K-048: Permissions Problems with FrontPage Extensions

PROBLEM:       Permission settings on FrontPage Extensions may allow
               defacements of web pages.
PLATFORM:      UNIX: The FrontPage Server Extensions on the following web
               servers: Apache 1.1.3 or 1.2, CERN 3.0, NCSA 1.5.2, Netscape
               Commerce Server 1.12, Netscape Communications Server 1.12,
               Netscape Enterprise 2.0 and 3.0, Netscape FastTrack 2.0.
               Windows NT: The FrontPage Server Extensions on the following
               web servers: Internet Information Server (IIS) 2.0 or higher,
               (including IIS 4.0), Netscape Commerce Server 1.12, Netscape
               Communications Server 1.12, Netscape Enterprise Server 2.0 and
               3.0, Netscape FastTrack 2.0, O'Reilly WebSite, FrontPage
               Personal Web Server.
               Windows 95/98: The FrontPage Extensions also run on Windows
               95/98, but this will be covered in a future bulletin.
DAMAGE:        A remote user may deface web pages of any given site when the
               permissions are not set properly.
SOLUTION:      Change the permissions on directories and files as indicated
               below.
VULNERABILITY  The risk is HIGH. Multiple web pages have been defaced by
ASSESSMENT:    exploiting the permissions of FrontPage Extensions.

Over the last few weeks, the Federal community, including DOE, has seen a
rash of web page defacements. Based on CIAC’s investigations into some of
these incidents, intruders achieved access because the permissions on the
Microsoft FrontPage Extensions had been inadvertently changed to allow an
outsider to have "execute access" to the remote web authoring extension.

TECHNICAL DETAILS:

Windows NT: The Microsoft FrontPage server extensions may be installed
automatically along with the Internet Information Server on a Windows NT
Server. They are also used by other web management packages such as Visual
InterDev. When they are installed automatically, the permissions are set to
prevent outside access. However, if you are not careful you can
inadvertently change the access permissions and allow any outsider to
change a website. The site becomes vulnerable to attack when the users are
given read execute (rx) access to the contents of the _vti_aut directory.
This usually occurs when the site administrator sets the permissions on the
webroot or _vti_bin directory and checks the check box "Replace permissions
on subdirectories". This gives normal users execute access to the admin.dll
and author.dll files.

There are three extensions in each root web of a website.
\webroot\_vti_bin\shtml.dll
\webroot\_vti_bin\_vti_adm\admin.dll
\webroot\_vti_bin\_vti_aut\author.dll

The extension shtml.dll handles user interactions with web forms and must
be accessible to the users of your website. The extension admin.dll
controls administration of a website and must be restricted to site
administrators. The extension author.dll allows remote authoring of web
pages on the site and access must be restricted to administrators and only
those authors who are allowed to change the web pages. The directory and
file permissions on these files and directories should be as follows:

_vti_bin   users (rx)(rx), authors (rx)(rx), administrators (rx)(rx)
_vti_adm   administrators (rx)(rx) no user or author access
_vti_aut   authors (rx)(rx), administrators (rx)(rx)

Here, users are the normal users of your web site including the web guest
user IUSR_ if anonymous access is allowed. Authors are those
people allowed to remotely change web pages and administrators are those
users allowed to administer the web site.

UNIX: Permissions on FrontPage Extensions on these systems are set in a
similar manner as though mention for Windows NT.

RECOMMENDATION:

Web site managers who are using FrontPage to administer their sites should
check these permissions to insure that only those who are allowed to author
or administer the site have access to the admin.dll and author.dll
extensions. Web site managers who are using IIS servers but not FrontPage
should check for the existence of the FrontPage extensions (check for the
_vti_* directories and for _vti_inf.html in the web root directory) and
remove them if FrontPage is not required.

OTHER IMPORTANT ISSUES:

1) Use care in tightening security through FrontPage Administrator. It’s
easy to block access to everyone.
2) Test your permissions by attempting to edit your web page as a regular
non-admin user. Put some character (like a dot) on the page and save it.
If that character appears on the web page, you have a permission
problem.  Apply this method remotely to see if an outsider can modify
the web page. If this works, then you still have a permission problem.
3) Make sure you set permissions correctly on all copies of _vti_bin and
its subdirectories.  There is one in each rootweb at a website.
4) Using scanners to identify this problem may not give you reliable
results.

For more information see:
http://officeupdate.microsoft.com/frontpage/wpp/serk98/
http://officeupdate.microsoft.com/frontpage/wpp/serk/scintro.htm

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