Steps to reproduce the issue

1. Set "URL Routing" to "Legacy" under Articles option.
2. Visit front-end and enter ANY URL. At the end of the URL add the ID of a published article.

Example:
http://site-root/any/url/here/8 (assuming that article with ID 8 is published)

Or if you have "Use URL Rewriting" to "No":
http://site-root/index.php/any/url/here/8 (assuming that article with ID 8 is published)

Expected result

404 Page

Actual result

The article with the ID that matches the number at the end of the URL is rendered.

System information (as much as possible)

Joomla 3.8.7 (Fresh installation, no extensions)

Additional comments

The issue is caused by com_content legacy router (components/com_content/helpers/legacyrouter.php) .
Not sure if the legacy routers or the rest core extensions have the same issue.