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SKILL: WAF Bypass Techniques It is listed under Coding & Dev Tools on SkillMD.

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ComeOnOliver (@comeonoliver) published this skill. Their other Agent Skills are listed on their SkillMD profile.

Offensive Waf Bypass by comeonoliver

Name: Offensive Waf Bypass
Author: ComeOnOliver

SKILL: WAF Bypass Techniques

Metadata

Skill Name: waf-bypass
Folder: offensive-waf-bypass
Source: https://github.com/SnailSploit/offensive-checklist/blob/main/waf-bypass.md

Description

WAF bypass techniques checklist: encoding bypass (URL/HTML/Unicode/double encoding), case variation, comment injection, HTTP header manipulation, chunked encoding, IP rotation, timing attacks, and payload obfuscation per WAF vendor. Use when WAF is blocking payloads during web app tests.

Trigger Phrases

Use this skill when the conversation involves any of: WAF bypass, web application firewall bypass, URL encoding, double encoding, Unicode bypass, comment injection, HTTP header bypass, chunked encoding, IP rotation, payload obfuscation, WAF evasion

Instructions for Claude

When this skill is active:

Load and apply the full methodology below as your operational checklist
Follow steps in order unless the user specifies otherwise
For each technique, consider applicability to the current target/context
Track which checklist items have been completed
Suggest next steps based on findings

Full Methodology

WAF Bypass Techniques

A Web Application Firewall (WAF) is a security tool that protects web applications from various attacks by analyzing HTTP requests and applying rules to identify and block suspicious traffic. This document outlines effective techniques to bypass WAF protections during security assessments.

graph TD
    A[Client] -->|HTTP Request| B[WAF]
    B -->|Filtered Request| C[Web Application]
    C -->|Response| D[WAF]
    D -->|Filtered Response| A

    E[Attacker] -->|Malicious Request| B
    B -->|Blocked| E

    F[Attacker with<br>Bypass Techniques] -->|Obfuscated<br>Malicious Request| B
    B -->|Request Appears Legitimate| C

    style B fill:#f9a,stroke:#333,color:#333
    style E fill:#f66,stroke:#333,color:#333
    style F fill:#f66,stroke:#333,color:#333

WAF Overview

WAFs operate in two primary models:

Negative Model (Blacklist-based): Uses pre-set signatures to block known malicious requests, effective against common vulnerabilities
Positive Model (Whitelist-based): Only allows requests that match specific patterns, blocking everything else by default

Popular WAFs

Cloudflare - Identified by __cf_bm, cf_clearance, cf_chl_* cookies and "/cdn-cgi/" routes (cookies rotate roughly every 30 min)
Akamai
Imperva/Incapsula - Look for "X-CDN: Incapsula" headers
AWS WAF - Commonly sets AWSALB or AWSALBCORS cookies
Sucuri - Check for "X-Sucuri-ID" headers
DataDome
F5 Networks
Barracuda
Fortinet
ModSecurity - Open-source WAF, commonly used with Apache
NAXSI - Open-source WAF for NGINX
Azure Front Door
Fastly Next-Gen WAF
Cloudflare AI WAF
Radware
Coraza - Modern open‑source WAF written in Go

Detection Methods

flowchart LR
    A[WAF Detection Methods] --> B[Control Page Analysis]
    A --> C[HTTP Header Inspection]
    A --> D[Cookie Analysis]
    A --> E[Route Examination]
    A --> F[JavaScript Object Analysis]

    B --> B1[Block Pages]
    B --> B2[Challenge Pages]
    B --> B3[CAPTCHA Systems]

    C --> C1[Custom Security Headers]
    C --> C2[Server Headers]
    C --> C3[CDN Markers]

    D --> D1[WAF-specific Cookies]
    D --> D2[Challenge Cookies]

    E --> E1[CDN Paths]
    E --> E2[WAF Asset Routes]

    F --> F1[Protection Objects]
    F --> F2[Challenge Scripts]

    style A fill:#f96,stroke:#333,stroke-width:2px,color:#333

Inspect Control Pages - Many WAFs display specific pages when blocking access
Analyze HTTP Headers - Check response headers for WAF-specific indicators
Examine Cookies - WAFs often set specific cookies (e.g., cf_clearance for Cloudflare)
Look for Specific Routes - e.g., /cdn-cgi/ for Cloudflare
Check JavaScript Objects - WAFs inject specific JS objects (e.g., \_cf_chl_opt)
JA3/JA4/TLS Fingerprints - Compare your client’s TLS fingerprint with common browser fingerprints; mismatches are often blocked.
HTTP/2/3 Support - Identify protocol negotiation; some WAF policies differ by protocol.

Fingerprinting WAFs

Some specific fingerprints of common WAFs:

Apache Generic: Writing method type in lowercase (e.g., get instead of GET)
IIS Generic: Using tabs before method (e.g., GET /login.php HTTP/1.1)
Cloudflare: Challenge pages with JavaScript verification
ModSecurity: Specific error messages and block pages
AWS WAF: AWSELB cookies and specific headers
Fastly Next‑Gen WAF: fastly-debug-* headers (when enabled), service IDs in responses
Cloudflare Bot Fight Mode/AI WAF: presence of Turnstile, managed challenge flows, Bot Management headers

Bypass Techniques

1. Use Residential IPs

Data center IPs are easily detected by WAFs
Residential IPs appear legitimate as they come from ISPs
Mobile IPs are useful for websites showing different content for mobile users
Use proxy rotation services to avoid IP-based rate limiting

2. Fortify Headless Browsers

Headless browsers often set special headers or variables that help WAFs recognize them as automation tools. Use the following libraries to make headless browsers appear more human-like:

undetected_chromedriver for Selenium
puppeteer-extra-plugin-stealth for Puppeteer/Playwright
playwright-extra with playwright-extra-plugin-stealth for Playwright

3. Use Web Scraping APIs

Services like ZenRows implement sophisticated anti-bot techniques including:

Premium proxies
JS rendering
WAF bypassing capabilities
CAPTCHA solving
Protocol emulation (HTTP/2 push/HTTP/3), humanization features

4. Call the Origin Server Directly

Use services like Shodan or tools like CloudFlair to find the origin server IP
Forge requests to make them appear as coming from a valid domain
Bypass the WAF layer completely by contacting the server directly
Check historical DNS records (e.g., with securitytrails API) – 2024 research found ~40 % of Fortune‑100 origins exposed via stale A records
Check Alt-Svc leakage for HTTP/3, misconfigured Workers/Edge redirects exposing bucket hostnames

sequenceDiagram
    participant Attacker
    participant WAF
    participant Origin as Origin Server

    Note over Attacker,Origin: Normal Route (Blocked)
    Attacker->>WAF: Malicious Request
    WAF->>Attacker: Request Blocked

    Note over Attacker,Origin: Origin Bypass
    Attacker->>Attacker: Find Origin IP (Shodan, etc.)
    Attacker->>Origin: Direct Request with Host Header
    Origin->>Attacker: Response (WAF Bypassed)

5. Utilize WAF Solvers

BypassWAF - Overcomes firewalls by looking for old DNS A records
Cfscrape - Python module to bypass Cloudflare protection
Cloudscraper - Python library to avoid Cloudflare waiting room

6. Reverse Engineer JavaScript Challenges

Analyze injected JavaScript snippets used by WAFs
Understand how the challenge works
Create custom solutions that satisfy the challenge requirements

7. CAPTCHA Bypass Techniques

Cloudflare Turnstile – low‑friction CAPTCHA replacement; see cloudflare_turnstile_bypass PoC (GUI‑driven, YMMV)
Use CAPTCHA solving services - Though often expensive and not always reliable
Utilize automated CAPTCHA solvers - Various libraries exist for different CAPTCHA types
Implement prevention measures - Use techniques that prevent CAPTCHAs from appearing

Cloudflare Turnstile Bypass

// Visibility bypass: Complete Turnstile in hidden iframe
const iframe = document.createElement("iframe");
iframe.style.display = "none";
iframe.src = "https://challenges.cloudflare.com/...";
document.body.appendChild(iframe);

// Token reuse test: Check if cf_clearance tokens are single-use
// Save token from successful solve, attempt reuse across sessions

// Timing attack: Solve challenge, delay submission to test token expiry
setTimeout(() => submitWithToken(token), 60000);

Tools:

cf-clearance-scraper (2024 fork with Turnstile support)
cloudflare-turnstile-solver - Automated solving with browser automation
turnstile-bypass - Research tool for testing Turnstile implementations

8. Avoid Honeypot Traps

Don't interact with invisible elements (display: none)
Skip elements with zero opacity or positioned off-screen
Analyze HTML structure to identify potential honeypots
Avoid filling hidden form fields

9. Defeat Browser Fingerprinting

Randomize or spoof canvas fingerprinting results
Use plugins that add noise to fingerprint data
Modify user agent and other HTTP headers periodically
Spoof hardware and software features to appear as different devices

10. TLS Fingerprinting Evasion

Modify TLS parameters during handshake
Use libraries that allow customization of SSL/TLS configuration
Match TLS fingerprints of legitimate browsers
Randomise both JA3 and JA4 fingerprints using libraries such as noble-tls or ja4py
Consider using the abuse-ssl-bypass-waf tool to find supported SSL/TLS ciphers
Align cipher suites, ALPN order, and signature algorithms with target browser versions.

11. Simulate Human Behavior

Add random delays between requests
Follow logical navigation patterns
Implement mouse movements and scrolling behavior
Interact with page elements naturally
Vary request patterns and timing
Use browser engines (Playwright/Selenium) with stealth plugins to match DOM APIs and canvas behavior.

mindmap
  root((WAF Bypass Techniques))
    Network Level
      Residential IPs
      Origin Server Direct
      IP Rotation
      Distributed Requests
    Browser Emulation
      Headless Browser Fortification
      TLS Fingerprint Evasion
      JS Challenge Solving
      Human Behavior Simulation
    Request Manipulation
      Header Manipulation
      Parameter Pollution
      HTTP Protocol Tricks
      Encoding Variations
    Attack Specific
      SQLi Bypasses
      XSS Obfuscation
      JSON-Based Injection
      Protocol Level Bypass
    Tools & Services
      WAF Solvers
      CAPTCHA Services
      Proxy Rotators
      Web Scraping APIs

12. SQL Injection Specific WAF Bypasses

Case variation: SeLeCt, UnIoN instead of SELECT, UNION
Comment injection: UN/**/ION SE/**/LECT to break up keywords
Alternate encodings:
- URL encoding: UNION → %55%4E%49%4F%4E
- Hex encoding: SELECT → 0x53454C454354
- Unicode encoding
Whitespace manipulation: UNION/**/SELECT or using tabs/newlines/carriage returns
Numeric representations:
- 1 → 1-0, 1+0, CHAR(49)
String concatenation:
- MySQL: CONCAT('a','b')
- Oracle: 'a'||'b'
- MSSQL: 'a'+'b'

Null byte injection:

%00' UNION SELECT password FROM Users WHERE username='xyz'--

Double encoding:

First pass: / → %2f
Second pass: %2f → %252f

SQLMAP tamper scripts:
- Use Atlas tool for suggesting tamper scripts
- Try multiple tamper scripts in combination
- Customize tamper scripts for specific WAFs
JSON-Based SQL Injection (CVE-2023-50969):
- Many WAFs (AWS, Cloudflare, F5, Imperva) don't properly support JSON syntax in SQL
- Example: {"id": {"$gt": "' OR 1=1--"}}
- Use SQLMap with JSON parameter support for exploitation
Leverage mixed encodings (half‑width Unicode, overlong UTF‑8), embedded comments, and case folding differences.

graph TD
    A[SQL Injection WAF Bypass] --> B[Syntax Manipulation]
    A --> C[Character Encodings]
    A --> D[Alternative Representations]
    A --> E[SQLMap Tamper Scripts]

    B --> B1[Case Variation<br>SeLeCt]
    B --> B2[Comment Insertion<br>UN/**/ION]
    B --> B3[Whitespace Manipulation<br>UNION++++SELECT]

    C --> C1[URL Encoding<br>%55%4E%49%4F%4E]
    C --> C2[Hex Encoding<br>0x53454C454354]
    C --> C3[Double Encoding<br>%252f]
    C --> C4[Unicode Encoding]

    D --> D1[String Alternatives]
    D --> D2[Numeric Alternatives]
    D --> D3[JSON-Based Injection]

    E --> E1[Multiple Script Chaining]
    E --> E2[WAF-Specific Scripts]

    style A fill:#f96,stroke:#333,stroke-width:2px,color:#333
    style B,C,D,E fill:#bbf,stroke:#333,color:#333

13. XSS-Specific WAF Bypasses

Context-Aware Payloads: Craft payloads based on where they will be inserted:

# HTML Context
<img src=x

# HTML Attribute Context
"

# JavaScript Context
';alert(1);//

Mutation XSS (mXSS): Use HTML parsing quirks to bypass sanitizers:
```
<noscript><p title="</noscript><img src=x
```
Alternative Tag Usage:
```
<svg
<body
<details open
```

JavaScript Obfuscation:

<script>eval(atob('YWxlcnQoMSk='))</script>
<img src=x

Avoiding Blacklisted Words:
```
<script>al\u0065rt(1)</script>
<svg
```

Protocol Obfuscation:

<a href="javas&#99;ript:alert(1)">Click Me</a>
<a href="data:text/html;base64,PHNjcmlwdD5hbGVydCgxKTwvc2NyaXB0Pg==">Click Me</a>

CSS-Based Attacks:

<style>@keyframes x{}</style><xss style="animation-name:x"
<div style="background-image:url('javascript:alert(1)')">

CSP Bypass Techniques:
- JSONP endpoint abuse: <script src="https://allowed-domain.com/jsonp?callback=alert(1)"></script>
- DOM clobbering: <form id=self><input name=location> then self.location
- Using allowed domains: Find script sources whitelisted in CSP that can be abused
- Abuse JSONP, postMessage, or gadget endpoints on allowed origins; check Trusted Types gaps.

Polyglot XSS: Payloads that work in multiple contexts:

jaVasCript:/*-/*`/*\`/*'/*"/**/(/* */oNcliCk=alert() )//%0D%0A%0D%0A//</stYle/</titLe/</teXtarEa/</scRipt/--!>\x3csVg/<sVg/oNloAd=alert()//>\x3e

14. HTTP Protocol Level Bypasses

HTTP Method Obfuscation:
- Using uncommon HTTP methods
- Modifying case of HTTP methods (e.g., gEt instead of GET)
- Adding tabs or spaces before HTTP methods
Request Header Manipulation:
- Adding excessive headers to confuse WAF processing
- Using duplicate headers with different values
- Adding headers that make the request appear to come from internal networks or spoof identity. Many applications, especially behind misconfigured reverse proxies, trust headers like X-Forwarded-For or X-Forwarded-Host to determine the client's IP or the requested host. Manipulating these can lead to various vulnerabilities:
  - Password Reset Poisoning: Injecting headers like X-Forwarded-Host: attacker.com can cause the application to generate password reset links pointing to the attacker's domain.
  - Bypassing IP Restrictions: Using X-Forwarded-For: <trusted_ip> or similar headers can bypass IP-based access controls if the server trusts the header.
  - Open Redirects: Manipulating host-related headers (X-Forwarded-Host, Referer) can redirect users to malicious sites.
  - SSRF: Headers like X-Forwarded-For or X-Real-IP can be manipulated to target internal IPs (e.g., 169.254.169.254 for AWS metadata service).
- Common Headers for Spoofing/Bypass: Attackers may use a variety of headers to manipulate server behavior or bypass WAF rules. Some common examples include:
```
X-Forwarded-Host: attacker.com
X-Forwarded-For: 127.0.0.1
X-Client-IP: 127.0.0.1
Client-IP: 127.0.0.1
X-Real-IP: 127.0.0.1
X-Originating-IP: 127.0.0.1
X-Remote-IP: 127.0.0.1
X-Remote-Addr: 127.0.0.1
Forwarded: for=127.0.0.1;host=attacker.com
Referer: attacker.com
Origin: null / attacker.com
# And many variations like X-Forwarded, X-Forwarded-By, etc.
```
HTTP Parameter Pollution:
- Using multiple parameters with the same name
```
?id=safe&id=malicious
```
- Mix sources (query, body, headers, cookies) to exploit precedence ambiguities across proxies/gateways.
HTTP RFC Inconsistencies:
- Exploiting differences in how servers handle HTTP specification
- Using newline variations (CR, LF, CRLF)
- Adding unexpected line breaks in headers
Host Header Spoofing:
- The Host header specifies the hostname the client wants to connect to, crucial for virtual hosting and reverse proxies.
- If a WAF/proxy relies solely on the Host header for filtering, it can be bypassed by sending a legitimate Host header value while the actual connection (e.g., SNI in TLS, IP address) points to a blocked or different target.
- Mechanism:
  1. Client connects to evil.example.com (e.g., IP address of evil.example.com).
  2. The SNI in the TLS handshake might be evil.example.com.
  3. The HTTP Host header is set to legit.example.net (a domain allowed by the WAF).
  4. The WAF sees Host: legit.example.net and may allow the request, which is then routed to evil.example.com by the underlying infrastructure if it uses the Host header for routing or if the IP was already resolved to evil.example.com.
- Example with curl:
```
# SNI will be evil.example.com (matching the URL)
# Host header is spoofed to legit.example.net
curl -H "Host: legit.example.net" https://evil.example.com
```
```
sequenceDiagram
    participant Alice as Alice (Attacker)
    participant Proxy as WAF/Proxy
    participant TargetServer as evil.example.com
    participant AllowedServer as legit.example.net

    Alice->>Proxy: TLS Handshake (SNI: evil.example.com, connects to IP of evil.example.com)
    Proxy-->>TargetServer: Forwards TLS Handshake
    TargetServer-->>Proxy: TLS Handshake Response
    Proxy-->>Alice: TLS Handshake Response

    Alice->>Proxy: HTTP Request (Host: legit.example.net) [Encrypted in TLS]
    Note over Proxy: Proxy inspects Host header 'legit.example.net' (Allowed)
    Proxy->>TargetServer: HTTP Request (Host: legit.example.net)
    TargetServer->>Proxy: HTTP Response
    Proxy->>Alice: HTTP Response
```
- Applicability:
  - Bypasses filters that only check the Host header.
  - Can bypass restrictions like file upload size limits if the spoofed Host points to a domain with fewer restrictions.
- Protection:
  - WAFs should verify that the hostname in the SNI and the Host header are identical.
  - If they mismatch, the request should be blocked (e.g., Fortinet's "Domain Fronting Protection").
  - This check prevents the simple Host Header Spoofing described.
  - Enforce strict ALPN and certificate SAN checks; for HTTP/2 enforce :authority ≈ SNI.

15. Advanced Techniques

HTTP Request Manipulation:
- Multiple Content-Encoding Headers: Send multiple headers with conflicting values
```
Content-Encoding: invalid
Content-Encoding: gzip
```
- 8k Bypass: Overloading request parameters to overwhelm WAF processing
- Unicode Normalization Exploitation: Using characters that normalize to malicious values
```
# Using NFKD normalization forms
unicodedata.normalize('NFKD', payload)
```
WAF Fuzzing Automation:
- Using tools like "Wafer" to automate WAF bypass discovery
- Systematically testing edge cases in WAF rule processing
- Combining multiple evasion techniques in automated sequences
- Add feedback‑driven payload evolution (genetic algorithms) to evade ML‑based detections.
Advanced Encoding Techniques:
- HTML Tag Encodings: Injecting symbols at specific points
- Specialized DOM Events: Triggering DOM events that bypass filtering
- Double Unicode Escaping: \u00\u0036\u0031 for character '1'
- MIME Smuggling: multipart boundary tricks, mixed Content‑Type declarations through chained proxies.
IP Fragmentation Attacks:
- Fragment HTTP requests across multiple packets
- WAFs may reassemble these differently than the target server
- Beware of modern middleboxes normalizing or dropping fragments; measure viability before use.
Timing-Based Techniques:
- Leveraging WAF timeout discrepancies
- Sending requests when WAF processing is at peak load
- Use request coalescing and long‑lived HTTP/2 streams to alter inspection timing.
Machine Learning-Based Evasion:
- Using ML to generate payloads that evade detection
- Adjusting payloads based on WAF feedback
- Gradually evolving attack patterns to avoid signature detection
- Cloudflare AI WAF Attack Score can be tricked by embedding payloads in lesser‑weighted features (e.g., multipart filenames)
- Vary entropy and payload tokenization to mislead n‑gram/embedding models.
Browser Bugs Exploitation:
- Leveraging browser-specific parsing quirks
- Using differences between how WAFs and browsers interpret content

16. HTTP/2, HTTP/3 & Advanced Protocol Bypasses

HTTP/2 :authority Header Bypass:
- HTTP/2 replaces the Host header with the :authority pseudo-header for the target URI. The :authority field includes the hostname and optionally the port.
- WAFs/proxies not correctly parsing the binary and compressed HTTP/2 protocol may fail to extract or compare the :authority field with the SNI.
- This can bypass domain fronting or Host header validation checks that rely on HTTP/1.1's Host header.
- For instance, some WAFs' domain fronting protection mechanisms may only be effective for HTTP/1.1 and not for HTTP/2.
- Example (curl automatically uses :authority for HTTP/2 when a Host header is provided):
```
curl --http2 -H "Host: legit-looking-domain.com" https://actual-target-server.com/index.html
```
HTTP/3 (QUIC) Bypass:
- HTTP/3 operates over QUIC (UDP), which multiplexes streams and integrates TLS 1.3+.
- If a WAF or proxy does not support or deeply inspect QUIC/HTTP/3 traffic, it cannot analyze the encrypted stream to apply policies based on HTTP content or SNI (within QUIC's TLS handshake).
- Enterprises may block outgoing UDP to non-standard ports, but HTTP/3 typically uses 443/UDP. If allowed, it can be a bypass vector.
- Servers can announce HTTP/3 support via the Alt-Svc HTTP header in responses to HTTP/1.1 or HTTP/2 requests.
- Example (curl):
```
curl --http3 https://example.net
```
HTTP/2 Rapid Reset flood (CVE‑2023‑44487): send a burst of RST_STREAM frames to overwhelm back‑ends while front‑end WAFs proxy at HTTP/1.1.
HTTP/2 CONTINUATION Flooding (2024 research): stress header compression/HPACK handling on middleboxes.

17. GraphQL‑Specific WAF Bypasses

Introspection & alias spamming: use deeply nested aliases to evade depth limits.
Batch operations: bundle many queries in one request to sneak payloads through superficial query‑string filters.
Query name tampering: mask malicious fields behind benign names.
Tools: graphql‑cop, inql, Escape p‑cli for automated fuzzing.
Test batched queries with differing operation names, file upload scalars, and custom directives; many WAFs only parse shallow GraphQL.

18. SNI (Server Name Indication) Manipulation & Evasion

Server Name Indication (SNI) is a TLS protocol extension where the client indicates the hostname it is attempting to connect to during the TLS handshake. This is crucial for servers hosting multiple websites on a single IP address. Since the SNI is typically sent in cleartext (in the ClientHello message, unless Encrypted Client Hello - ECH - is used), web filters can inspect it to enforce policies.

SNI spoofing is a technique to bypass web filters that rely solely or primarily on the SNI value to allow or block requests. The attacker sends an SNI for a whitelisted domain while directing the actual traffic to a different, potentially blocked, server IP address.

Mechanism:

The client initiates a TLS handshake, providing an SNI of an allowed domain (e.g., allowed.example.com) to the web filter/proxy.
Simultaneously, the client directs the actual TCP connection to the IP address of the target server (e.g., 198.51.100.23 for blocked.example.org).
The HTTP Host header in the subsequent (encrypted) HTTP request might also be set to the actual target server (blocked.example.org).

Examples:

Using openssl:

# Connects to IP 198.51.100.23 (blocked.example.org)
# but presents 'allowed.example.com' as SNI
openssl s_client -connect 198.51.100.23:443 -servername allowed.example.com

Using curl:
```
# Tells curl to connect to 198.51.100.23 (IP of blocked.example.org)
# for any request to allowed.example.com.
# The SNI will be 'allowed.example.com'.
# The Host header is explicitly set to 'blocked.example.org'.
curl -k --connect-to allowed.example.com::198.51.100.23 \
     -H "Host: blocked.example.org" \
     https://allowed.example.com
```
- -k: Ignores certificate warnings (since the presented cert for blocked.example.org won't match allowed.example.com).
- --connect-to host1:port1:host2:port2: Resolves host1:port1 to host2:port2. Here, allowed.example.com (default port 443) is directed to 198.51.100.23 (default port 443 if not specified after IP).
- -H "Host: blocked.example.org": Sets the HTTP Host header for the application layer.

Applicability & Proxy Types:

Basic SNI Filters: Effective against filters that only check the SNI value.
Transparent Proxies: Can be effective as the client controls the DNS resolution (or IP choice) and the initial TLS handshake.
Explicit Proxies (Normal/Non-Transparent):
- Can still be effective. The client can specify the target IP address in the HTTP CONNECT request to the proxy.
- The client then performs the TLS handshake through the proxy, sending the spoofed SNI.
- Example with curl and an explicit proxy:
```
curl --proxy http://proxy.internal:8080 -k \
     --connect-to allowed.example.com::198.51.100.23 \
     -H "Host: blocked.example.org" \
     https://allowed.example.com
```
- Proxy Behavior Variance: Some proxies might honor the IP in the CONNECT request, while others might resolve the hostname from the SNI themselves, potentially thwarting the bypass.
TLS Inspecting Proxies: SNI spoofing can sometimes work even if the proxy performs TLS inspection, provided the proxy forwards the client's spoofed SNI and connects to the IP specified in the CONNECT request (rather than resolving the spoofed SNI).

Limitations and Protections:

SNI vs. Certificate Validation: Advanced WAFs/filters compare the SNI with the Subject Alternative Name (SAN) or Common Name (CN) in the server's presented TLS certificate.
- If there's a mismatch, the connection might be flagged or dropped (e.g., FortiGate's "Server certificate SNI check" set to "Strict").
Bypassing Certificate Checks: To bypass this, an attacker would need to control the server certificate of the target (blocked.example.org) to match the spoofed SNI (allowed.example.com). This could involve using a self-signed certificate for allowed.example.com on the attacker-controlled server.
Certificate Chain Verification: Filters should always validate the entire certificate chain and reject connections with untrusted certificates (e.g., self-signed ones if not explicitly trusted by the filter/client policy).
Omitting SNI Bypass:
- The SNI extension in the TLS ClientHello is optional according to RFC 6066.
- Clients can omit SNI by connecting directly to the server's IP address instead of using a hostname.
- Mechanism:
  1. Client establishes a TCP connection to the server's IP address.
  2. TLS handshake begins; ClientHello is sent without the SNI extension.
  3. The server might return a default certificate (e.g., for a generic CDN hostname), potentially causing a mismatch warning that the client might need to ignore (e.g., curl --insecure).
  4. The HTTP Host header in the encrypted request can still be set to the desired target hostname.
- Example (curl):
```
# Request to IP 151.101.194.133, no SNI sent.
# Spoofed Host header: compass-test.global.ssl.fastly.net
curl --insecure -v -H "Host: compass-test.global.ssl.fastly.net" https://151.101.194.133/index.html
```
- Impact: This can bypass WAFs/proxies that rely on the presence of SNI for policy enforcement or domain fronting detection (which compares SNI to Host header).
  - Explicit proxies might block direct IP connections in CONNECT requests if configured to do so.
  - Transparent proxies might be more susceptible if they only inspect SNI.
Encrypted Client Hello (ECH) Bypass:
- ECH (Encrypted Client Hello, successor to ESNI) encrypts most of the ClientHello message, including the SNI.
- This prevents network intermediaries (WAFs, proxies, ISPs) from snooping on the SNI and thus determining the intended destination server.
- If a WAF relies on SNI for filtering or routing, ECH effectively bypasses this by making the SNI unreadable.
- Mitigation (Defensive): Defenders might choose to block ECH connections or attempt to strip ECH information from DNS HTTPS records. However, this can break legitimate ECH-enabled clients and has privacy implications.
- Note: ECH support is expanding in modern browsers; expect increased prevalence in 2025.

19. Domain Fronting

Domain fronting is a technique to bypass web filters and censorship by masking the true destination of a connection. It leverages Content Delivery Networks (CDNs) by sending a request with an SNI (Server Name Indication) of a legitimate, allowed domain, while the HTTP Host header specifies a different, potentially blocked, domain that is also served by the same CDN.