Frontend Security Best Practices in 2025

By 2025, client-side code is no longer just a UI layer. Modern frontend apps manage sessions, call APIs, handle sensitive data, and store authentication tokens. Whether you're building with React, Vue, Angular, or another stack, your frontend is part of the attack surface—and everything in the browser can be compromised.

We’re seeing rising threats like XSS attacks, session leaks, and insecure third-party SDKs—especially in fintech, healthcare, and marketplace platforms.

During our audits, we regularly find critical bugs in code that passes all CI checks but remains vulnerable due to ill-conceived architectural decisions, missing web security headers, or poorly configured security policies such as CORS and CSP.

Dmitry Karpunin Head of Frontend, Evrone

Top 3 Frontend Risk Zones Every Product Should Audit

• Insecure Token Storage and Cookie Handling

One of the most common issues is storing access tokens in localStorage or accessible JavaScript cookies, which makes them vulnerable to XSS or malicious browser extensions.

Best practice: use HttpOnly, Secure cookies with SameSite=Strict when possible. This keeps tokens out of JavaScript and reduces the attack surface.

We check:

Presence of HttpOnly, Secure, SameSite=Strict

Token leaks into third-party scripts or SDKs

Correct implementation of silent refresh and logout flows

• Rendering Vulnerabilities and User-Injected HTML

Any time user input is rendered into the DOM, there’s a risk of XSS—especially in React, Vue, or Angular projects using custom widgets, markdown editors, or visual WYSIWYG components.

We perform:

Static analysis of dangerous insertions (e.g., dangerouslySetInnerHTML)

Integration of XSS protection libraries: DOMPurify, sanitize-html, etc.

Code review of raw HTML logic

XSS protection testing for React and Vue

• Missing or Misconfigured Content Security Policy (CSP)

CSP is one of the most effective defenses against XSS and malicious script injection. But it’s often disabled or misconfigured due to conflicts with analytics or chat scripts.

We help implement a balanced CSP strategy

• Use CSP headers or meta http-equiv="Content-Security-Policy" depending on architecture
• Configure rules to block inline scripts, eval, and unsafe sources
• Align policy with essential tools (chat, analytics, payments)
• Integrate CSP validation into CI/CD to prevent regressions

We also check other critical web security headers: X-Content-Type-Options, X-Frame-Options, Referrer-Policy

When It Makes Sense to Run a Frontend Security Audit

Consider a security audit if you:

• Handle sensitive user data
• Are preparing for GDPR, HIPAA, or a funding round
• Doubt your architecture would pass an external security review
• Rely on third-party SDKs (analytics, payments, chats)
• Aren’t sure if you follow current best practices for token storage, CSP, CORS, XSS

How We Integrate Security Without Blocking Development

Security initiatives often fail because they disrupt established workflows. We embed best practices directly into your development lifecycle:

• ESLint rules for XSS and DOM usage
• Pre-commit hooks to catch potential vulnerabilities
• Secure components for rendering HTML
• Third-party vulnerability scanning in CI (e.g., npm audit, Snyk)
• Automated checks for web security headers and CSP
• Optional AES encryption for client-to-API communication
• Trusted libraries for HTML sanitization (DOMPurify, sanitize-html)

What You Risk by Ignoring This

Client-side vulnerabilities are rarely visible at launch—but they accumulate. Each new SDK, form, or logic workaround increases your exposure. A single flaw can lead to token theft, session hijacking, or even regulatory sanctions if your payment system is compromised.

When You Should Talk to Us

If your product:

• Handles sensitive data
• Uses third-party libraries or scripts
• Hasn’t had a frontend security audit in the last 12 months

— it's time for a conversation. We won’t just patch holes — we’ll help embed frontend security into your development pipeline, sustainably.