To (Deny, then Allow), or (Allow, then Deny)?
The real problem with the above “all known” qualifier is that it is an allow, then deny model. In other words, we first give everyone access to every file and then deny access to specific files by adding a line of code. Consider the logic for a password authentication script. We have essentially two choices: • First allow all access, then deny any username/password combination that DOES NOT match the approved list. • First deny all access, then allow any username/password combination that DOES match the approved list. Obviously the second method is better. A passing familiarity with regular expressions shows that the first method is much more difficult to write securely. It fails anew each time a new variation of some attack is developed, and tends to require constant revisions. Over time, such revisions become so complex that the authentication system itself becomes a source of vulnerabilities. Conceptually, the second method is an example of building a strong fence around your site (
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