cis-apache24-5.13

Ensure Access to Inappropriate File Extensions Is Restricted (Manual)

Profile Applicability

Level 2

Description

Restrict access to inappropriate file extensions that are not expected to be a legitimate part of web sites using the FilesMatch directive.

Rationale

There are many files that are often left within the web server document root that could provide an attacker with sensitive information. Most often these files are mistakenly left behind after installation, troubleshooting, or backing up files before editing. Regardless of the reason for their creation, these files can still be served by Apache even when there is no hyperlink pointing to them. The web administrators should use the FilesMatch directive to restrict access to only those file extensions that are appropriate for the web server. Rather than create a list of potentially inappropriate file extensions such as .bak, .config, .old, etc, it is recommended instead that a white list of the appropriate and expected file extensions for the web server be created, reviewed and restricted with a FilesMatch directive.

Audit Procedure

Perform the following steps to determine if the recommended state is implemented:

1. Verify that the FilesMatch directive that denies access to all files is present as shown in step 3 of the remediation.
2. Verify that there is another FilesMatch directive similar to the one in step 4 of the remediation, with an expression that matches the approved file extensions.

Remediation

Perform the following to implement the recommended state:

1. Compile a list of existing file extension on the web server. The following find/awk command may be useful, but is likely to need some customization according to the appropriate webroot directory for your web server. Please note that the find command skips over any files without a dot (.) in the file name, as these are not expected to be appropriate web content.

   find */htdocs -type f -name '*.*' | awk -F. '{print $NF}' | sort -u
   

2. Review the list of existing file extensions, for appropriate content for the web server, remove those that are inappropriate and add any additional file extensions expected to be added to the web server in the near future.

3. Add the FilesMatch directive below which denies access to all files by default.

   # Block all files by default, unless specifically allowed.
   <FilesMatch "^.*
   quot;>
       Require all denied
   </FilesMatch>
   

4. Add another a FilesMatch directive that allows access to those file extensions specifically allowed from the review process in step 2. An example FilesMatch directive is below. The file extension in the regular expression should match your approved list, and not necessarily the expression below.

   # Allow files with specifically approved extensions
   # Such as (css, htm; html; js; pdf; txt; xml; xsl; gif; ico|jpe?g|png) 
   quot;>
   # images (gif; ico; jpeg; jpg; png; ...), multimedia
   <FilesMatch "^.*\.(css|html?|js|pdf|txt|xml|xsl|gif|ico|jpe?g|png)
   quot;>
       Require all granted
   </FilesMatch>
   

Default Value

There are no restrictions on file extensions in the default configuration.

References

1. https://httpd.apache.org/docs/2.4/mod/core.html#filesmatch

CIS Controls

v8:

• 9.4 Restrict Unnecessary or Unauthorized Browser and Email Client Extensions
  • Restrict, either through uninstalling or disabling, any unauthorized or unnecessary browser or email client plugins, extensions, and add-on applications.

v7:

• 18.2 Ensure Explicit Error Checking is Performed for All In-house Developed Software
  • For in-house developed software, ensure that explicit error checking is performed and documented for all input, including for size, data type, and acceptable ranges or formats.