What can anyone tell me about a file named "agreement.execds.pdf"?
My email filter will not let this file pass because it blocks incoming EXE files. Is this an executable file? The file is actually a signed PDF document and a Norton scan says the file is safe.
Would ".execds.pdf" be considered a valid Adobe extension for a signed PDF document? I have looked at the document and it appears that it is just a signed contract sent to my client that they were expecting.
I've seen this before, and it is a result of an unintelligent filter. It is picking up on the ".exe" in the middle of the file name.
agreement .exe cds.pdf
If you rename the file it should pass through easily.
It might seem silly that these filters will happily allow a real .exe to come through as a .jpg or whatever else, however they do still provide a level of protection. Your regular everyday user will still be largely protected because their attempts to open an exe with .jpg extension won't execute the exe, and will just give them an error.
The reason it plays safe and sill picks up the .exe in the middle of the filename is because users are dumb. They might realize that in order to view cool_cat_picture.exe.jpg they just have to rename it themselves. Without any .exe at all there is no prompt for the user and significantly lower chance of it being run.
Can a file named “agreement.execds.pdf” really contain an executable file?
In general yes.
An executable file can be given any name, including any file name extension (the file type at the end of the name).
In this case, probably no. If a file name has the word "exe" somewhere within, has not bearing on what file type it is, and it does not change how the operating system would handle that file. If file identification tools says this is a pdf, it probably is a pdf.
(That said, pdf files can theoretically be crafted to exploiting bugs in pdf readers. If a pdf document is safe or not with your pdf reader, will mostly not be indicated by any word "safe", "unsafe" or "exe" in the middle of the file name.)
Like previous answers actually have already answered: A file is just bits. Down to the bits there's no real difference between an image and an executable. In fact you can create polyglot files that are for example both valid GIFs and valid executables. The file extension is only a hint for the user to tell the user how he should interpret the bits in the file. On windows *.exe means that this file should be treated as an executable but it might as well just contain random garbage.
On most systems you have a file browser (explorer) that has a list of file types associated with a specific application so when you double click on a file it will open it with the preferred application for the given file type. That's all there is to file types. Other than that they have absolutely no special meaning. However, because it is easy to trick people into opening attachments. Opening a *.exe is really dangerous so to prevent people from accidentally opening them e-mail filters will just throw files away that have a certain file extension. Some e-mail filters also try to guess the file type based on file headers (magic bytes) etc. and throw potentially dangerous attachments away.
Your e-mail filter might just throw everything away that contains .exe somewhere or it might have detected malware in the PDF - we can only guess. (That's right: every file can contain malware).
