < minSize) return false;
     return data->SecurityCookie != 0;
    }
    
    void MappedImage::ProcessResults()
    {
     printf("%d-bit binary\n", bitness_);
     auto Characteristics = GetCharacteristics();
     printf("Large address aware: %s\n",
        (Characteristics & IMAGE_FILE_LARGE_ADDRESS_AWARE)
        ? "Yes" : "No");
    
     auto DllCharacteristics = bitness_ == 32
        ? GetDllCharacteristics<IMAGE_NT_HEADERS32>()
        : GetDllCharacteristics<IMAGE_NT_HEADERS64>();
    
     printf("ASLR: %s\n",
        (DllCharacteristics & IMAGE_DLLCHARACTERISTICS_DYNAMIC_BASE)
        ? "Yes" : "No");
     printf("ASLR^2: %s\n",
        (DllCharacteristics & IMAGE_DLLCHARACTERISTICS_HIGH_ENTROPY_VA)
        ? "Yes" : "No");
     printf("DEP: %s\n",
        (DllCharacteristics & IMAGE_DLLCHARACTERISTICS_NX_COMPAT)
        ? "Yes" : "No");
     printf("TS Aware: %s\n",
        (DllCharacteristics & IMAGE_DLLCHARACTERISTICS_TERMINAL_SERVER_AWARE)
        ? "Yes" : "No");
    
     bool hasSecurityCookie =
        bitness_ == 32 ? HasSecurityCookie<IMAGE_LOAD_CONFIG_DIRECTORY32>()
                       : HasSecurityCookie<IMAGE_LOAD_CONFIG_DIRECTORY64>();
     printf("/GS: %s\n", hasSecurityCookie
        ? "Yes" : "No");
    }
    
    int __cdecl main(int argc, char**argv)
    {
     MappedImage mappedImage;
     if (mappedImage.MapImage(argv[1])) {
      mappedImage.ProcessResults();
     }
     return 0;
    }
    

    Let's see what happened.

    First we use the Map­Image method to load the binary and map it into memory. While we're at it, we sniff at the headers to determine whether it is a 32-bit or 64-bit binary.

    The Get­Characteristics method merely extracts the Characteristics from the File­Header. This is easy because the File­Header is the same for 32-bit and 64-bit binaries.

    The Get­Dll­Characteristics method has two versions depending on the image bitness. In both cases, it extracts the Dll­Characteristics field, but the location of the field depends on the structure.

    The Has­Security­Cookie method also has two versions depending on the image bitness. The minimum size necessary to get OS-assisted stack overflow protection is the size that encompasses the Security­Cookie member, and in order to get that extra protection, the member needs to be nonzero.

    What is OS-assisted stack overflow protection?

    First, I'm going to assume that you've read Compiler Security Checks In Depth.

    Okay, welcome back.

    In theory, /GS could be implemented entirely in application code, with no need for operating system assistance. And in fact, that's what happens when the executable is run on older versions of Windows (like Windows 98 or Windows 2000). But the module can tell the operating system, "Hey, here is where I put my security cookie," and if the operating system understands this field, then it will go in and make the security cookie even more randomer than random by mixing in some cryptographically secure random bits.

    Okay, so that's the program. Note that some of these flags are meaningless in DLLs, so be careful to interpret the output correctly.

  • The Old New Thing

    MapGenericMask is just a convenience function for converting generic access to specific access, according to the instructions you provide

    • 5 Comments

    For some reason, people call the Map­Generic­Mask function in order to calculate what access mask to pass to request access to something. That's not what Map­Generic­Mask is for. The Map­Generic­Mask function is to assist the server side of the access, not the client side.

    As the documentation says, the Map­Generic­Mask function maps the generic access rights in an access mask to specific and standard access rights. The function applies a mapping supplied in a GENERIC_MAPPING structure.

    This is further explained in the remarks:

    After calling the Map­Generic­Mask function, the access mask pointed to by the Access­Mask parameter has none of its generic bits (GenericRead, GenericWrite, GenericExecute, or GenericAll) or undefined bits¹ set, although it can have other bits set. If bits other than the generic bits are provided on input, this function does not clear them.

    What this function does is take the Access­Mask parameter and convert all of the GENERIC_* bits into object-specific bits, as defined by the GENERIC_MAPPING.

    In other words, the code for the Map­Generic­Mask function looks basically like this:

    void MapGenericMask(
      PDWORD AccessMask,
      PGENERIC_MAPPING GenericMapping
    )
    {
     if (*AccessMask & GENERIC_READ)
         *AccessMask |= GenericMapping->GenericRead;
    
     if (*AccessMask & GENERIC_WRITE)
         *AccessMask |= GenericMapping->GenericWrite;
    
     if (*AccessMask & GENERIC_EXECUTE)
         *AccessMask |= GenericMapping->GenericExecute;
    
     if (*AccessMask & GENERIC_ALL)
         *AccessMask |= GenericMapping->GenericAll;
    
     *AccessMask &= ~(GENERIC_READ | GENERIC_WRITE |
                      GENERIC_WRITE | GENERIC_ALL);
    }
    

    The function takes the access mask and sees if any of the generic access bits are set. If so, then it replaces them with the corresponding specific access bits provided by the Generic­Mapping parameter.

    Note that this function doesn't tell you anything you don't already know. It's just a helper function to make access checks easier to implement: If you are a component that manages an object and you need to perform an access check, you use Map­Generic­Access to convert all the generic access requests into specific requests according to the rules you specified in your GENERIC_MAPPING, and the rest of your code only needs to deal with specific requests.

    For example, we saw some time ago that a hypothetical Gizmo object could map GENERIC_READ to operations that query information from a Gizmo without modifying it, map GENERIC_WRITE to operations that alter the gizmo properties, map GENERIC_EXECUTE to operations that enable or disable the Gizmo, and map GENERIC_ALL to include all Gizmo operations.

    And if you needed to do a security check on a Gizmo, you would do something like this:

    BOOL IsGizmoAccessGranted(
     HTOKEN Token,
     PSECURITY_DESCRIPTOR SecurityDescriptor,
     DWORD AccessDesired,
     PDWORD AccessAllowed)
    {
     MapGenericMask(&AccessDesired, &GizmoGenericMapping);
    
     BOOL AccessGranted = FALSE;
     PRIVILEGE_SET PrivilegeSet;
     DWORD PrivilegeSetSize = sizeof(PrivilegeSet);
    
     return AccessCheck(SecurityDescriptor,
                Token,
                AccessDesired,
                &GizmoGenericMapping,
                &PrivilegeSet,
                &PrivilegeSetSize,
                AccessAllowed,
                &AccessGranted) && AccessGranted;
    }
    

    When somebody wants to access a Gizmo, you use Map­Generic­Mask to convert all the generic requests to specific requests. You then pass that request to Access­Check, along with token for the user making the request and the security descriptor for the widget. The Access­Check function does the heavy lifting of seeing which ACEs apply to the user specified by the token, then verifying that all the requested accesses have an Allow ACE, and that none of the requested accesses have a Deny ACE. It also takes care of the MAXIMMUM_ALLOWED access request by simply returning all the accesses that are allowed and not denied.

    The point of the Map­Generic­Mask function is to make life a little easier for the code that enforces security.

    On the other hand, the Map­Generic­Mask function is not particularly useful on the side that is requesting access. If you are requesting generic read access, just pass GENERIC_READ. The code that does the security check will convert the GENERIC_READ into the access masks that are specific to the object you are trying to access. (And it will most likely use the Map­Generic­Mask function to do it.)

    ¹ That extra phrase "or undefined bits" is contradicted by the subsequent sentence "If bits other than the generic bits are provided on input, the function does not clear them." The second sentence is correct; the extra phrase "or undefined bits" is incorrect and should be removed.

  • The Old New Thing

    Low-level hooks have thread affinity, so make sure you keep an eye on the thread

    • 4 Comments

    A customer was having a problem with their automated testing tool.

    We have an automation testing tool that, among other things, installs a low-level mouse hook. Sometimes, the hook takes too long to process an action, and it gets unhooked. We have a watchdog thread that tries to detect when this has happened, and in response, it kicks off a task on the thread pool to re-register the low-level hook. The call to register the low-level hook succeeds, but the hook apparently didn't get installed correctly because it never fires. What are we doing wrong?

    Recall that low-level hooks have thread affinity. This is spelled out in the documentation.

    This hook is called in the context of the thread that installed it. The call is made by sending a message to the thread that installed the hook. Therefore, the thread that installed the hook must have a message loop.

    So there are two mistakes here.

    First, the hook is installed from a thread pool task, which means that the hook is associated with the thread pool thread. One of the characteristics of the thread pool is that threads come and go based on demand. If there is no thread pool activity for a while, the thread pool will probably start trimming threads, and it it decides to get rid of the thread that installed the hook, and the hook disappears with it.

    The second mistake is that the hook is installed from a thread pool task. Sure, the hook registers successfully, but then when you return back to the thread pool, there's no guarantee that anybody on that thread is going to pump messages any time soon.

    Indeed, odds are that it won't.

    Tasks queued up on the thread pool tend not to be UI tasks, because, well, they're on the thread pool, not the UI thread. Therefore, there is no expectation that they will pump messages. Furthermore, if the thread goes idle, the thread pool is probably not going to pump messages; it's just going to put the thread to sleep until the next task is queued up.

    The customer thanked us for the explanation. I'm not sure what they are going to do about it, but I hope they're going to solve their problem not by patching up their watchdog thread but rather by fixing their low-level mouse hook so it doesn't exceeed the low-level hook timeout. For example, they could have the low-level hook post its events to another thread, then return immediately. That other thread can then do the expensive processing asynchronously. (This assumes that they are using the low-level hook only for monitoring the mouse rather than trying to intercept and block it.)

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