In the ever-evolving landscape of web security, Cross-Site Scripting (XSS) stands as one of the most pernicious vulnerabilities. XSS allows attackers to inject malicious scripts into web pages which then run on another user’s browser. These injected scripts can lead to a variety of malicious actions, such as stealing session cookies or defacing web pages. To counteract these vulnerabilities, developers deploy multiple techniques. But as developers fortify defenses, attackers refine their techniques to bypass these security measures. This article will explore some techniques used to bypass XSS filters and how developers can stay vigilant.

Technique: Attackers set a limit on the payload’s length, hoping the filter doesn’t recognize lengthy malicious scripts.

def filter_input(data):
    if len(data) > 50: # Assuming filter has a set length limit of 50 characters
        return "Data too long"
    # ... further processing
# Attackers payload
payload = "<img src=x onerror=alert('XSS')>"
filter_input(payload)

Output: If the length of the payload is under the limit, it could bypass the filter.

Technique: Filters block all event handlers to prevent malicious scripts using them.

def filter_input(data):
    event_handlers = ["onerror", "onload", "onclick"]
    for handler in event_handlers:
        if handler in data:
            return "Suspicious event handler detected"
    # ... further processing
# Attackers payload
payload = "<img src=x onerror=alert('XSS')>"
filter_input(payload)

Output: If the payload uses an event handler not in the filter’s list, it might bypass the filter.

Technique: Filters block certain HTML tags like <script> and <iframe>.

def filter_input(data):
    disallowed_tags = ["<script>", "<iframe>"]
    for tag in disallowed_tags:
        if tag in data:
            return "Blocked tag detected"
    # ... further processing
# Attackers payload
payload = "<script>alert('XSS')</script>"…