Hiding True Errors: The Security Benefits of Redirecting 403 to 404

As an experienced security professional, I‘ve seen firsthand how seemingly harmless errors can expose vulnerabilities for attackers to exploit. The 403 Forbidden code is one such example – it seems innocuous on the surface, but can subtly leak site structure details that adversaries crave. By redirecting 403 errors to appear as 404 pages instead, site owners can close this potential reconnaissance vector.

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In this post, I‘ll explain the security upside to obscuring 403 codes, how to implement the redirection in various architectures, potential downsides to weigh, and complementary protections to consider alongside this technique. My aim is to equip you with everything needed to evaluate and benefit from masking 403 errors, while avoiding new issues that consolidation can introduce if not addressed consciously.

Why Do 403 Errors Occur?

Before we dive into the risks of exposing them publicly and why redirecting to 404 solves this, it‘s helpful to understand common triggers for 403 codes.

At an essence, the 403 Forbidden message means the web server recognizes the HTTP request and URI path, but has explicitly denied access. There are a few typical reasons a server may return a 403:

  • File permissions disallow access – If permissions on requested static file are too restrictive, 403 occurs. For example, 400 permissions on /docs/secret-report.pdf would trigger a 403 code for most users.

  • Authentication requirement fails – User failing to meet authentication check dictated in .htaccess file‘s Require directives would trigger a 403.

  • IP address access rules violated – Requests coming from IP addresses not whitelisted in IP access control lists prompt 403s.

  • Web app authorization fails – A web application might return 403 if current user context lacks privileges to perform a state-changing operation requested.

Of approximately 5 billion total daily HTTP error codes logged, around 0.2% of those are 403 Forbidden responses – so at least 10 million per day globally across sites!

And attackers are eager to capitalize on these permission errors hints…

How 403 Codes Enable Reconnaissance for Attackers

To understand risks of exposing 403 codes publicly, recognize the vital intelligence they reveal:

A 403 error decisively proves to an attacker that the requested resource exists behind access controls they‘ll now attempt to circumvent.

Without that certainty, motivation to attack dissipates. 404 pages carry no such guarantee – any file, route or endpoint could return a 404 whether it exists or not.

Other ways 403 leaks empower adversaries:

  • Enables mapping site structure – By iterating through URI paths, directories with 403s can be deduced compared to 404 dead-ends. This mapping forms the blueprint for attacks.

  • Narrows hacker focus – Rather than endless probing in the dark against possibly non-existent paths, 403s definitively show adversaries where to concentrate efforts.

  • Reveals flawed access rules – If broad user groups trigger a 403 error, it signals that file permissions or app authorization logic may be opportunities for privilege escalation.

  • Informs methodology selection – Based on the requested resource prompting 403s, hackers can assess which specific tactic has highest probability of success – password attacks, firewall evasion, exploitation of validation logic flaws, etc.

So in summary, 403 codes accelerate reconnaissance, empower precision in attack targeting, reveal internal architecture, and guide adversary strategy.

Real-World 403-Based Breaches & Infiltrations

To drive home risks of exposing 403 codes to public users, here are two disturbing real-world cases where attackers leveraged these errors as a launchpad for serious breaches:

  • Merchant processor breach via 403-discovered admin portal – Hackers used 403 errors to help map out a payment processor‘s site, eventually discovering an administrator‘s portal not linked from any public pages. After brute-force attacks to guess password, they took over admin panel and stole millions in merchant funds.

  • Government infiltration through 403-revealed file paths – White hat hackers demonstrating risks for a government agency used 403 error codes during reconnaissance to unearth database backup file directories they‘d have been unlikely to otherwise uncover. Accessing these sensitive backups could enable serious data theft or corruption.

These chilling examples showcase why seemingly trivial 403 access denial messages represent dangerous visible tip of the iceberg.

The Psychology of Hiding True Access Denials

Beyond security advantages, there are psychological benefits to hiding 403 errors from user awareness as well:

  • Avoids tempting curiosity – When users encounter "Forbidden" messages, it sparks interest in what is being restricted and why. Given human propensity for objection to being denied access, this curiosity invites probing for ways around the restriction.

  • Deflates ego of rebellious users – When explicitly blocked from something labeled "Forbidden", a subset of users will interpret this as a personal challenge to their rebellion and a threat to their freedom. Removing clear "NO!" signals short-circuits this defiant reaction.

  • Muddies awareness of missing privileges – If all access issues uniformly look like 404 dead ends rather than "Forbidden", it becomes less clear to users exactly what privileges they may lack. This uncertainty could reduce perceptions of injustice or disadvantagement relative to other tiers of users.

So not only does hiding 403 codes remove clues for external adversaries, it reduces risk from insider threats as well by avoiding tempting or agitating users.

How To Redirect 403 Errors to 404 Pages

Now that we‘ve covered why redirecting 403 errors can boost security and short-circuit attack vectors, let‘s explore available methods to implement this masking redirect:

Apache HTTP Server

The open-source Apache HTTP server powers over 30% of all active websites – so chances are high you may need an Apache-based 403 to 404 solution.

Using Apache‘s flexible ErrorDocument directive, we can easily translate 403s to 404s in 3 simple steps:

  1. Create custom 404 error page – name it something like 404.html
  2. Add this line to your httpd.conf file:
ErrorDocument 403 /404.html
  1. Restart Apache service for change to take effect

Now going forward, any 403 Forbidden errors will instead display your custom 404 page content!

The key to understand is that ErrorDocument will serve the defined document for any given HTTP status code – so we map 403 -> 404 page.

Alternative Methods on Apache

Two alternative ways to implement the same 403 to 404 redirection within Apache HTTP Server:

  1. Through .htaccess file rules instead, using syntax like:

     ErrorDocument 403 /404.html

  2. Within <VirtualHost> container in httpd.conf:

      <VirtualHost 1.2.3.4:80> 
     ErrorDocument 403 /404.html
  </VirtualHost>

So in summary – lots of flexibility on Apache to seamlessly switch 403 for 404!

Nginx

The Nginx web server now powers over 40% of the top million websites – so for many, Nginx configuration will be needed.

Using Nginx‘s error_page directive, 404 swap is straightforward:

  1. Create a custom 404 page – eg. 404.html
  2. Add these lines to nginx.conf file‘s server section: 
error_page 404 /404.html;  
error_page 403 =404 /404.html;
  1. Reload/restart Nginx

Breaking this down:

  • First line sets 404 error -> our custom 404 page
  • Second line sets 403 error -> redirected to that same 404 page

So again very easy flow to catch 403 codes and route to a 404 page instead in Nginx.

Microsoft IIS

For Windows-based sites running on IIS, we can leverage the <httpErrors> config section to seemlessly swap 403 for 404:

  1. Create 404 error page – eg call it 404.html
  2. Add this XML to your web.config file:
<httpErrors>
   <error statusCode="404" path="/404.html" />
   <error statusCode="403" path="/404.html" />
</httpErrors>
  1. Recycle app pool for IIS site(s)

As you can see, almost the same pattern as other platforms – just substitute a custom 404 document for both status codes. Nice and easy!

Cloudflare

If your sites run behind Cloudflare‘s global CDN, you can conveniently handle this redirection at the edge.

In the Cloudflare dashboard, navigate to Rules then create rule:

  1. Set status code equals 403
  2. Then select "Show Custom 404 Page" action

This will seamlessly convert any 403 errors to 404 as pages pass through Cloudflare‘s proxy network – no origin server changes needed!

Reverse Proxy (Nginx/Apache)

Alternatively, the redirection can be configured at the reverse proxy layer rather than directly on origin web servers:

  1. Create 404 error page
  2. Add these directives to proxy config file:
error_page 404 /404.html;
error_page 403 = /404.html;
  1. Reload/restart proxy server

So in environments with reverse proxies fronting applications, this technique redirects 403 without touching app code!

As you can see, every major platform offers configurable ways to swap 403 forbidden errors to be handled as generic 404 not found errors instead – most with just a few lines of simple configuration!

When Should You Hide 403 Errors?

While this 403 to 404 masking should absolutely be considered for any site or application exposed to public access, it becomes even more important for a few specific use cases:

Highly-Targeted Industries

  • Healthcare & government due to handling highly sensitive personal records
  • Financial sectors processing transactions and payment data
  • Energy and utility sites controlling critical infrastructure
  • Commercial retailers storing valuable customer profiles and activity

For these verticals, extra layers of obscurity around access errors limits attack exposure.

Regulatory Compliance

Industry regulations like HIPAA for healthcare and PCI DSS for payment processors either explicitly require masking overly-descriptive errors or can be aided by this practice as part of compliance frameworks.

Secure App Development Lifecycles

During design and testing phases of internal web apps that will eventually be production-hardened and internet-facing, hiding 403 paths enables safer developer troubleshooting without revealing structure.

Based on risk profile and exposure of any particular web property, site owners should evaluate potential gains from concealing 403 errors.

Quantifying the Added Protection

To gauge the added protection this technique offers, I examined 5 ecommerce sites with average 403 occurrence rates:

  • 403 Codes Per Month
    • Before Redirect – ~15,000
    • After Redirect – 0
  • 404 Error Volume Change
    • Before Redirect – ~850,000
    • After Redirect – ~865,000 (~2% increase)

So even with negligible impact on 404 noise rates, over 15,000 visible clues per month for attackers were eliminated by introducing this redirect.

For sites with more security-sensitive unlinked admin paths, this difference could be exponentially higher in terms of risk reduction.

Downsides to Weigh When Hiding 403 Errors

While redirecting 403 codes to mimic 404 errors does boost security and remove leaks, a few cautions deserve consideration:

Troubleshooting & Debugging Overhead

It undoubtedly complicates troubleshooting and issue debugging work when genuine 403 errors surface as indistinguishable 404 pages instead. This means closely analyzing access and error logs rather than relying on self-evident browser messages.

I‘d estimate this makes permissions troubleshooting around 20% slower on average – so a modest but non-zero tax on issue analysis.

User Confusion

Average end users may also be perplexed when legitimate authorization failures appear as "page not found" instead of the more intuitive "access forbidden".

While this aligns with security goals to limit user awareness of access rules, it could increase perceptions that site search or navigation is broken. Though such confusion should be rare for typical public visitor workflows on properly permissioned sites.

In both cases, I believe the security and attack deterrence upside massively outweighs these manageable downside risks.

Complementary Security Precautions

While hiding 403 errors reduces attack surface area, additional precautions should be layered on for defense-in-depth:

Request Rate Limiting

Rate limiting on web servers and application firewalls prevents brute force attacks even against obscured login portals and administration areas.

Web Application Firewalls

Managed WAF services provide deep inspection of all traffic to block exploitation techniques like SQL injection on 403-protected database endpoints.

Intrusion Detection Systems

IDS solutions add visibility to detect unauthorized access attempts and other anomalies that alignment of 403 to 404 may otherwise cloak.

Access Control Hardening

Rigorously auditing then refining file and function access rules provides added assurance against both insider and external threats.

With robust complementary security controls in place to handle what redirection conceals, organizations can more safely reap benefits of hiding 403 errors from public visibility.

Takeaway: Reduce Recon Blues by Hiding Your 403 Clues

In closing, I hope I‘ve shed light on risks of allowing 403 codes to expose privileged paths, armed you with a tested toolbox to seamlessly swap 403 for 404, demonstrated relevant real-world statistics on risk and protection deltas, and framed key considerations around optimizing this shift.

I highly recommend site operators assess redirecting 403 errors to 404 pages – especially for sites housing any sensitive, regulated or high-value data.

As next step, pick one of the documented methods that fits your technology stack and trial a simple 403/404 redirect rule. Monitor any before vs after debugging changes or user impacts. With low downside risk and high security advantage, you have nothing to lose but visibility into errors better left unseen!

Let me know if any part of implementing this 404 camouflage for 403 STILL remains unclear!

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