Re: partial URLs ? (was <p> ... </p>)

Daniel W. Connolly (
Wed, 20 Dec 1995 10:24:36 -0500

In message <>, Jon Wallis writes:
>At 13:19 19/12/95 -0600, BearHeart/Bill Weinman wrote:
>>At 10:40 am 12/19/95 -0800, Walter Ian Kaye wrote:
>>><A HREF="index.html"><IMG SRC="../gifs/btnhome3.gif" ALT="[Home]"
>>><A HREF="../map.html"><IMG SRC="../gifs/btnmap3.gif" ALT="[Index]"
>>>(I'm gonna be changing the form and cgi soon, btw, cuz Lynx doesn't like
>>>partial URLs -- tho' Netscape handles this form perfectly.)
>> The problem with the parial URLs may be the "../" references.
>> Some servers, and perhaps some browsers too, disallow them because
>>they've been abused to get around security measures.
>That really shouldn't be a problem if the system is set up right - but since
>so many systems are poorly set up in terms of security I can believe it.

I think there are two issues that are getting confused here:
(1) whether it's OK to use ../../ in an HREF or SRC attribute
in an HTML document,
(2) whether it's OK to _send_ ../../ in the path field of
and HTTP request.

(1) is cool, (2) is not.

For example, if the example above was fetched from,
then to fetch the [Home] image, the client must combine the value of the HREF
attribute with the base URL as per RFC1808, yielding:

To access the resource at that address, it makes a TCP connection to port 80
of, and sends:

GET /a/gifs/btnhome3.gif HTTP/1.0
Accept: image/*

What's _not_ cool is to try to sidestep the processing of .. on the client side;
that is, to just combine the base and HREF into:

(which is _not_ a well-formed HTTP url) and send:

GET /a/b/../gifs/btnhome3.gif HTTP/1.0

This is illegal because it is a potential secruity risk. Consider a server
whose document root is /usr/local/etc/httpd/docs/ and a client who sends:

GET /../../../../etc/passwd HTTP/1.0
Accept: text/plain

a naive server implementation might just do:
and give away a bunch of sensitive info.

In stead, any server that sees /../ in the HTTP path is supposed to
issue a 403 Unauthorized response. (Is this in the HTTP specs somewhere?
YIKES! I can't find it in draft-ietf-http-v10-spec-02.txt!!!

HTTP-WG folks: this should be addressed in the HTTP 1.0 spec, no?