Optimization: e.g. stop sending X-Frame-Options for CSS

README.md

@@ -38,7 +38,8 @@ X-Content-Type-Options: nosniff <-----------
 ## Key Features
 
 *   Plug-n-Play: the default set of security headers can be enabled with `security_headers on;` in your NGINX configuration
-*   Sends `X-Content-Type-Options` only for appropriate MIME types, preserving unnecessary bits from being transferred for non-JS and non-CSS resources
+*   Sends `X-Content-Type-Options` only for relevant MIME types (CSS/JS), preserving unnecessary headers from being sent for HTML documents
+*   Similiarly, sends HTML-only relevant headers for relevant types and skips sending for others e.g. `X-Frame-Options` is useless for CSS
 *   Plays well with conditional `GET` requests: the security headers are not included there unnecessarily
 *   Does not suffer the `add_header` directive's pitfalls
 *   Hides `X-Powered-By`, which often leaks PHP version information
@@ -74,8 +75,18 @@ Enables hiding headers which leak software information:
 *   `X-Page-Speed`
 *   `X-Varnish`
 
-Next are the common security headers being set. It's worth noting that special value of `omit` for directives below
-will disable sending a particular header by the module (useful if you want to let your backend app to send it). 
+It's worth noting that some of those headers bear functional use, e.g. [`X-Page-Speed` docs](https://www.modpagespeed.com/doc/configuration#XHeaderValue) mention:
+
+> ... it is used to prevent infinite loops and unnecessary rewrites when PageSpeed 
+> fetches resources from an origin that also uses PageSpeed
+
+So it's best to specify `hide_server_tokens on;` in a front-facing NGINX insances, e.g.
+the one being accessed by actual browsers, and not the ones consumed by Varnish or other software.
+
+In most cases you will be just fine with `security_headers on;` and `hide_server_tokens on;`, without any adjustments.
+
+For fine-tuning, use the header-specific directives below. 
+A special value `omit` disables sending a particular header by the module (useful if you want to let your backend app to send it). 
 
 ### `security_headers_xss`
 
@@ -141,6 +152,7 @@ To compile the module into NGINX, run:
     make 
     make install
 
-Or you can compile it as dynamic module. In that case, use `--add-dynamic-module` instead, and load the module after compilation via:
+Or you can compile it as dynamic module. In that case, use `--add-dynamic-module` instead, and load the module after 
+compilation by adding to `nginx.conf`:
 
-    load_module modules/ngx_http_security_headers_module.so;
+    load_module /path/to/ngx_http_security_headers_module.so;

src/ngx_http_security_headers_module.c

@@ -37,7 +37,10 @@ typedef struct {
     ngx_uint_t                 rp;
 
     ngx_hash_t                 nosniff_types;
-    ngx_array_t                *types_keys;
+    ngx_array_t                *nosniff_types_keys;
+
+    ngx_hash_t                 text_types;
+    ngx_array_t                *text_types_keys;
 
 } ngx_http_security_headers_loc_conf_t;
 
@@ -99,6 +102,14 @@ ngx_str_t  ngx_http_security_headers_default_nosniff_types[] = {
     ngx_null_string
 };
 
+ngx_str_t  ngx_http_security_headers_default_text_types[] = {
+    ngx_string("text/html"),
+    ngx_string("application/xhtml+xml"),
+    ngx_string("text/xml"),
+    ngx_string("text/plain"),
+    ngx_null_string
+};
+
 static ngx_command_t  ngx_http_security_headers_commands[] = {
 
     { ngx_string( "security_headers" ),
@@ -119,7 +130,7 @@ static ngx_command_t  ngx_http_security_headers_commands[] = {
       NGX_HTTP_MAIN_CONF|NGX_HTTP_SRV_CONF|NGX_HTTP_LOC_CONF|NGX_CONF_1MORE,
       ngx_http_types_slot,
       NGX_HTTP_LOC_CONF_OFFSET,
-      offsetof(ngx_http_security_headers_loc_conf_t, types_keys),
+      offsetof(ngx_http_security_headers_loc_conf_t, nosniff_types_keys),
       &ngx_http_security_headers_default_nosniff_types[0] },
 
     { ngx_string("security_headers_xss"),
@@ -143,6 +154,13 @@ static ngx_command_t  ngx_http_security_headers_commands[] = {
       offsetof(ngx_http_security_headers_loc_conf_t, rp),
       ngx_http_referrer_policy },
 
+    { ngx_string("security_headers_text_types"),
+      NGX_HTTP_MAIN_CONF|NGX_HTTP_SRV_CONF|NGX_HTTP_LOC_CONF|NGX_CONF_1MORE,
+      ngx_http_types_slot,
+      NGX_HTTP_LOC_CONF_OFFSET,
+      offsetof(ngx_http_security_headers_loc_conf_t, text_types_keys),
+      &ngx_http_security_headers_default_text_types[0] },
+
       ngx_null_command
 };
 
@@ -242,7 +260,8 @@ ngx_http_security_headers_filter(ngx_http_request_t *r)
 
     /* Add X-XSS-Protection */
     if (r->headers_out.status != NGX_HTTP_NOT_MODIFIED
-        && NGX_HTTP_SECURITY_HEADER_OMIT != slcf->xss)
+        && NGX_HTTP_SECURITY_HEADER_OMIT != slcf->xss
+        && ngx_http_test_content_type(r, &slcf->text_types) != NULL)
     {
         ngx_str_set(&key, "X-XSS-Protection");
         if (NGX_HTTP_XSS_HEADER_ON == slcf->xss) {
@@ -265,7 +284,8 @@ ngx_http_security_headers_filter(ngx_http_request_t *r)
 
     /* Add X-Frame-Options */
     if (r->headers_out.status != NGX_HTTP_NOT_MODIFIED
-        && NGX_HTTP_SECURITY_HEADER_OMIT != slcf->fo)
+        && NGX_HTTP_SECURITY_HEADER_OMIT != slcf->fo
+        && ngx_http_test_content_type(r, &slcf->text_types) != NULL)
     {
         ngx_str_set(&key, "X-Frame-Options");
         if (NGX_HTTP_FO_HEADER_SAME == slcf->fo) {
@@ -339,14 +359,22 @@ ngx_http_security_headers_merge_loc_conf(ngx_conf_t *cf, void *parent,
     ngx_conf_merge_value(conf->hide_server_tokens,
                          prev->hide_server_tokens, 0 );
 
-    if (ngx_http_merge_types(cf, &conf->types_keys, &conf->nosniff_types,
-                             &prev->types_keys, &prev->nosniff_types,
+    if (ngx_http_merge_types(cf, &conf->nosniff_types_keys, &conf->nosniff_types,
+                             &prev->nosniff_types_keys, &prev->nosniff_types,
                              ngx_http_security_headers_default_nosniff_types)
         != NGX_OK)
     {
         return NGX_CONF_ERROR;
     }
 
+    if (ngx_http_merge_types(cf, &conf->text_types_keys, &conf->text_types,
+                             &prev->text_types_keys, &prev->text_types,
+                             ngx_http_security_headers_default_text_types)
+        != NGX_OK)
+    {
+        return NGX_CONF_ERROR;
+    }
+
     ngx_conf_merge_uint_value(conf->xss, prev->xss,
                               NGX_HTTP_XSS_HEADER_BLOCK);
     ngx_conf_merge_uint_value(conf->fo, prev->fo,

t/headers.t

@@ -23,6 +23,7 @@ hello world
 === TEST 2: no nosniff for html
 --- config
     security_headers on;
+    charset utf-8;
     location = /hello {
         return 200 "hello world\n";
     }
@@ -31,6 +32,7 @@ hello world
 --- response_body
 hello world
 --- response_headers
+content-type: text/plain; charset=utf-8
 !x-content-type-options
 x-frame-options: SAMEORIGIN
 x-xss-protection: 1; mode=block
@@ -140,4 +142,21 @@ hello world
 !x-content-type-options
 x-frame-options: SAMEORIGIN
 x-xss-protection: 1; mode=block
-referrer-policy: origin
+referrer-policy: origin
+
+=== TEST 8: X-Frame-Options should not be sent for CSS (even when encoding specified)
+--- config
+    security_headers on;
+    charset utf-8;
+    charset_types text/css;
+    location = /hello.css {
+        default_type "text/css";
+        return 200 "hello world\n";
+    }
+--- request
+    GET /hello.css
+--- response_body
+hello world
+--- response_headers
+content-type: text/css; charset=utf-8
+!x-frame-options