The difference between WMS 1.1.1 and 1.3.0 is two fold.
CHANGE NO 1 - CRS/SRS Usage
Use SRS for 1.1.1
Use CRS for 1.3.0
CHANGE No 2 - WMS 1.3.0 ONLY
The order of parameters for BBOX depends on whether the CRS definition has flipped axes. You will see this in the GetCapabilities request at 1.3.0 - the response should show the flipped axes.
BBOX=xmin,ymin,xmax,ymax NON-FLIPPED
BBOX=ymin,xmin,ymax,xmax FLIPPED
I have made a list of EPSG codes that need to be flipped by creating a SpatiaLite 4.3.0 database and then saving this SQL request to file:
SELECT auth_srid, has_flipped_axes, ref_sys_name, axis_1_name, axis_1_orientation, axis_2_name, axis_2_orientation FROM "spatial_ref_sys_all" WHERE auth_name = "epsg";
You will then see that EPSG:4326 needs to have flipped axes.
4326 1 WGS 84 Latitude North Longitude East
THIS IS THE CORRECTED 1.3.0 REQUEST
Change is BBOX=24,-126,50,-66
http://mesonet.agron.iastate.edu/cgi-bin/mapserv/mapserv?map=/mesonet/www/apps/iemwebsite/data/wms/goes/conus_ir.map&SERVICE=WMS&REQUEST=GetMap&VERSION=1.3.0&WIDTH=256&HEIGHT=256&FORMAT=image/png&TRANSPARENT=TRUE&BBOX=24,-126,50,-66&LAYERS=conus_ir_4km_900913,conus_ir_4km&CRS=EPSG:4326&STYLES&
Here is an example of a WMS from my config file...
<layer label="NWS RADAR" type="wms" autorefresh="66" skipgetcapabilities="true" url="http://mesonet.agron.iastate.edu/cgi-bin/wms/nexrad/n0q.cgi" version="1.1.1" visible="false" visiblelayers="nexrad-n0q-900913" alpha=".5"/>
That is a RADAR layer I'm getting from the University of Iowa. Here is one of my own...
<layer label="COUNTY BOUNDARY" wkid="3857" type="wms" skipgetcapabilities="true" url="http://maps.ironcountywi.org:8888/cgi-bin/mapserv.exe?map=C:/Projects/county_webmerc.map" version="1.3.0" visible="true" visiblelayers="COUNTY"/>
My layer is in lat-lon, so I added the [wkid="3857"] entry to request the correct projection.
If you have the [skipgetcapabilities="true"] entry, then it doesn't need a cross-site script.
Best Answer
Maybe this will help you: https://stackoverflow.com/questions/25261945/cannot-access-wms-getmap-as-anonymous-in-geoserver
It is another problem, but read the last comment on the page and try to configurate the "Allowing anonymous read-only access" (http://docs.geoserver.org/stable/en/user/security/service.html)