XSS Filter Bypass Methodology

Overview

This skill provides a systematic approach for bypassing HTML/JavaScript sanitization filters in authorized security testing contexts (CTF challenges, penetration testing, security research). The methodology emphasizes understanding filter mechanisms before attempting bypasses, avoiding trial-and-error approaches in favor of systematic analysis.

Phase 1: Filter Analysis

Before attempting any bypasses, thoroughly analyze the filter implementation:

Identify the Sanitization Library

• Determine which library performs sanitization (BeautifulSoup, DOMPurify, html-sanitizer, etc.)
• Identify the parser being used (html.parser, lxml, html5lib for BeautifulSoup)
• Research known quirks and bypass techniques for that specific library/parser combination

Map Filter Behavior

Create a systematic map of what the filter blocks vs. preserves:

1. Blocked Elements: Test which HTML tags are removed

   • Script-related: <script>, <noscript>
   • Frame-related: <iframe>, <frame>, <object>, <embed>
   • Other dangerous: <base>, <link>, <meta>

2. Blocked Attributes: Test which attributes are stripped

   • Event handlers: onclick, onload, onerror, onmouseover, etc.
   • URL attributes with javascript: href="javascript:...", src="javascript:..."

3. Preserved Elements: Identify what passes through unchanged

   • Standard HTML elements: <div>, <span>, <p>, <img>, <a>, <style>
   • SVG elements: <svg>, <animate>, <set>
   • Math elements: <math>

Examine Filter Implementation

• Read the filter source code if available
• Look for regex-based filtering (often bypassable)
• Check if filtering is case-sensitive
• Determine if the filter runs once or recursively

Phase 2: Systematic Bypass Categories

Organize bypass attempts by category rather than random trial-and-error:

Category 1: Parser Differential Exploits

Different parsers interpret malformed HTML differently. The server-side sanitizer may parse HTML differently than the browser:

• Nested tag confusion: <noscript><style></noscript><img src=x onerror=alert(1)></style>
• Comment injection: <!--<script>-->alert(1)<!--</script>-->
• Encoding mismatches: UTF-7, charset switching

Category 2: Alternative JavaScript Execution Vectors

If <script> is blocked, identify other execution paths:

• SVG with script: <svg><script>alert(1)</script></svg>
• SVG event handlers: <svg onload=alert(1)>
• SVG animate: <svg><animate onbegin=alert(1)>
• Math elements: <math><maction actiontype="statusline#http://evil">

Category 3: Event Handler Variations

If standard event handlers are blocked:

• Less common events: onfocus, onblur, onanimationend, ontransitionend
• Attribute injection: Breaking out of attribute context
• Data attributes with event delegation

Category 4: URL-Based Execution

• javascript: protocol in href: <a href="javascript:alert(1)">
• data: URLs: <a href="data:text/html,<script>alert(1)</script>">
• Encoded payloads: URL encoding, HTML entities, mixed encoding

Category 5: CSS-Based Attacks

• CSS expressions (legacy IE): <div style="background:expression(alert(1))">
• CSS injection for data exfiltration
• @import with data URLs

Phase 3: Testing Methodology

Build a Testing Harness First

Before testing individual payloads, create infrastructure for efficient testing:

# Example: Test multiple payloads at once
payloads = [
    '<script>alert(1)</script>',
    '<img src=x onerror=alert(1)>',
    '<svg onload=alert(1)>',
    # ... more payloads
]

for payload in payloads:
    filtered = apply_filter(payload)
    print(f"Input: {payload}")
    print(f"Output: {filtered}")
    print(f"Preserved: {payload == filtered}")
    print("---")

Two-Stage Verification

1. Stage 1 - Filter preservation: Does the payload survive the filter?
2. Stage 2 - Browser execution: Does the filtered payload execute in the browser?

Run Stage 1 tests first to eliminate non-viable candidates before slower browser testing.

Document All Attempts

Maintain a log of:

• What was tried
• Why it failed (filtered out vs. didn't execute)
• Insights gained for next attempt

Phase 4: Verification and Validation

Multiple Verification Steps

• Run the verification test multiple times
• Check all success criteria, not just the primary indicator
• Examine the filtered output for anomalies (duplicate tags, malformed HTML)

Cross-Browser Considerations

• A bypass working in Chrome may not work in Firefox or Safari
• Identify which browser the test environment uses
• Document browser-specific behavior

Handle Verification Discrepancies

If initial tests pass but final verification fails:

• Re-read the task requirements
• Check for additional validation steps
• Examine timing issues or race conditions
• Verify the test environment matches expectations

Common Pitfalls to Avoid

Premature Success Declaration

• Do not celebrate after a single test pass
• Run additional verification rounds
• Check the overall task status, not just test output

Workarounds vs. Understanding

• Avoid hacky workarounds that mask underlying issues
• If the test expects files in unexpected locations, understand why before copying files
• Workarounds may introduce inconsistencies

Inefficient Trial-and-Error

• Do not try random XSS vectors without a systematic framework
• Research before attempting; look up known bypass techniques first
• Understand why previous attempts failed before trying similar approaches

Ignoring Malformed Output

• Pay attention to duplicate or malformed tags in filtered output
• Malformed output may indicate an unstable bypass
• Question whether the solution is reliable

Missing Root Cause Analysis

When a bypass works, understand WHY:

• How does the sanitizer parse the payload?
• What browser behavior enables execution?
• Is this a stable, reliable technique or a fragile edge case?

Reference Resources

For authorized security testing contexts, these resources provide bypass techniques:

• OWASP XSS Filter Evasion Cheat Sheet
• PortSwigger Web Security Academy
• HTML5 Security Cheatsheet
• BeautifulSoup documentation on parser differences
• Browser-specific parsing quirks documentation