Re: "tinyurl" url masking

From: Laurian Gridinoc (
Date: 09/13/05

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    Date: Tue, 13 Sep 2005 02:54:07 +0300
    To: Lincoln Yeoh <>


    On 9/10/05, Lincoln Yeoh <> wrote:
    > Background:
    > A number of sites allow minimally controlled 3rd parties to post links to
    > images which other 3rd parties can view, and the only filtering used is
    > some pattern matching to ensure that a url has the "correct" extension.

    Wrong approach, URLs are opaque, the application should send a HEAD
    request to the URL in order to get a mime type and validate it.
    > However such filtering has problems if the "image" url actually redirects
    > to a url to a target site that does some naughty stuff.

    Two cases,

    1. mime type mismatch -- if you have an <img src="foo.jpg"> and at
    the src URL you get anything but not image/* mime type, the browser
    would not render anything, nor redirect you there -- Netscape
    Communicator had some issues with this.

    2. triggering unwanted actions by URL, as you pointed out:

    > Goes to
    > Which seems to log one out of google mail :).

    Which shows that Gmail (which I use to write this email) is badly
    designed, logout should be used via a POST only...

    And here let's blame cookies again... if the session would be in the
    URL, you won't be able to guess the session id of your target...
    > Some url shortening pages send a metarefresh page instead, which helps
    > prevent them being abused in this way, but of course it means users of such
    > services have to wait or make an additional click.
    And the metarefresh is intended for user-agents that know or expect text/html.
    The issue is interesting, there are a lot of webapplications that
    break the semantics of GET, and a lot of other webapplications allow
    displaying images from other domains... what to fix?

    I think that it would be easier to fix the issue in the browser, to
    have browsers not use cookies over a redirect? (thinking only at
    session related scenarios)


    Laurian Gridinoc,

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