There are lots of way to increase the security on the Apache Web Server and there is no one size fits all. Some hardening steps will beak specific needs.
But before you start, as advised in the Apache Setup, ensure Apache works with its intended integrated purpose in a test environment. Verify vanilla and then basic hardening first. Then carefully apply each security setting and test at intervals.
Use the Center for Internet Security Security and search "apache benchmark" and look for your version of Apache to get hardening documentation.
I'll create my own abridged version in the future with additional perspective of impact to web applications.
These items are best practice and but may impact integrated modules. If there are any known impacts, I will document them here too.
Control what external domains iframes can communicate to on your your website.
Risk - internally developers may introduce iframe code to subvert your website or collect sensitive data. Externally, if your application is open ton an injection attack, a malicious iframe may be placed on your website.
Possible Impact - The success of this policy is dependant have a proper inventory of external domains used by iframes.
Considerations - If the website is an application, you may want to use code to set headers instead of using the web server.
CORS (Cross-Origin Resource Sharing) allows a domain to set policies to control if resources on the server can be access by a web page hosted on a different domain.
Risk - an overly permissive CORS can allow a malicious application leverage assets on your website leading to spoofing, data theft, relay and other attacks.
Etag (entity tag) was introduced to help improve caching. However, besides not being very effective in an enterprise clusters environments), it also provides sensitive information like inode number, multipart MIME boundary and child processes. It allows hackers to uniquely identify a particular server.
Unless you have a compelling reason you may,
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The advanced hardening is more likely to break your system so apply carefully one step at a time.
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