DLL injection may sound like a new medical procedure, but it's actually a way of attaching a dynamic link library, or DLL, to certain system actions in Windows. Many programs employ DLL injection to trap program functions
DLL injection has both legitimate and illegitimate uses. A macro recorder is one example of a legitimate DLL injection. But a key logger that's been installed without your (or your supervisor's) knowledge would be a problem. Because DLL injection isn't obvious—it doesn't usually show up as a process in the Task Manager—it can be hard to tell if injection is taking place, or how legitimate it might be.
Programmer Nir Sofer has written a tool, InjectedDLL, that takes a good deal of the mystery out of injected DLLs. Run it and you're presented with a report that lists all of the injected DLLs currently at work in the system: the image name, pathname and file attributes; a description (if available); any security-signing information; code revision; and so on.
The report can be saved as HTML or copied piecemeal or en masse to the clipboard. The program itself requires no installation -- it can be unpacked and run in any directory -- and there are translations available for several languages.
One quick way to tell if a given injected DLL is malicious or at least unwanted is to look at the company/product name signing. If there isn't one, or if it's a misspelling or variant on something else (such as "Micorsoft") there's a chance the DLL in question isn't legit.
Another interesting aspect of the program is that when it's first loaded, it forces a small window to open and then close directly under the cursor, which is attached to a process called dummywin.exe. This is deliberate. Some DLLs are only injected when the mouse cursor moves over a window, so this is a way to force that action to take place so the DLL in question will show up in the report. Note: On some particularly fast machines, the window may open and close too quickly for you to see.
About the author: Serdar Yegulalp is editor of the Windows Power Users Newsletter. Check it out for the latest advice and musings on the world of Windows network administrators.
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This was first published in July 2006