A vulnerability exists in Microsoft Internet Information Server (IIS) that could disclose sensitive information contained in CGI-type files. Typically a CGI/script file on a web server should only be executable and not readable to remote users. Sensitive information contained in CGI-type files file might include user credentials for access to a back-end database.
This is a variation of the vulnerability previously discussed in VU#35085 and Microsoft Security Bulletin MS00-031.
IIS contains a technology called ISAPI (Internet Services Application Programming Interface) which is designed to allow programmers to write custom applications which run on IIS. One ISAPI extension uses .HTR scripts to provide a password management interface via IIS. The .HTR ISAPI extension is implemented in the file ISM.DLL. When IIS receives a request for a file ending in .HTR it performs some pre-processing of the request then passes it off to ISM.DLL. If a malicious user crafts an HTTP request for an existing non-.HTR file and appends "+.HTR" to the request, ISM.DLL truncates the "+.HTR" from the file name and processes the requested file as if it were an .HTR file. If the requested file is not an .HTR file then the contents of the file will be returned to the requestor, with the exception of text enclosed in script delimiters "<% %>".
The contents of files may be disclosed to unauthenticated remote users. Target files may be CGI scripts that contain sensitive configuration information such as database access credentials. This specific vulnerability does not allow an attacker to access files located outside the root directory of the web server.
Disable .HTR script mapping if not needed, as described in Microsoft Security Bulletin MS01-004:
It may be possible to use an application layer filter to detect and block malicious requests.
The CERT Coordination Center thanks the IBASE Security Team and Microsoft Product Security for information used in this document.
This document was written by Art Manion.
|Date First Published:||2001-06-15|
|Date Last Updated:||2001-08-07 13:09 UTC|