Dear Paweł Goleń,
--Tuesday, March 13, 2007, 11:34:53 PM, you wrote to bugtraq@xxxxxxxxxxxxxxxxx:
>>
>> Process 1: Opens file for reading with
>> FILE_SHARE_WRITE|FILE_SHARE_READ|FILE_SHARE_DELETE
>> Process 2: Opens file for writing with FILE_SHARE_NONE and _succeeds_.
>>
>> With valid mandatory locking implementation process 2 _must fail_.
PG> 3APA3A, from one hand you are right this may be considered to be
PG> vulnerability in Windows mandatory file locks. But I'm not sure if file
PG> locks in Windows are mandatory. I've never considered "share modes" to
PG> be security feature.
It was advised in original article.
PG> In order to be sure I'm creating not opening file I would probably
PG> used CREATE_NEW as value for dwCreationDisposition attribute AND
PG> FILE_SHARE_NONE to prevent others processess to open my file.
...
PG> Am I correct or I'm missing something?
You are right, and again it was advised in article.
You've only missed the fact, sometimes you need to create a file with
given name. The examples were demonstrated - file copy operation,
archive extraction, restore from backup, file replications and creation
of any persistent file (e.g. new document). And you get a problem - what
to do with existing file, you can not simply create new one.
At my opinion, if CREATE_NEW fails because file exists and user asks to
overwrite file, application should try to remove existing file and
re-create it with CREATE_NEW and fail, if second attempt with CREATE_NEW
fails. But: ALL TESTED APPLICATIONS FAILED to act like this. It's true
even for application you may expect to operate in secure way, because
they restore original file permissions and may be used to copy secret
information.
Namely:
xcopy (standard utility) with /Y opens existing file without attempt
to delete it.
robocopy (from resource kit) opens existing file
ntbackup (if "replace file" option is on during restore) opens existing
file
rar opens existing file
PG> And one question - which flag for dwCreationDisposition is used for
PG> example by Microsoft World during creating temporary files.
According to tests I recently made, Word correctly behaves with both
original file (it doesn't edit original file, but renames it, creates
new one and copies content) and temporary file (also new file is
created). It may be slow, but it's safe :) It may be possible to catch
race condition between old file is renamed and new one is created, but
it's a bit harder to test.
--
~/ZARAZA http://securityvulns.com/
