Mailchimp’s Security Issues: Why We Need a Better Newsletter Solution

A B Rating From CodeFrog

While building CodeFrog’s landing page, we integrated Mailchimp for newsletter signups. However, our security scans revealed a critical issue that prevents us from achieving an A rating: Mailchimp’s validation script violates Content Security Policy (CSP) requirements.

The Security Violation

Mailchimp’s mc-validate.js script injects inline styles dynamically, which requires the 'unsafe-inline' directive in our CSP style-src policy. This is a security anti-pattern because:

1. CSP Bypass Risk: 'unsafe-inline' allows any inline styles, defeating the purpose of CSP protection against XSS attacks
2. Nonce Incompatibility: Even when using CSP nonces for our own styles, Mailchimp’s dynamically injected styles can’t use nonces, forcing us to allow all inline styles
3. Security Rating Impact: Security scanners (like our own Mega Report) flag this as a MEDIUM severity issue, preventing an A rating

The Error

When we removed 'unsafe-inline' to improve security, the browser console shows:

Applying inline style violates the following Content Security Policy directive: 
`style-src 'self' 'nonce-...'`. The action has been blocked.

This error originates from mc-validate.js:164, confirming that Mailchimp’s script requires unsafe inline styles to function.

Why This Matters

Content Security Policy is a critical security feature that helps prevent:

• Cross-Site Scripting (XSS) attacks
• Data injection attacks
• Code injection vulnerabilities

By requiring 'unsafe-inline', Mailchimp forces us to weaken our security posture, which is unacceptable for a security-focused tool like CodeFrog.

What We Need

We’re looking for a newsletter service that:

✅ Free tier (or very affordable) for up to 500 subscribers
✅ CSP-compliant – doesn’t require 'unsafe-inline'
✅ Secure by default – supports nonces or external stylesheets
✅ Easy integration – simple embed or API
✅ Reliable – good deliverability and uptime

Call for Recommendations

We’re reaching out to the developer community for recommendations. If you know of a newsletter service that:

• Respects modern web security standards
• Works with strict CSP policies
• Offers a free tier for small lists
• Provides good deliverability

Please share your suggestions! We’re particularly interested in:

• Self-hosted solutions (if they’re easy to set up)
• Modern alternatives that prioritize security
• Services that use external stylesheets or support CSP nonces
• Any workarounds for making Mailchimp CSP-compliant (if they exist)

The Solution: CSP Hash Instead of Unsafe-Inline

Good news! We found a solution that doesn’t require 'unsafe-inline'. When we removed 'unsafe-inline' from our CSP, the browser console error message actually provided the answer:

Applying inline style violates the following Content Security Policy directive: 
'style-src 'self' 'nonce-...''. Either the 'unsafe-inline' keyword, a hash 
('sha256-iIHQ0a6ntSSQhfMwBwjvXvp+zrKJldURld+iiblnEKo='), or a nonce 
('nonce-...') is required to enable inline execution.

The browser helpfully suggested using a hash for the specific inline style instead of allowing all inline styles. This is a much better security approach!

What We Changed

Instead of using 'unsafe-inline' in our CSP style-src directive, we now use:

style-src 'self' 'nonce-{style_nonce}' 'sha256-iIHQ0a6ntSSQhfMwBwjvXvp+zrKJldURld+iiblnEKo='

This hash is specific to the inline style that Mailchimp’s mc-validate.js script injects. By using the hash, we:

✅ Allow only that specific style – not arbitrary inline styles
✅ Maintain strict CSP – no 'unsafe-inline' directive
✅ Pass security scans – scanners don’t flag specific hashes
✅ Keep Mailchimp working – the validation script functions correctly

Why This Is Better

Using a hash is more secure than 'unsafe-inline' because:

• It’s whitelist-based: Only the exact style with that hash is allowed
• It’s CSP-compliant: Security scanners accept hashes as a valid, secure approach
• It’s maintainable: If Mailchimp changes their script, we’ll get a new error with a new hash to add

Remaining Issue

While we’ve solved the CSP style-src issue, there’s still one security concern:

⚠️ SRI (Subresource Integrity) Missing: Mailchimp’s script is loaded from their S3 bucket without an integrity attribute. This means we can’t verify the script hasn’t been tampered with. However, this is a known limitation because:

• Mailchimp’s script is dynamically generated and changes frequently
• Their S3 bucket doesn’t send CORS headers, which would be required for SRI
• Adding crossorigin="anonymous" would break script loading due to CORS issues

This is a MEDIUM severity issue that prevents a perfect A+ rating, but it’s an acceptable trade-off given the constraints.

Conclusion

As a security-focused developer tool, CodeFrog needs to maintain the highest security standards. We’ve successfully resolved the CSP 'unsafe-inline' issue by using a hash-based approach, which is more secure and CSP-compliant.

The remaining SRI issue is a known limitation with third-party scripts that don’t support CORS, and we’ve documented it appropriately. We’re now much closer to that A security rating!

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Update: We solved the CSP issue using a hash-based approach! The browser’s error message provided the exact hash we needed. Mailchimp now works with strict CSP without requiring 'unsafe-inline'.