Re: Windows MS-DOS Device Name DoS vulnerabilities

[Alun Jones <alun () texis com>]

At 04:46 AM 7/6/2001, 3APA3A wrote:
> I completely disagree with your paper. It puts software developers and
> users into false sense of security. Right now SECURITY.NNOV is working
> out  few  MS-DOS  Device Name issues with vendors (not only in Windows
> 95/98/ME  but  also  in  NT/2000),  and  the  problem is definitely in
> software,  not  in  operation system, because operation system behaves
> exactly  as  expected  and  documented.  Later  we  will  publish  our
> advisory.  Software  MUST check type of file it tries to access BEFORE
> it  access  it,  if  this  can cause access to special device. Special
> devices  under  Windows  allow raw access to ports, drives, tapes, etc
> and  impact  of  such access can be same with impact of accessing /dev
> under unix.

The hole that ByteRage discussed was most _definitely_ a flaw in system 
security, wherein merely trying to open a file with a path name that 
contained two DDNs would cause a BSoD.  Your suggested workaround of 
GetFileType() requires a handle to an open file, which you cannot get 
without (tada!) opening the file, which itself would have caused the crash.

> MS  patched one hole, which causes Windows 95/98/ME to crash then some
> API call refer to any special device. This patch doesn't solve problem
> of  special devices, because _successful_ access to such devices under
> Windows can lead to much greater impact.

This is a second issue with device names, and is not touched on by 
ByteRage's initial posting in this thread.  You aren't disagreeing with 
ByteRage, you're posting something additional that just happens to share 
the detail of access to devices through file-like paths.

> Also, enumeration of special device names is bad idea. New versions of
> Windows     can     introduce     new     devices.    Eugene    Roshal
> (http://www.rarsoft.com),  developer  of  well-known utilities Far and
> Rar,  recommends  use  of GetFileType() API. In MS source examples you
> can find a lot of:

Enumeration of special devices, by an API, would be most useful, because 
then an application could prohibit access to device names _prior_ to having 
to open the file.  Should there be future bugs discovered in opening device 
specifiers, it'd be nice to be able to say "filter all input paths and 
remove any specification that matches a device name".  Currently, there is 
no API to do this.

> According to  Mr.  Roshal  FILE_TYPE_CHAR and FILE_TYPE_PIPE probably
> refer to special device names.

According to the documentation:

FILE_TYPE_UNKNOWN - The type of the specified file is unknown.
FILE_TYPE_DISK - The specified file is a disk file.
FILE_TYPE_CHAR - The specified file is a character file, typically an LPT 
device or a console.
FILE_TYPE_PIPE - The specified file is either a named or anonymous pipe.

> Also,  `prn'  and  `lpt1'  are just a sample of the special names. Any
> device driver which can be reached by opening a special file name will
> cause  such  problems;  thus the list of the offending names cannot be
> known  in advance, since additional device drivers can be installed on
> the target system.

A list compiled programmatically at the start of operation would be 
somewhat more useful.  While it's true that additional device drivers can 
be installed while a program is running, I wonder if these are given DDNs?

> In  addition,  the  file-name  extension  is ignored when the basename
> matches.  So  `aux.lst', `prn.c', `con.foo', and an infinite number of
> other  similar  names--all  of them are prone to this problem. Some of
> the  devices  will actually wedge the DOS box ... kids, don't try that
> at home!

The person quoted here doesn't indicate whether it is merely opening the 
device files, or trying to access (read or write) their contents, is what 
will "wedge the DOS box" - if the former, then GetFileType is sadly of no 
assistance.

Alun.
~~~~

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