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Outdated Allowlist Challenge

ItemDetail
CategoryUnvalidated Redirects
DifficultyEasy (1-Star)
Juice Shop Flagscore-board#Outdated Allowlist
Tools UsedBrowser DevTools / Burp Suite
StatusSolved

1. Vulnerability Explanation​

The Juice Shop previously accepted cryptocurrency donations (Bitcoin, Dash, Ether) but later removed these options when no donations were received.

However, the developers did not fully clean up the code. The old redirect URLs still remain inside the frontend JavaScript redirect allowlist.

Visiting these cryptocurrency URLs directly does not solve the challenge.
The challenge requires being redirected to them through the application's own redirect mechanism.

2. Security Impact​

  • Outdated allowlist entries expose old, unmaintained URLs
  • Demonstrates poor code cleanup practices
  • Can be abused for phishing or redirect abuse
  • Shows that removed features may still leave traces in production code

3. Step-by-Step Exploitation​

Step 1: Find the redirect allowlist​

  • Open Browser DevTools (F12)
  • Go to Sources tab
  • Search for redirectAllowlist

You will find something similar to:

const redirectAllowlist = new Set([
  'https://github.com/bkimminich/juice-shop',
  'https://blockchain.info/address/1AbKfgvw9psQ41NbLi8kufDQTezwG8DRZm',
  'https://explorer.dash.org/address/Xr556RzuwX6hg5EGpkybbv5RanJoZN17kW',
  'https://etherscan.io/address/0x0f933ab9fcaaa782d0279c300d73750e1311eae6'
])

Outdated crypto addresses:

  • Bitcoin: https://blockchain.info/address/1AbKfgvw9psQ41NbLi8kufDQTezwG8DRZm
  • Dash: https://explorer.dash.org/address/Xr556RzuwX6hg5EGpkybbv5RanJoZN17kW
  • Ether: https://etherscan.io/address/0x0f933ab9fcaaa782d0279c300d73750e1311eae6

Step 2: Intercept with Burp Suite​

  • Configure browser to route traffic through Burp Suite (127.0.0.1:8080)
  • Turn Intercept ON in the Proxy tab

Step 3: Trigger a redirect​

Navigate to any page using redirects (for example the GitHub link).

The request looks like:

GET /redirect?to=https://github.com/bkimminich/juice-shop

Step 4: Modify the redirect target​

Change the to parameter to one of the outdated crypto URLs.

Example:

GET /redirect?to=https://blockchain.info/address/1AbKfgvw9psQ41NbLi8kufDQTezwG8DRZm

Step 5: Forward the request​

  • Click Forward in Burp Suite

Step 6: Verify​

  • Check the Score Board
  • Challenge is marked as Solved

4. Why This Works​

The isRedirectAllowed() function uses:

url.includes(allowedUrl)

to validate redirect targets.

This means any URL containing one of the allowlisted strings can pass validation, including these outdated cryptocurrency URLs.

The challenge only triggers when the redirect occurs through the /redirect endpoint, not by directly visiting the URLs.

5. Mitigation​

  • Remove outdated allowlist entries completely
  • Use exact URL matching instead of includes()
  • Apply proper code cleanup when deprecating features

6. Key Takeaways​

  • Old code often leaves traces after features are removed
  • Search JavaScript files for hidden allowlists and URLs
  • Burp Suite is useful for modifying redirect parameters on the fly
  • includes() can be dangerous for URL validation