Limit origin check to when host is loopback.

This should still prevent the exploit without breaking things for people
who use reverse proxies.

server.py

@@ -12,6 +12,8 @@
 import glob
 import struct
 import ssl
+import socket
+import ipaddress
 from PIL import Image, ImageOps
 from PIL.PngImagePlugin import PngInfo
 from io import BytesIO
@@ -80,6 +82,32 @@ async def cors_middleware(request: web.Request, handler):
 
     return cors_middleware
 
+def is_loopback(host):
+    if host is None:
+        return False
+    try:
+        if ipaddress.ip_address(host).is_loopback:
+            return True
+        else:
+            return False
+    except:
+        pass
+
+    loopback = False
+    for family in (socket.AF_INET, socket.AF_INET6):
+        try:
+            r = socket.getaddrinfo(host, None, family, socket.SOCK_STREAM)
+            for family, _, _, _, sockaddr in r:
+                if not ipaddress.ip_address(sockaddr[0]).is_loopback:
+                    return loopback
+                else:
+                    loopback = True
+        except socket.gaierror:
+            pass
+
+    return loopback
+
+
 def create_origin_only_middleware():
     @web.middleware
     async def origin_only_middleware(request: web.Request, handler):
@@ -93,12 +121,16 @@ async def origin_only_middleware(request: web.Request, handler):
             parsed = urllib.parse.urlparse(origin)
             origin_domain = parsed.netloc.lower()
             host_domain_parsed = urllib.parse.urlsplit('//' + host_domain)
+
+            #limit the check to when the host domain is localhost, this makes it slightly less safe but should still prevent the exploit
+            loopback = is_loopback(host_domain_parsed.hostname)
+
             if parsed.port is None: #if origin doesn't have a port strip it from the host to handle weird browsers, same for host
                 host_domain = host_domain_parsed.hostname
             if host_domain_parsed.port is None:
                 origin_domain = parsed.hostname
 
-            if host_domain is not None and origin_domain is not None and len(host_domain) > 0 and len(origin_domain) > 0:
+            if loopback and host_domain is not None and origin_domain is not None and len(host_domain) > 0 and len(origin_domain) > 0:
                 if host_domain != origin_domain:
                     logging.warning("WARNING: request with non matching host and origin {} != {}, returning 403".format(host_domain, origin_domain))
                     return web.Response(status=403)