Possible bug when inserting JIRA Gadget into Confluence 3.3 Rich Text Editor

Reported by Nick Venegas (I was able to reproduce the same symptoms locally):

Hi

I've come across a problem in adding a gadget served from a JIRA
4.2-SNAPSHOT (via trusted apps) to Confluence 3.3.

Confluence 3.1.2 and 3.2.1_01 do not exhibit the problem:

Steps
1. Run JIRA 4.2-SNAPSHOT standalone
(https://jira.bamboo.atlassian.com/browse/QA-ENT-717/artifact/Standalone/atlassian-jira-enterprise-4.2-SNAPSHOT)
2. Run Confluence 3.3 standalone
(http://filehost/releases/confluence/downloads/confluence-3.3-std.tar.gz)
choosing the evaluation install during setup.
3. Setup trusted apps between the two instances.
4. Add the JIRA Pie Chart gadget as an external gadget in Confluence
(e.g., http://cardinal.sydney.atlassian.com:8880/rest/gadgets/1.0/g/com.atlassian.jira.gadgets:pie-chart-gadget/gadgets/piechart-gadget.xml)
5. In confluence, add a new page and in RTE mode, insert the JIRA pie
chart gadget.

Expected

• The JIRA pie chart gadget macro text should be inserted into the RTE textarea.

Actual

• After clicking insert at step 5, nothing is added to the RTE textarea
• Instead, there is a 403 response to a POST to
  http://cardinal.sydney.atlassian.com:7073/json/convertwikimarkuptoxhtmlwithoutpagewithspacekey.action
• Adding any other macro after this 403 response also generates a 403 response.

If you use the wiki markup editor instead of the RTE, then inserting
the gadget works as normal.

I came across this when verifying the bugfix for
http://jdog.atlassian.com/browse/JRADEV-1903 (which is covered by
http://jira.atlassian.com/browse/CONF-20106).

Any ideas on what is going wrong here? I'm not sure if it's a bug on
the JIRA side or not.

If you want a quick reproduction of the bug, feel free ping me and I
can setup up the instances for you to try it against.

Cheers
Nic