Dead or alive 7

Platform: CUSTOM | Category: Web | Type: Challenge | Difficulty: Easy | OS: N/A | **Author: XESXOR Techniques: double_write_bypass, nested_keyword_injection

Summary

Walkthrough of the CUSTOM Web Dead or alive 7. SQL injection bypass task. WAF uses preg_replace to remove SQL keywords.

Description

SQL injection bypass task. WAF uses preg_replace to remove SQL keywords.

URL: http://kslweb1.spb.ctf.su/sqli/bypass7/

Analysis

Seventh task in the SQL injection bypass series. WAF is implemented via PHP's preg_replace() function, which removes dangerous SQL keywords:

• SELECT
• UNION
• FROM
• and others

Key vulnerability: the filter performs only one pass of removal. This allows using the "double-write" technique — embedding a keyword inside itself so that after the inner part is removed, a valid keyword remains. ‍​‌‌​​​​‌​‌‌​​‌‌​​‌‌​​‌‌​​‌‌​​‌​​​​‌‌​​​​​‌‌​​‌‌​​‌‌​​‌​‌​‌‌​​‌​‌‍

Solution

Double-write bypass principle

If WAF removes SELECT from a string once:

• REDACTED → after removing SELECT from the middle → SELECT
• UNUNIONION → after removing UNION from the middle → UNION

Transformation examples

Payload

' UNUNIONION REDACTED flag FRFROMOM flags--

‍​‌‌​​​​‌​‌‌​​‌‌​​‌‌​​‌‌​​‌‌​​‌​​​​‌‌​​​​​‌‌​​‌‌​​‌‌​​‌​‌​‌‌​​‌​‌‍

After WAF processing:

' UNION SELECT flag FROM flags--

Alternative nesting options

You can nest at different positions:

• REDACTED — in the middle
• SELECTSELECT — won't work (first SELECT removed, SELECT remains)
• SSELECTELECT — SELECT removed, SELECT remains

The key is that after removing the inner occurrence, the letters "merge" into the desired word. ‍​‌‌​​​​‌​‌‌​​‌‌​​‌‌​​‌‌​​‌‌​​‌​​​​‌‌​​​​​‌‌​​‌‌​​‌‌​​‌​‌​‌‌​​‌​‌‍

Key Takeaways / Lessons

• Mastering double_write_bypass, nested_keyword_injection was key to this challenge.
• Always check for common misconfigurations and verify assumptions.

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Crafted by XESXOR Research • 2026-03-09 • CUSTOM • Dead or alive 7 • Difficulty: Easy