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wp-csp-core-patches for WordPress v. 5.5.1

WordPress' core templates contain some inline styling and scripting elements that must be dealt with in order to implement a proper Content Security Policy (CSP).

By applying these patches, a nonce attribute is added to inline <script> and <style> tags in the relevant source files. The attribute value is a secret placeholder value that the web server will replace with a random nonce that changes with each request. The nonce proves that inline scripts and styles originates from the web application itself, so that it will not be blocked when the client enforces the Content Security Policy. See my blog posting about this project for more details.

Please note that the elements patched here are the ones causing trouble for my specific WordPress installation. However, there might be even more unpatched inline scripts and styles in then core that I have not yet encountered. Feel fre to create an issue or pull request where you suggest additional patches.

How to install the patches

Before you start, make a backup copy of your WordPress directory!

  1. Clone the Git repository and cd into the base directory:

    #> git clone https://github.com/espena/wp-csp-core-patches.git
#> cd wp-csp-core-patches

  2. Run make to build the patches and write the output files to wp-csp-patch. This will also generate the secret nonce placeholder that will be known to you only:

    #> make

  3. Move wp-csp-patch to WordPress' root directory:

    #> mv wp-csp-patch /path/to/wordpress/root/.

  4. From WordPress' root directory, run the apply.sh script:

    #> cd /path/to/wordpress/root/
#> wp-csp-patch/apply.sh

  5. Restore ownership of patched files. The owner of the patced files may have changed after the patch. Reset the owner to the correct one, usually www-data, for all php files in wp-includes:

    #> cd /path/to/wordpress/root/
#> sudo chown www-data:www-data wp-includes/*.php
#> sudo chown www-data:www-data wp-includes/widgets/*.php

  6. Update Nginx. Insert the generated nonce placeholder into the site configuration file. See your secret nonce placeholder value along with suggested configuration parameters in wp-csp-patch/nginx_site_config.txt.

  7. Restart Nginx.

NOTE

I know! Don't Hack Wordpress' Core -- although that's exactly what these patches do. There are a couple of reasons why. First and foremost, the patches do not change anything substantial in the code. It merely appends an extra, static attribute to inline <script> and <style> tags. Updating or reinstalling WordPress will revert the patches, which is fine. Then you will have to run the apply.sh script again, or bail out and allow unsafe-inline in your CSP header. The patch files can be easily inspected to check what modifications are made. That is good from a security perspective. Besides, this is a temporary fix, as I expect inline scripts and styles to be removed from WordPress within a few iterations.

Nginx setup

For this to work, Nginx must be configured with two additional modules:

  • ngx_set_misc for random nonce generation
  • http_sub_module for search and replace functionality

Please refer to Scott Helme's excellent description on how to implement CSP Nonce support in Nginx for details about compiling Nginx with these two modules.

Suggested server settings for your Nginx site configuration can be found in the generated file wp-csp-patch/nginx_site.conf.

Basically, the web server searches for the secret placeholder string and replaces it with a one-shot nonce value that is referred to by the CSP header. This confirms to the client that the inline <script> or <style> tags in question are legitimate and not from someone attempting an injection attack.

License and disclaimer

wp-csp-core-patches is written by Espen Andersen, and released under the GNU General Public License v. 2.0.

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

Although the author has attempted to find and correct any bugs in the free software programs, the author is not responsible for any damage or losses of any kind caused by the use or misuse of the programs. The author is under no obligation to provide support, service, corrections, or upgrades to the free software programs.