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Zero Stars Challenge

ItemDetail
CategoryImproper Input Validation
DifficultyMedium (1-Star)
Juice Shop Flagscore-board#Zero Stars
Tools UsedBurp Suite (Proxy & Repeater)
StatusSolved

1. Vulnerability Explanation​

The UI only allows ratings between 1–5 stars, but the backend fails to validate the rating value.
An attacker can send 0 directly to the API, and the server accepts it.

2. Security Impact​

  • Attackers can artificially deflate product reputations
  • Demonstrates broken trust boundary between client and server

3. Step-by-Step Exploitation​

Step 1: Intercept the request​

  • Configure browser to use Burp Suite proxy
  • Navigate to Contact Us page
  • Turn Intercept ON in Burp

Step 2: Submit normal feedback​

  • Enter a comment and select 1 star
  • Click Submit

Step 3: Modify the request​

Burp captures this request:

POST /api/Feedbacks HTTP/1.1
Host: localhost:3000

{"message":"Test","rating":1}

Change rating from 1 to 0:

{"message":"Zero stars!","rating":0}

Step 4: Forward​

  • Click Forward in Burp

Step 5: Verify​

  • Check the Score Board
  • Challenge is marked as Solved

4. Why This Works​

The server trusts client-side input. Since there is no validation such as:

if rating < 1:
    return error

the backend accepts the value 0.

5. Mitigation​

Always validate input on the server side.
Reject any rating outside the 1–5 range.

6. Key Takeaways​

  • UI restrictions are for usability, not security
  • Use Burp Suite to inspect what the browser really sends
  • Never trust client-side validation alone