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101 lines
4.0 KiB
Markdown
101 lines
4.0 KiB
Markdown
---
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title: "Slightly Improving Mailcow Security"
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date: 2022-07-18
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tags: ['Applications', 'Linux', 'Security']
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author: Tommy
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---
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Mailcow is a fairly popular self-hosted mail server. If you use it, there are a few ways you can improve its security by following these steps.
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## Postfix Configuration
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Consider disabling weak ciphers and TLS versions below 1.2 in `data/conf/postfix/extra.cf`:
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```
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tls_high_cipherlist = ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-CHACHA20-POLY1305:ECDHE-RSA-CHACHA20-POLY1305:ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-GCM-SHA256
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tls_preempt_cipherlist = yes
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smtp_tls_protocols = !SSLv2, !SSLv3, !TLSv1, !TLSv1.1
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smtp_tls_ciphers = high
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smtp_tls_mandatory_ciphers = high
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smtpd_tls_protocols = !SSLv2, !SSLv3, !TLSv1, !TLSv1.1
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smtpd_tls_ciphers = high
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smtpd_tls_mandatory_ciphers = high
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```
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## NGINX Configuration
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These security configurations can be added/modified in `data/conf/nginx/includes/site-defaults.conf`.
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### SSL Ciphers
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Consider only supporting strong ciphers:
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```
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ssl_ciphers ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-CHACHA20-POLY1305:ECDHE-RSA-CHACHA20-POLY1305:DHE-RSA-AES128-GCM-SHA256:DHE-RSA-AES256-GCM-SHA384;
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```
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And prioritize ChaCha ciphers:
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```
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ssl_conf_command Options PrioritizeChaCha;
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```
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### HSTS
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Consider adding `includeSubDomains;` and `preload;` to the HSTS configuration if all of your services are using HTTPS:
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```
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add_header Strict-Transport-Security "max-age=63072000; includeSubDomains; preload";
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```
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### X-XSS-Protection
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We will setup Content Security, so this header is no longer needed. In fact, it may do [more harm than good](https://github.com/helmetjs/helmet/issues/230). Change the setting to `0`:
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```
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add_header X-XSS-Protection "0";
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```
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### Permission Policy
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Mailcow does not need any special permissions to operate, except for USB which is needed to access your FIDO2 keys if you use them.
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Add this header to deny other permissions:
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```
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add_header Permissions-Policy "accelerometer=(), ambient-light-sensor=(), autoplay=(), battery=(), camera=(), clipboard-read=(), display-capture=(), document-domain=(), encrypted-media=(), fullscreen=(), geolocation=(), gyroscope=(), hid=(), idle-detection=(), interest-cohort=(), magnetometer=(), microphone=(), midi=(), payment=(), picture-in-picture=(), screen-wake-lock=(), serial=(), usb=(), sync-xhr=(), xr-spatial-tracking=()";
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```
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### Content Security Policy
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Use the following as your [Content Security Policy](https://developer.mozilla.org/en-US/docs/Web/HTTP/CSP#:~:text=Content%20Security%20Policy%20(CSP)%20is,site%20defacement%2C%20to%20malware%20distribution.):
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#### If you use Gravatar with SOGo
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```
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add_header Content-Security-Policy "default-src 'none'; connect-src 'self' https://api.github.com https://www.gravatar.com; font-src 'self' https://fonts.gstatic.com; img-src 'self' data: https://www.gravatar.com; script-src 'self' 'unsafe-inline'; style-src 'self' 'unsafe-inline' https://fonts.googleapis.com; frame-ancestors 'none'; upgrade-insecure-requests; block-all-mixed-content; base-uri 'none'";
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```
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#### If you do not use Gravatar with SOGo
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```
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add_header Content-Security-Policy "default-src 'none'; connect-src 'self' https://api.github.com; font-src 'self' https://fonts.gstatic.com; img-src 'self' data:; script-src 'self' 'unsafe-inline'; style-src 'self' 'unsafe-inline' https://fonts.googleapis.com; frame-ancestors 'none'; upgrade-insecure-requests; block-all-mixed-content; base-uri 'none'";
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```
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### Cross-Origin Resource, Opener, and Embedder Policies
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Mailcow does not use any cross site scripts, or documents. Thus, you should set CORP and COOP headers to their strictest configuration:
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```
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add_header Cross-Origin-Resource-Policy same-origin;
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add_header Cross-Origin-Opener-Policy same-origin;
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```
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If you do not use Gravatar with SOGo, you can also set COEP to require-corp since image embedding will not be used either:
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```
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add_header Cross-Origin-Embedder-Policy require-corp;
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```
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