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Message ID: 900     Entry time: 26 Aug 2013
Author: Konstantin Olchanski 
Topic: Bug Fix 
Subject: Enable cross-site requests in mhttpd 
Javascript "AJAX" functions (and their MIDAS wrappers - ODBGet/ODBSet) are subject to something called 
"same origin policy" intended to prevent something called "cross-site scripting attacks", i.e. see

In practice it means that if you load the MIDAS custom web page from and try to access 
mhttpd at, ODBSet/ODBGet will not work.

I always thought that this meant that the requests are blocked by the browser and are a form of 
protection of the web server - only scripts loaded from mhttpd can do AJAX (ODBGet/ODBSet) to mhttpd.

It turns out that I was wrong. This is what actually happens: the "cross-site" requests are still sent to the 
server (mhttpd), the response it received, parsed and discarded if "same origin" conditions are not met.

This means that the "same origin" policy does not protect mhttpd at all - any script from any page 
anywhere can issue AJAX requests into any mhttpd, these requests will be successfully sent, received
and processed by mhttpd, including requests for writing into ODB ("jset" command using the HTTP GET 

So for the case of MIDAS, "same origin" does not prevent malicious (or buggy) scripts from writing into the 
wrong mhttpd of the wrong experiment.

All it does is prevent desired and intentional access to mhttpd (ODBGet) from scripts that happen to have 
been loaded outside of mhttpd (i.e. from a developer own test page).

Then it turns out that there is an "official" way to disable this unwanted protection policy, called CORS, see

I have now implemented this in mhttpd and added an mhttpd.js function ODBSetURL() to explicitly set the 
URL of mhttpd that we want to talk to.

This work is on the feature/ajax branch, to be merged soon. For the impatient, here is what you need to 
do in mhttpd:

diff --git a/src/mhttpd.cxx b/src/mhttpd.cxx
index 1d9d1cc..0460cec 100755
--- a/src/mhttpd.cxx
+++ b/src/mhttpd.cxx
@@ -1070,6 +1070,7 @@ void show_text_header()
    rsprintf("HTTP/1.0 200 Document follows\r\n");
    rsprintf("Server: MIDAS HTTP %d\r\n", mhttpd_revision());
+   rsprintf("Access-Control-Allow-Origin: *\r\n");
    rsprintf("Pragma: no-cache\r\n");
    rsprintf("Expires: Fri, 01 Jan 1983 00:00:00 GMT\r\n");
    rsprintf("Content-Type: text/plain; charset=iso-8859-1\r\n\r\n");

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