||Home : Advisories : File Access issue with Internet Information Server|
||File Access issue with Internet Information Server
||8th July 1998
Microsoft Security Bulletin (MS98-003)
File Access issue with Internet Information Server
Last Revision: July 8, 1998
Recently Paul Ashton reported an issue on the NTBugtraq mailing
list (http://www.ntbugtraq.com) that affects Microsoft Internet
Information Server (IIS). Web clients that connect to IIS can read
the contents of any NTFS file in an IIS v-root directory to which
they have been granted "read access". They can read these files
even if the file is marked for "applications mappings", such as
used with Active Server Pages scripts.
The purpose of this bulletin is to inform Microsoft customers of this
issue, its applicability to Microsoft products, and the availability
of countermeasures Microsoft has developed to further secure its
The native Microsoft(r) Windows NT(r) file system, NTFS, supports
multiple data streams within a file. The main data stream, which stores
the primary content has an attribute called $DATA. Accessing this NTFS
stream via IIS from a browser may display the contents of a file that
is normally set to be acted upon by an Application Mapping.
For example, .ASP files are mapped such that they are executed by
the Active Server Pages scripting agent on the server, rather than
simply returning the contents of a file, as is done with standard
.htm files. Normally direct contents of the these script-mapped
files should not be returned to the user. However, by requesting the
file using the its complete data stream name, a web browser could
obtain the contents of the script file. In some cases, the file
might contain sensitive information such as embedded passwords or
other sensitive "business logic" information.
This issue does not give the user, who was able to access the script
file, the ability to alter the script on the server, or force the server
to run any arbitrary code. The only exposure here is to the plain text
contents of the script file.
The issue is a result of how IIS parses filenames. The fix involves
IIS supporting NTFS alternate data streams by asking Windows NT to
canonicalize the filename.
For the problem to occur:
- The user must know the name of the file
- The ACLs on the file must allow the user read access
- The file must reside on an NTFS partition
Affected Software Versions
- Microsoft Internet Information Server versions 1.0, 2.0, 3.0, 4.0
- Microsoft Peer Web Server versions 2.0, 3.0
- Microsoft Personal Web Server version 4.0 on Windows NT 4.0 Workstation
What Microsoft is Doing
The Microsoft Product Security Response Team has produced a hotfix for
Microsoft Internet Information Server versions 3.0 and 4.0.
Additionally, some administrative workarounds are included below.
What customers should do
Microsoft strongly recommends that customers using IIS versions 3.0
and 4.0 should apply the hotfix.
Customers running previous versions of IIS should upgrade to a more
recent version (3.0 or 4.0).
The following hotfixes are available from the Microsoft FTP download
IIS 3.0 (Intel x86) hotfix /iis3-datafix/iis3fixi.exe
IIS 3.0 (Alpha) hotfix /iis3-datafix/iis3fixa.exe
IIS 4.0 (Intel x86) hotfix /iis4-datafix/iis4fixi.exe
IIS 4.0 (Alpha) hotfix /iis4-datafix/iis4fixa.exe
As localized versions of this hotfix are produced, they will appear
in the respective language directories under
Customers who cannot apply the hot fix can use the following workaround
to temporarily address this issue:
Normally, web users do not need "read" access to script files, such
as .ASP files. They simply need "execute" permissions. Removing "read"
access to these files for non-administrative users will remove this
For additional protection, the Application Maps can be modified in
IIS 4.0 to take into account the existence of the alternate data
streams. More details on this workaround are available in the
Microsoft Knowledge Base article Q188806 (see the "More Information"
section below for the URL).
In addition, the following practices can help to further improve
security for your IIS servers:
- Periodically review the users and groups who have access to the web
server: Review the users and groups and their permissions to ensure
that only valid users have the appropriate permissions.
- Use auditing to detect for suspicious activity: Apply auditing
controls on sensitive files and review these logs periodically to
detect suspicious or unauthorized behavior.
- Set "read" and "execute" permissions appropriately: ASP and other
script files do not need to be readable by users that access them
through IIS, rather they need to be executable. Thus, it is
advisable to remove "read" access from these files for normal users.
Please see the following references for more information related to
- Microsoft Security Bulletin 98-003, File Access issue with Internet
Information Server (the web-posted version of this bulletin),
- Microsoft Knowledge Base article Q188806, NTFS Alternate Data Stream
Name of a File May Return Source,
- Microsoft Knowledge Base article Q105763, HOWTO: Use NTFS Alternate
July 2, 1998: Bulletin Created
July 6, 1998: Updated information on the availability of hotfix for IIS
4.0 and Alpha version as well. Added additional information
on workaround, and more thorough issue description.
July 8, 1998: Updated to include information about localized versions of
the hotfix. Updated information about products affected.
For additional information on security with Microsoft products, please visit
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