proxy's http response to http request results in empty download

[chrysn]

when a resource is requested using https and a proxy server, and the proxy declines the CONNECT request with 403, a download window to a file name taken from the url is opened, with unknown mime type and (upon saving) empty body.

the behavior was discovered when and can be reproduced by using midori behind a privoxy proxy (with default settings), and accessing a google ad-foo infested page like http://slashdot.net/. using javascript, an iframe gets constructed there, which accesses something like [1], which results in a download dialog popping up. sniffing between the proxy and the browser reveals a dialog roughly like this:

> CONNECT googleads.g.doubleclick.net:443 ...
< HTTP/1.1 403 Request blocked by Privoxy
< Content-Type: image/png
<
< an empty image

i don't know what the http spec on proxying says on the topic (whether it's ok to take the unencrypted response for the encrypted request), but opening a download popup certainly isn't the right reaction.

[1] https://googleads.g.doubleclick.net/pagead/viewthroughconversion/1002083962/?random=1448530200142&cv=8&fst=1448530200142&num=1&fmt=1&label=G_uGCOaBlAQQ-qzq3QM&guid=ON&u_h=768&u_w=1366&u_ah=768&u_aw=1366&u_cd=24&u_his=3&u_tz=0&u_java=true&frm=0&url=http%3A//slashdot.org/%3Fsource%3Dautorefresh&ref=http%3A//slashdot.org/&tiba=Slashdot%20%2816%29