Think your WAF has you covered? Think again. Not monitoring JavaScript this holiday season is a critical oversight that allows attackers to steal payment data while WAFs and intrusion detection systems are unaware. With the 2025 shopping season just weeks away, you need to close the visibility gap now.
Get your complete holiday security handbook here.
bottom line up front
The 2024 holiday season saw a major attack on website code. The Polyfill.io breach affected more than 500,000 websites, and the September Cisco Magecart attack targeted holiday shoppers. These attacks exploited weaknesses in third-party code and online stores during peak shopping periods when attacks increased by 690%.
Looking ahead to 2025: What security measures and monitoring should online retailers take today to prevent similar attacks while using the necessary third-party tools?
As holiday shopping traffic increases, companies are hardening their servers and networks, but critical weaknesses remain unmonitored. It is a browser environment where malicious code runs covertly on a user’s device, stealing data and bypassing standard security.
Client-side security gap
Recent industry research has revealed the worrying extent of this security gap.
These statistics highlight a fundamental shift in the threat landscape. As organizations strengthen their server-side defenses through WAFs, intrusion detection systems, and endpoint protection, attackers have adapted by targeting browser environments where traditional monitoring tools are inadequate because:
Visibility limitations: Server-side monitoring tools cannot monitor JavaScript execution within a user’s browser. WAF and network monitoring solutions miss attacks that operate entirely within the client environment. Encrypted traffic: Modern web traffic is encrypted over HTTPS, making it difficult for network monitoring tools to inspect the content of data sent to third-party domains. Dynamic nature: Client-side code can change its behavior based on user actions, time of day, or other factors, making static analysis insufficient. Compliance gap: Regulations such as PCI DSS 4.0.1 now focus on client-side risks, but guidance on client-side data protection remains limited.
Understand client-side attack vectors
E-skimming (Magecart)
Perhaps the most notorious client-side threat, Magecart attacks involve injecting malicious JavaScript into e-commerce sites to steal payment card data. The 2018 British Airways breach, which exposed the payment details of 380,000 customers, exemplifies how a single compromised script can bypass robust server security. The attack operated undetected for two weeks, collecting data directly from checkout forms before sending it to attacker-controlled servers.
Supply chain compromise
Modern web applications rely heavily on third-party services, analytics platforms, payment processors, chat widgets, and advertising networks. Each represents a potential entry point. The 2019 Ticketmaster breach occurred when attackers compromised a customer support chat tool, demonstrating how a single third-party script can expose the entire platform.
Shadow scripts and script sprawl
Many organizations don’t have complete visibility into all the JavaScript code running on their pages. Scripts can dynamically load other scripts, creating a complex web of dependencies that security teams have a hard time tracking. This “shadow scripting” phenomenon means that unauthorized code can be executed without explicit authorization or oversight.
Working with sessions and cookies
Client-side attacks can intercept authentication tokens, manipulate session data, and extract sensitive information from cookies and local storage. Unlike server-side attacks that leave network logs, these operations occur entirely within the user’s browser, making them difficult to detect without special monitoring.
Real-world holiday attacks: Lessons from 2024
The 2024 holiday season provided a clear example of increased client-side threats. The infamous Polyfill.io supply chain attack, which began in February 2024 and affected more than 100,000 websites by the end of the year, demonstrated how compromised third-party scripts can redirect users to malicious sites. Similarly, the September 2024 Cisco Magecart attack targeted holiday shoppers via merchandise stores, highlighting how vulnerable even large organizations are to payment data theft during peak times.
Beyond these high-profile incidents, the pervasive nature of client-side threats was clear. Compromised Kuwaiti e-commerce site Shrwaa.com hosted malicious JavaScript files throughout 2024 that undetected and infected other sites, exposing a “shadow scripting” issue. A variant of the Grelos skimmer further reveals session and cookie manipulation, deploying fake payment forms on small, trusted e-commerce sites just before Black Friday and Cyber Monday. These incidents highlight the critical need for robust client-side security measures.
Holiday season increases risk
Several factors make the holiday shopping period particularly vulnerable.
Increased motivation for attacks: Increased transaction volume creates lucrative targets, and on Cyber Monday 2024, Cloudflare’s network saw 5.4 trillion daily requests and 5% were blocked as potential attacks.
Code freeze period: Many organizations implement development freezes during peak seasons, limiting their ability to quickly respond to newly discovered vulnerabilities.
Third-party dependencies: Holiday promotions often require integration with additional marketing tools, payment options, and analytics platforms, increasing the attack surface.
Resource constraints: Security teams can be stretched thin, and most organizations reduce after-hours SOC staffing levels by up to 50% during holidays and weekends.
Implementing effective client-side security
1. Implement a content security policy (CSP)
To visualize script execution without loss of functionality, start CSP in report-only mode.
This approach allows you to quickly understand the behavior of your scripts while giving you time to refine your policies.
CSP traps to avoid: When implementing CSP, you may run into broken functionality in legacy scripts. An attractive and simple solution is to add “unsafe-inline” to your policy. This allows all inline JavaScript execution. However, this one directive completely weakens CSP protection. This is the same as leaving your front door unlocked because one key doesn’t work. Instead, use nonces (cryptographic tokens) for legitimate inline scripts.
2. Implementing Subresource Integrity (SRI)
Implement SRI tags to ensure that third-party scripts have not been tampered with.
3. Conduct regular script audits
Maintain a comprehensive inventory of all third-party scripts, including:
Purpose and business justification Data access permissions Update and patching procedures Vendor security measures Alternative solutions if the service is compromised
4. Implement client-side monitoring
From browser-based CSP validators to Web Exposure management solutions to commercial Runtime Application Self-Protection (RASP) solutions, we deploy specialized client-side monitoring tools that can monitor JavaScript execution in real-time and detect:
Unexpected data collection or submission DOM manipulation attempts New or modified scripts Suspicious network requests
5. Establish incident response procedures
Develop specific playbooks to respond to client-side incidents, such as:
Script isolation and removal instructions Customer communication templates Vendor contact information and escalation paths Regulatory notification requirements
Implementation challenges and solutions
Although the benefits of client-side security are clear, implementation can be fraught with obstacles. Here’s how to solve common challenges:
Legacy system compatibility
Implement CSP in stages, starting with the highest-risk pages Use CSP reports to identify problematic scripts before deploying Consider deploying a reverse proxy to insert security headers without changing the application
Performance impact
Test thoroughly using report-only mode first. Monitor that SRI checks add minimal overhead (typically less than 5ms per script). Track real user metrics like page load times during your rollout.
vendor resistance
Include security requirements in vendor contracts upfront Frame requirements to protect both parties’ reputations Track vendor security posture Maintain a risk register Document uncooperative vendors as highest-risk dependencies
resource limits
Consider a managed security service that focuses on client-side protection Start with free browser-based tools and CSP reporting analyzers Prioritize automated script inventory, monitoring, and alerts Budget 6-12 hours per month for initial setup and ongoing monitoring, or budget 1-2 days per quarter for a comprehensive audit in an enterprise environment with 50+ third-party scripts
organizational buy-in
Build a business case around the cost of a breach (average Magecart attack: $3.9 million) and monitoring investment ($10,000-$50,000 per year) Organizations with dedicated client-side monitoring detect breaches 5.3 months faster than the industry average (7.5 months of detection time reduced to 2.2 months), significantly limiting data breaches and regulatory penalties IT Provide client-side security as revenue protection rather than overhead Secure executive sponsorship before the holiday freeze period Emphasize that prevention is less disruptive than actively responding to a breach during peak season
I’m looking forward to it
Client-side security represents a fundamental change in the way web application protection is approached. As the attack surface continues to evolve, organizations must adapt their security strategies to incorporate comprehensive monitoring and protection of client environments.
The holiday shopping season brings both urgency and opportunity. There is an urgency to address these vulnerabilities before traffic peaks, and an opportunity to implement monitoring that provides valuable insight into normal and suspicious script behavior.
To be successful, you must move beyond the traditional perimeter-focused security model and adopt a more holistic approach that protects data wherever it travels, including inside a user’s browser. Organizations that make this transition will not only protect their customers during the busy holiday season, but will also establish a more resilient security posture for the year ahead.
Download the complete Holiday Season Security Handbook to ensure your organization is prepared for the 2025 shopping season.
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