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:
- CSP Bypass Risk:
'unsafe-inline' allows any inline styles, defeating the purpose of CSP protection against XSS attacks
- 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
- 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!
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'.
