NT Help File Attach - 06 Dec 1999

Windows help system uses a HELPFILE.CNT file as table of contents
metafile for creating HELPFILE.GID which is needed to view table of contents for HELPFILE.HLP.

If you delete previously created HELPFILE.GID and edit HELPFILE.CNT, you can change a topic action to run an executable instead of viewing help for that topic. When victim user uses help system and chooses the infected topic, help system runs an executable from path.

BTW that :Title tag in .CNT files has a kind of buffer overflow. Buffer size
is ~256 bytes. I think it triggers when the created
.GID file is opened.

DEMONSTRATION

1) Delete C:\Program Files\Microsoft Office\Office\WDMAIN8.GID
(kill winhlp32.exe process if necessary)

2) Edit C:\Program Files\Microsoft Office\Office\WDMAIN8.CNT
which is a text file. You should change the line which has
something like:

3)  3 Word 97 new [email protected]>REF

to read:

3 Word 97 new features=!EF("CMD.EXE","",1)

4) Run WinWord and select Help|Contents from menubar.
5) Find topic "Word 97 new features" and select it.
6) You should see CMD.EXE to run.

DEFENSE

David LeBlanc commented on BugTraq:

"I don"t think you have to delete the .gid file for this to happen - it is
just an index for the find feature. I used to write help systems, and am very familiar with what can be done from a help system. .hlp and .cnt files can both be used in a number of ways to make system calls and to execute arbitrary binaries, as well as call into DLLs. I can also call one .hlp file from another, and IIRC, can call more than one .hlp file from a given .cnt file (which is a text file and easily edited).

If you have a multi-user system, you need to secure all .hlp and .cnt files the same as you would .exe files. If you"re worried about .gid files, open the associated .hlp file, choose "find", create the database, and then secure it."

VENDOR RESPONSE