"Most web application security experts frown on the practice of passing session or authentication tokens in a URL through the use of URL rewriting. Usually these tokens are passed between the server and the browser through HTTP cookies, but in cases where users configure their browsers to not accept cookies, this is impossible. Some web application frameworks – including ASP.NET – will detect this condition and revert to the cookieless URL rewriting method for passing session tokens. For example, a user who requests the page http://www.contoso.com/welcome.aspx would be redirected to http://www.contoso.com/{SID}/welcome.aspx, where {SID} is that user’s unique session identifier.

Again, most web application security people will tell you that this technique is fraught with peril. It can lead to session hijacking vulnerabilities (a man-in-the-middle sniffs the session identifier out of the URL) as well as session fixation vulnerabilities (an attacker creates his own session and tricks a victim into using it)"

Read more: http://blogs.msdn.com/sdl/archive/2009/04/09/improving-security-with-url-rewriting.aspx