offensive-waf-bypass

name	offensive-waf-bypass
description	WAF bypass techniques checklist for Azure Front Door and Application Gateway. Covers encoding bypass, header manipulation, and Azure-specific evasion strategies.
domain	cybersecurity
subdomain	web-security
tags	["waf","azure-front-door","application-gateway","evasion","web-security"]
version	1.0
author	openshield
license	Apache-2.0
nist_csf	["PR.AC-3","PR.AC-5","SI-3","PR.DS-1","DE.CM-1"]

WAF Bypass Techniques — Azure Focused

When to Use

• When testing web applications protected by Azure Front Door WAF or Application Gateway WAF.
• When payloads are being blocked by default CRS (Common Rule Set) rules.
• When validating the effectiveness of custom WAF rules and rate limiting.
• When assessing if the backend origin can be reached directly, bypassing the WAF.

Key Concepts

Term	Definition
CRS	Common Rule Set, the base rules used by Azure WAF (e.g., OWASP 3.2)
Front Door	Azure's global CDN and security layer with integrated WAF
App Gateway	Azure's regional load balancer with integrated WAF
Origin Bypass	Directly connecting to the backend IP/hostname to skip WAF inspection

Evasion Strategies for Azure WAF

1. Origin Bypass (Direct-to-IP)

If the backend application (App Service, VM) is not restricted to Front Door/App Gateway IPs, an attacker can bypass the WAF entirely.

# Test if origin is reachable directly
curl -v https://<backend-ip-or-dns>/api/test

2. Encoding and Obfuscation

Azure WAF (CRS) can sometimes be bypassed using specific encoding combinations that the WAF doesn't normalize but the backend does.

• Double URL Encoding: UNION → %2555%254E%2549%254F%254E
• Case Variation: SeLeCt instead of SELECT
• Comment Injection: UN/**/ION SE/**/LECT

3. Header Manipulation

Some WAF rules can be confused by malformed or duplicate headers.

• Duplicate Headers: X-Forwarded-For: 1.2.3.4 followed by X-Forwarded-For: 127.0.0.1
• Large Headers: Sending headers larger than the WAF's buffer (e.g., > 8KB).

Remediation Reference

• Restrict Access to WAF IPs: Ensure backend resources only accept traffic from Azure Front Door or App Gateway Service Tags.
• Enforce WAF Prevention Mode: Ensure WAF is in "Prevention" mode, not just "Detection".
• Keep CRS Updated: Regularly update to the latest CRS version (e.g., 3.2 or Managed Rulesets).