# Security Headers Fix for PDF and File Previews

## Problem
PDF previews and embedded content were failing with the error:
```
Refused to display 'http://localhost:8080/' in a frame because it set 'X-Frame-Options' to 'deny'.
```

This was preventing:
- PDF file previews in iframes
- Document previews
- Any embedded same-origin content

## Root Cause
The security headers were set too restrictively:
- `X-Frame-Options: DENY` - Completely blocked all framing
- `Content-Security-Policy` with `frame-ancestors 'none'` - Also blocked framing in CSP

## Solution Applied

### Changes Made

#### 1. **main.go** (Line 109)
```diff
- c.Writer.Header().Set("X-Frame-Options", "DENY")
+ c.Writer.Header().Set("X-Frame-Options", "SAMEORIGIN")
```

#### 2. **internal/middleware/security_headers.go** (Line 15)
```diff
- c.Header("X-Frame-Options", "DENY")
+ c.Header("X-Frame-Options", "SAMEORIGIN")
```

#### 3. **internal/middleware/security_headers.go** (Lines 59, 74)
```diff
Production CSP:
- "frame-ancestors 'none';"
+ "frame-ancestors 'self';"

Development CSP:
- "frame-ancestors 'none';"
+ "frame-ancestors 'self';"
```

#### 4. **internal/config/config.go** (Line 110)
```diff
- ContentSecurityPolicy: getEnv("CONTENT_SECURITY_POLICY", "default-src 'self' data: blob: https: http:; img-src * data: blob:; style-src 'self' 'unsafe-inline' https: http:; script-src 'self' 'unsafe-inline' 'unsafe-eval' https: http:; connect-src *;"),
+ ContentSecurityPolicy: getEnv("CONTENT_SECURITY_POLICY", "default-src 'self' data: blob: https: http:; img-src * data: blob:; style-src 'self' 'unsafe-inline' https: http:; script-src 'self' 'unsafe-inline' 'unsafe-eval' https: http:; connect-src *; frame-ancestors 'self';"),
```

## Security Impact

### ✅ What's Still Protected:
- **Clickjacking from external sites**: Only same-origin framing is allowed
- **XSS attacks**: CSP and other headers remain in place
- **MIME sniffing**: X-Content-Type-Options still set
- **HTTPS enforcement**: HSTS still active
- **All other security measures**: Unchanged

### ✅ What's Now Allowed:
- **PDF previews**: Can now be embedded in iframes on same domain
- **Document previews**: Word, Excel, PowerPoint previews work
- **Image previews**: Enhanced preview capabilities
- **Same-origin embedding**: Any same-origin content can be framed

### 🔒 Security Comparison:

| Header | Before | After | Protection Level |
|--------|--------|-------|------------------|
| X-Frame-Options | DENY | SAMEORIGIN | Still protected against external clickjacking |
| CSP frame-ancestors | 'none' | 'self' | Still protected against external embedding |

## Verification

Test the headers are correctly applied:
```bash
curl -I http://localhost:8080/api/v1/settings | grep -E "X-Frame-Options|Content-Security-Policy"
```

Expected output:
```
X-Frame-Options: SAMEORIGIN
Content-Security-Policy: ...frame-ancestors 'self';
```

## Files Modified
1. `main.go`
2. `internal/middleware/security_headers.go`
3. `internal/config/config.go`

## Deployment
Backend was rebuilt and restarted:
```bash
docker compose build backend --no-cache
docker compose up -d backend
```

## Testing Checklist

- ✅ PDF files can be previewed in iframes
- ✅ Word documents (.doc, .docx) preview correctly
- ✅ Excel spreadsheets (.xls, .xlsx) preview correctly
- ✅ PowerPoint presentations (.ppt, .pptx) preview correctly
- ✅ Text files (.txt) preview correctly
- ✅ Images still display correctly
- ✅ External sites cannot frame the application
- ✅ Same-origin iframes work correctly
- ✅ YouTube embeds still work
- ✅ All other security headers remain active

## Additional Notes

### Why SAMEORIGIN?
- **DENY**: Blocks all framing, including same-origin (too restrictive for previews)
- **SAMEORIGIN**: Allows same-origin framing but blocks external sites (balanced security)
- **ALLOW-FROM**: Deprecated and not widely supported

### CSP frame-ancestors
- **'none'**: Blocks all framing (equivalent to X-Frame-Options: DENY)
- **'self'**: Allows same-origin framing only (equivalent to X-Frame-Options: SAMEORIGIN)
- More modern and flexible than X-Frame-Options

## Browser Compatibility
- **X-Frame-Options**: Supported by all browsers
- **CSP frame-ancestors**: Supported by all modern browsers, overrides X-Frame-Options when present

## Future Enhancements (Optional)
- Implement per-route frame options for even finer control
- Add specific iframe sandbox attributes for enhanced security
- Consider using `<embed>` or `<object>` tags with additional security attributes

## References
- [MDN: X-Frame-Options](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/X-Frame-Options)
- [MDN: CSP frame-ancestors](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/frame-ancestors)
- [OWASP: Clickjacking Defense](https://cheatsheetseries.owasp.org/cheatsheets/Clickjacking_Defense_Cheat_Sheet.html)
