Merge pull request #3498 from edx/max/sec-220

SEC-220: Block external logged in checking

playbooks/roles/nginx/templates/edx/app/nginx/sites-available/lms.j2

@@ -116,6 +116,20 @@ error_page {{ k }} {{ v }};
   {% include "python_lib.zip.j2" %}
   {% include "common-settings.j2" %}
 
+  {% if NGINX_EDXAPP_EMBARGO_CIDRS -%}
+  #only redirect to embargo when $embargo == true and $uri != /embargo
+  #this is a hack to do multiple conditionals
+  if ( $embargo ) {
+    set $do_embargo "A";
+  }
+  if ( $uri != "/embargo" ) {
+    set $do_embargo "${do_embargo}B";
+  }
+  if ( $do_embargo = "AB" ) {
+    return 302 /embargo;
+  }
+  {% endif -%}
+
   location @proxy_to_lms_app {
     {% if NGINX_SET_X_FORWARDED_HEADERS %}
     proxy_set_header X-Forwarded-Proto $scheme;
@@ -140,11 +154,22 @@ error_page {{ k }} {{ v }};
     {% if EDXAPP_LMS_ENABLE_BASIC_AUTH|bool %}
       {% include "basic-auth.j2" %}
     {% endif %}
-    {% if NGINX_EDXAPP_EMBARGO_CIDRS -%}
-    if ( $embargo ) {
-      return 302 /embargo;
+
+    try_files $uri @proxy_to_lms_app;
+  }
+
+  # /login?next=<any image> can be used by 3rd party sites in <img> tags to
+  # determine whether a user on their site is logged into edX.
+  # The most common image to use is favicon.ico.
+  location /login {
+    {% if EDXAPP_LMS_ENABLE_BASIC_AUTH|bool %}
+      {% include "basic-auth.j2" %}
+    {% endif %}
+
+    if ( $arg_next = "favicon.ico" ) {
+      return 403;
     }
-    {% endif -%}
+
     try_files $uri @proxy_to_lms_app;
   }