Reflected XSS Flaw Enables Attackers to Evade Amazon CloudFront Protection Using Safari

A recent bug bounty discovery has drawn attention to a browser-specific reflected Cross-Site Scripting (XSS) vulnerability on help-ads.target.com.

This flaw was found to bypass Amazon CloudFront’s Web Application Firewall (WAF) protections but could only be exploited on the Safari browser. The finding highlights the importance of testing for diverse browser behaviors during security assessments.

Discovery

While exploring the help-ads.target.com domain, a bug bounty hunter attempted to inject a classic XSS payload using a simple HTML anchor:

<a href="https://gbhackers.com/reflected-xss-flaw-evade-amazon-cloudfront/JavaScript:alert(1)">Test XSS</a>

Amazon CloudFront’s WAF blocked this and other standard payloads, appearing to provide robust protection against reflected XSS.

However, during deeper testing, it was observed that the parameter s in the URL was reflecting user input without proper sanitization.

This led the researcher to experiment with payload variants that could take advantage of subtle parsing differences in different browsers.

To bypass the WAF, various encoding techniques and payloads were tried, such as JavaScript event handlers with inline actions:

<a onclick="alert(1)">Click me</a>

And hexadecimal encoding to obfuscate the alert function:

<a href="https://gbhackers.com/reflected-xss-flaw-evade-amazon-cloudfront/javascript:\u0061lert(1)">Click me</a>

Despite these attempts, most browsers and the WAF continued to block execution, demonstrating their resilience against common XSS techniques.

However, the researcher found that certain payloads especially those crafted around Safari’s handling of reflected content were not stopped by CloudFront’s screening.

Successful Exploitation

A breakthrough occurred when a unique payload, specifically targeting how Safari interprets URLs and reflected script blocks, successfully executed.

This payload exploited subtle differences in Safari’s input parsing, allowing the reflected JavaScript to run:

<a href="https://gbhackers.com/reflected-xss-flaw-evade-amazon-cloudfront/javascript:alert(document.cookie)">Test XSS</a>

While other browsers would block or neutralize this payload, Safari allowed it to execute when clicking the crafted link.

The vulnerability was only exploitable in Safari, demonstrating that WAF solutions like CloudFront may not fully account for browser-specific behaviors during filtering.

This vulnerability means that attackers could potentially trick Safari users into running malicious scripts by sending them crafted links.

Sensitive information such as cookies could then be exposed or hijacked. Once confirmed, the bug bounty hunter responsibly reported the issue to the platform owners, allowing them to fix the flaw before any real-world exploitation could occur.

This case underscores the importance of:

• Testing across multiple browsers during security assessments.
• Avoiding assumptions about WAF coverage, as some browser-specific quirks can evade generic protections.
• Rigorous output encoding and sanitization, rather than relying solely on WAFs.

Bug bounty hunters and security teams should keep browser diversity in mind and update defenses to address subtle interpretation differences, particularly as attackers increasingly exploit client-side behaviors that differ between browsers.

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