SKILL: WAF Bypass Techniques — Evasion Playbook
AI LOAD INSTRUCTION: Covers WAF identification, generic bypass categories (encoding, protocol abuse, HTTP/2, parameter pollution), and a decision tree. For product-specific bypasses (Cloudflare, AWS WAF, ModSecurity, Akamai, etc.), load WAF_PRODUCT_MATRIX.md. Base models often suggest basic encoding but miss protocol-level bypasses and WAF behavioral quirks.
0. RELATED ROUTING
- sqli-sql-injection for payloads to deliver after bypassing WAF
- xss-cross-site-scripting for XSS payloads that need WAF evasion
- request-smuggling when smuggling can route requests around WAF entirely
- http-parameter-pollution HPP is itself a WAF bypass primitive
- csp-bypass-advanced when WAF blocks inline scripts but CSP bypass is available
Product-Specific Reference
Load WAF_PRODUCT_MATRIX.md when you need per-product bypass techniques for Cloudflare, AWS WAF, ModSecurity CRS, Akamai, Imperva, F5 BIG-IP, or Sucuri.
1. PHASE 0 — IDENTIFY THE WAF
Before bypassing, know what you're fighting.
1.1 Tools
| Tool | Usage |
|---|
| Fingerprint WAF vendor from response headers/behavior |
nmap --script=http-waf-detect
| NSE script for WAF detection |
| Manual header inspection | , , , (Cloudflare), , |
1.2 Behavioral Fingerprinting
1. Send benign request → record baseline response (status, headers, body size)
2. Send obvious attack: /?q=<script>alert(1)</script>
3. Compare: 403? Custom block page? Redirect? Connection reset?
4. Block page content reveals WAF: "Cloudflare", "Access Denied (Imperva)", "ModSecurity"
5. If transparent proxy: check response time difference (WAF adds latency)
2. GENERIC BYPASS CATEGORIES
2.1 Encoding Bypasses
| Technique | Example | Bypasses |
|---|
| URL encoding | | Basic string matching |
| Double URL encoding | | WAFs that decode once, app decodes twice |
| Unicode encoding | | IIS-specific Unicode normalization |
| HTML entities | or | WAFs not performing HTML entity decoding |
| Hex encoding (SQL) | = | WAFs matching SQL keywords |
| Octal encoding | | Rare but some parsers handle it |
| Overlong UTF-8 | (invalid encoding for ) | Legacy parsers with loose UTF-8 handling |
| Mixed case | , | Case-sensitive rule matching |
| Null byte | | WAFs that stop parsing at null |
2.2 Chunked Transfer Encoding
Split the payload across HTTP chunks so no single chunk contains the blocked pattern:
http
POST /search HTTP/1.1
3
sel
3
ect
1
4
from
0
WAFs that inspect the full body may not reassemble chunks before matching.
2.3 HTTP/2 Binary Format Bypasses
HTTP/2 transmits headers as binary HPACK-encoded frames. Some WAFs only inspect after downgrading to HTTP/1.1:
- Header names can contain characters illegal in HTTP/1.1
- Pseudo-headers (, ) bypass header-based WAF rules
- H2 → H1 downgrade may introduce request smuggling (see request-smuggling)
2.4 HTTP Parameter Pollution (HPP)
Different servers handle duplicate parameters differently:
| Server | Behavior for |
|---|
| PHP/Apache | Last value: |
| ASP.NET/IIS | Concatenated: |
| Python/Flask | First value: |
| Node.js/Express | Array: |
WAF checks
(benign), app uses
(malicious). Or combine:
→ ASP.NET sees
.
2.5 IP Source Spoofing (Bypass IP-Based Rules)
Headers trusted by some WAFs/apps for client IP:
X-Forwarded-For: 127.0.0.1
X-Real-IP: 127.0.0.1
X-Originating-IP: 127.0.0.1
True-Client-IP: 127.0.0.1
CF-Connecting-IP: 127.0.0.1
X-Client-IP: 127.0.0.1
Forwarded: for=127.0.0.1
Use case: WAF whitelists internal IPs or has different rule sets per source.
2.6 Path Normalization Tricks
| Technique | Example | Effect |
|---|
| Dot segments | or | WAF sees different path than app |
| Double slash | | Some normalizers collapse, WAFs may not |
| URL encoding path | | WAF sees encoded, app decodes |
| Null byte in path | | Legacy: app truncates at null, WAF sees .jpg |
| Backslash (IIS) | | IIS treats as |
| Trailing dot/space | or | OS-level normalization (Windows) |
| Semicolon (Tomcat) | | Tomcat strips after , WAF may not |
2.7 Content-Type Manipulation
WAFs often have format-specific parsers. Switching Content-Type can bypass rules:
Default: Content-Type: application/x-www-form-urlencoded → WAF parses params
Switch: Content-Type: application/json → WAF may not parse JSON body
Switch: Content-Type: multipart/form-data → WAF may not inspect all parts
Switch: Content-Type: text/xml → WAF expects XML, payload in different format
Trick: If app accepts both JSON and form-urlencoded, use JSON — WAFs often have weaker JSON inspection rules.
2.8 Multipart Boundary Abuse
http
------WAFBypass
<script>alert(1)</script>
------WAFBypass--
Variations: long boundary strings, boundary with special characters, missing final boundary, nested multipart.
2.9 Newline & Whitespace Injection
sql
-- SQL keyword splitting
SEL
ECT * FROM users
-- SQL comment insertion
SEL/**/ECT * FR/**/OM users
UN/**/ION SEL/**/ECT 1,2,3
-- Tab/vertical tab as separator
SELECT\t*\tFROM\tusers
2.10 Keyword Splitting & Alternative Syntax
| Blocked | Alternative |
|---|
| , |
| , , ` |
| , <img src=x onerror=alert(1)>
|
| , , (Chrome) |
| , |
| , |
| BENCHMARK(5000000,SHA1('x'))
|
3. PROTOCOL-LEVEL BYPASS TECHNIQUES
3.1 Request Line Abuse
http
GET /path?q=attack HTTP/1.1 ← WAF inspects
vs.
http
GET http://target.com/path?q=attack HTTP/1.1 ← Absolute URI: some WAFs miss the path
3.2 Header Injection via CRLF
If WAF inspects original headers but app processes injected ones:
X-Custom: value\r\nX-Forwarded-For: 127.0.0.1
3.3 Connection-State Bypass
1. Establish connection through WAF (normal request)
2. On same keep-alive connection, send attack request
3. Some WAFs reduce inspection on subsequent requests in same connection
4. WAF BYPASS DECISION TREE
Payload blocked by WAF?
├── Identify WAF (wafw00f, response headers, block page)
│
├── Try encoding bypasses
│ ├── URL encode payload → still blocked?
│ ├── Double URL encode → still blocked?
│ ├── Unicode/overlong UTF-8 → still blocked?
│ ├── Mixed case keywords → still blocked?
│ └── HTML entities (for XSS) → still blocked?
│
├── Try protocol-level bypasses
│ ├── Switch Content-Type (JSON, multipart, XML)
│ │ └── App accepts alternate format? → re-send payload
│ ├── HTTP Parameter Pollution (duplicate params)
│ ├── Chunked Transfer-Encoding to split payload
│ ├── HTTP/2 direct if available (binary framing bypass)
│ └── Request line: absolute URI format
│
├── Try path-based bypasses
│ ├── Path normalization (/./path, //path, ;param)
│ ├── Different HTTP method (POST vs PUT vs PATCH)
│ └── Alternate endpoint serving same function
│
├── Try payload mutation
│ ├── SQL: comments (/**/), alternative functions, hex literals
│ ├── XSS: alternative tags/events, JS template literals
│ ├── RCE: wildcard abuse, string concatenation, variable expansion
│ └── Check WAF_PRODUCT_MATRIX.md for vendor-specific mutations
│
├── Try IP-source bypass
│ ├── X-Forwarded-For / True-Client-IP spoofing
│ ├── Access origin server directly (bypass CDN)
│ └── Find origin IP (Shodan, historical DNS, email headers)
│
└── Try request smuggling to skip WAF entirely
└── See ../request-smuggling/SKILL.md
5. COMMON MISTAKES & TRICK NOTES
- Test bypass with actual exploitation, not just 200 OK: WAF may return 200 but strip the payload silently.
- WAFs often have size limits: Very large request bodies (>8KB–128KB depending on WAF) may bypass inspection entirely.
- Rate limiting ≠ WAF: Getting 429s is rate limiting, not payload blocking. Different bypass needed.
- CDN caching: If the WAF is at CDN level, cached responses bypass WAF on subsequent requests. Poison cache with clean request, exploit cache.
- Origin server direct access: If you find the origin IP behind CDN/WAF, connect directly — WAF is bypassed completely.
- Multipart file upload fields: WAFs often skip inspection of file content in multipart uploads — embed payload in filename or file content if reflected.
6. DEFENSE PERSPECTIVE
| Measure | Notes |
|---|
| WAF + application-level input validation | WAF is a layer, not a fix |
| Parameterized queries | Eliminates SQLi regardless of WAF |
| CSP + output encoding | Eliminates XSS regardless of WAF |
| Regularly update WAF rules | Vendor signatures lag behind new bypasses |
| Deny by default, not block-list | Allowlist valid input patterns |
| Log and alert on WAF blocks | Bypass attempts are visible in logs |