Create 07_header_manipulation.md

07_header_manipulation.md

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+# Nginx Header Manipulation
+Instructions to manipulate headers in nginx
+
+## Hide Webserver-Details
+The simplest (and anyhow recommended) configuration is to remove the webserver details.
+This will set the server header to `nginx` only (without any further details).
+This is done e.g. in the main configuration: `/etc/nginx/nginx.conf`
+
+```bash
+sudo vim /etc/nginx/nginx.conf
+```
+
+and uncomment or add following line:
+
+```nginx
+http {
+
+        ##
+        # Basic Settings
+        ##
+
+        [...]
+        server_tokens off; # Recommended practice is to turn this off
+        [...]
+}
+```
+
+## Hide/Replace Webserver at all
+To remove or add headers at all, the "headers more module" is needed.
+On debian/ubuntu the this module is included in the package `nginx-extras`.
+To verify if the package is already installed run:
+
+```bash
+sudo apt list nginx-extras --installed
+```
+
+to install the package run:
+
+```bash
+sudo apt install nginx-extras -y
+```
+
+to remove the Server header at all, modify the site configuration:
+
+```bash
+sudo vim /etc/nginx/sites-available/*site-name*
+```
+
+and add the following line to the server section:
+
+```nginx
+server {
+    [...]
+    more_clear_headers Server;
+    [...]
+}
+```
+
+to replace it with a custom value, use following statement:
+
+```nginx
+server {
+    [...]
+    more_set_headers 'Server: Webserver';
+    [...]
+}
+```
+
+## Security Header
+
+### X-Frame-Options
+instructs the browser to not show the website in an iframe.
+**Note:** The modern way is using `frame-ancestors` within the CSP header.
+edit the site configuration:
+
+```nginx
+server {
+    [...]
+    add_header X-Frame-Options DENY always;          # disable being loaded as iframe at all
+    add_header X-Frame-Options SAMEORIGIN always;    # only from same domain allowed
+    [...]
+}
+```
+
+### CSP
+
+#### XSS/CSP Demo
+Using Demo-App [Addressbook](enable_php.md)
+
+Using Instructions [XSS-Demo](xss_demo.md)
+
+#### Preparing Website for exercise
+as simple playground...add following code-block to the index.html (or index.nginx-debian.html) before **`</body>`**
+
+```html
+<input id="hello_world" type="button" value="Hello World!" />
+
+<script nonce="nginx_nonce">
+document.getElementById("hello_world").addEventListener('click', hello);
+
+function hello() {
+        alert("hello world!");
+}
+</script>
+```
+
+and add a style tag (within head-section):
+
+```html
+<style nonce="nginx_nonce">
+  h1 {color:red;}
+</style>
+```
+
+#### CSP general
+
+Set the Content Security Policy. Edit the site configuration.
+
+```nginx
+server {
+    [...]
+    add_header Content-Security-Policy "default-src 'self';" always;    # only resources from our webserver are loaded. Inline script/css tags are blocked as well!
+    [...]
+}
+```
+
+afterwards compare the website to before (comment/uncomment the CSP header).
+
+#### CSP nonce
+a simple example to generate a dynamic nonce per request...
+edit the site configuration.
+
+```nginx
+server {
+    [...]
+    # nonce
+    set $nonce $request_id;
+    add_header Content-Security-Policy "default-src 'self' 'nonce-$nonce';" always;    # only resources from our webserver are loaded
+    sub_filter 'nginx_nonce' "$nonce";
+    sub_filter_once off;
+    sub_filter_types text/html;
+    [...]
+}
+```
+
+Note: *nginx_nonce* has to match to the value provided in the *script* and *style* tag.
+
+Note2: Test the website again. The style and script tags should now be allowed, as they are whitelisted by using the nonce tag.
+
+#### CSP hash
+nonce requires will and option to modify the source code of webapp...
+another option is using hash-values in the CSP header.
+
+comment the lines from above to have the style and script stop working again.
+
+get hash-values from the debug-console of the browser and add it to the CSP instruction.
+![firefox debug-console](./images/get_csp_hash.png)
+
+or (for external resources) calculate them using:
+
+```bash
+curl -s https://*domain*/*path-to-resource* | openssl dgst -sha384 -binary | openssl base64 -A
+```
+
+```nginx
+server {
+    [...]
+    # csp hash
+    add_header Content-Security-Policy "default-src 'self' 'nonce-$nonce' 'sha256-4qxDpGEJUcxjIP3NOEWlTKBLTDQ5y6fmRuEEO6ZT9Q0=' 'sha256-IKS/bMyKMqxTwEMnsaKruaZIbhySJGL+EebDepTqwUM=';" always;    # only resources from our webserver are loaded
+    [...]
+}
+```
+
+Note: Test the website again. The style and script tags should now be allowed, as they are whitelisted by providing their hash-value.