The Rejetto HTTP File Server (HFS) version 2.x is vulnerable to an unauthenticated server side template injection (SSTI) vulnerability. A remote unauthenticated attacker can execute code with the privileges of the user account running the HFS.exe server process. The vulnerability has been confirmed to work against version 2.4.0 RC7 and 2.3m. The Rejetto HTTP File Server (HFS) version 2.x is no longer supported by the maintainers and no patch is available. Users are recommended to upgrade to version 3.x.

The server uses a default template when rendering the content for a HTTP response. This template when rendered will include the content of a request’s search query parameter. It is this search query parameter that lets us supply a value that will not be escaped correctly, and ultimately results in an SSTI vulnerability.

Under normal operation any user supplied content will be escaped, so any symbols, which are normally encoded as %symbol-name%, and any macros, which are normally encoded as {:macro-name:} will be escaped to prevent SSTI.

However we can force a percent symbol to become un-escaped. This allows us to embed any symbol in the content being processed. We can do this via the sequence %25x%25symbol-name%25.

We can leverage this to force the %url% symbol to become unescaped. When the %url% symbol is processed by the server, it will echo back the remainder of the URL into the server side content. By forcing the remainder of the URL in the HTTP request to not be correctly URL-encoded, we can now include characters such as additional % or } characters.

To inject arbitrary macros, we first need to close the default template MARKER_QUOTE sequence ({:) by writing an unexpected MARKER_UNQUOTE (:}) sequence, however this will still be filtered. To bypass this filtering, we can leverage the %host% symbol and an empty host header value. So :%host%} will become :} and this will not be escaped. After this happens we can perform an arbitrary template injection containing a sequence of any HFS symbols or macros we want.

Finally we can execute an arbitrary operating system command by using the exec macro as shown below. As the search query parameter is processed several times by the default template, we avoid executing our command several times by issuing a break macro which will stop all processing.

$ echo -ne "GET /?search=%25x%25url%25:%host%}{.exec|notepad.}{.break.} HTTP/1.1\r\nHost:\r\n\r\n" | nc 192.168.86.35 80

I have rated the exploitability of this vulnerability as very high, as it is trivial to exploit by a remote unauthenticated attacker. I have rated the attacker value as low as this is not an enterprise web server.