Use Content-Security-Policy frame-ancestors

X-Frame-Options has been deprecated by Content Security Policy Level 2.
We will need to provide the option for older browsers for the time
being, but frame-ancestors is preferred on newer ones.

README.md

@@ -47,8 +47,8 @@ remoteuploads = true
 - ```-metapath meta/``` -- Path to store information about uploads (default is meta/)
 - ```-maxsize 4294967296``` -- maximum upload file size in bytes (default 4GB)
 - ```-allowhotlink``` -- Allow file hotlinking
-- ```-contentsecuritypolicy "..."``` -- Content-Security-Policy header for pages (default is "default-src 'self'; img-src 'self' data:; style-src 'self' 'unsafe-inline'; referrer origin;")
-- ```-filecontentsecuritypolicy "..."``` -- Content-Security-Policy header for files (default is "default-src 'none'; img-src 'self'; object-src 'self'; media-src 'self'; referrer origin;")
+- ```-contentsecuritypolicy "..."``` -- Content-Security-Policy header for pages (default is "default-src 'self'; img-src 'self' data:; style-src 'self' 'unsafe-inline'; frame-ancestors 'self'; referrer origin;")
+- ```-filecontentsecuritypolicy "..."``` -- Content-Security-Policy header for files (default is "default-src 'none'; img-src 'self'; object-src 'self'; media-src 'self'; style-src 'self' 'unsafe-inline'; frame-ancestors 'self'; referrer origin;")
 - ```-xframeoptions "..." ``` -- X-Frame-Options header (default is "SAMEORIGIN")
 - ```-remoteuploads``` -- (optionally) enable remote uploads (/upload?url=https://...) 
 - ```-nologs``` -- (optionally) disable request logs in stdout

server.go

@@ -212,10 +212,10 @@ func main() {
 	flag.StringVar(&Config.remoteAuthFile, "remoteauthfile", "",
 		"path to a file containing newline-separated scrypted auth keys for remote uploads")
 	flag.StringVar(&Config.contentSecurityPolicy, "contentsecuritypolicy",
-		"default-src 'self'; img-src 'self' data:; style-src 'self' 'unsafe-inline'; referrer origin;",
+		"default-src 'self'; img-src 'self' data:; style-src 'self' 'unsafe-inline'; frame-ancestors 'self'; referrer origin;",
 		"value of default Content-Security-Policy header")
 	flag.StringVar(&Config.fileContentSecurityPolicy, "filecontentsecuritypolicy",
-		"default-src 'none'; img-src 'self'; object-src 'self'; media-src 'self'; style-src 'self' 'unsafe-inline'; referrer origin;",
+		"default-src 'none'; img-src 'self'; object-src 'self'; media-src 'self'; style-src 'self' 'unsafe-inline'; frame-ancestors 'self'; referrer origin;",
 		"value of Content-Security-Policy header for file access")
 	flag.StringVar(&Config.xFrameOptions, "xframeoptions", "SAMEORIGIN",
 		"value of X-Frame-Options header")