Spam Protection SI-8

Source: NIST SP 800-53 Rev.5, System and Information Integrity family NIST SP 800-53 Rev.5. The baseline shows the lowest SP 800-53B baseline (Low / Moderate / High) this control appears in NIST SP 800-53B.

Control guide — plain-English, per NIST SP 800-53

SI-8 (Spam Protection) is about stopping unsolicited and malicious email — the spam and phishing that carry so many attacks — at the points where mail enters and leaves. You run spam-filtering mechanisms at those boundaries and keep them updated as new releases arrive. It is a technical control in the Moderate baseline, and it matters most on any host that sends or receives mail.

What good looks like

• Run a spam filter at the mail boundary — on Linux, SpamAssassin or rspamd in front of your mail server (for example Postfix).
• Filter at both entry and exit so you catch inbound spam and stop your own host being used to send it.
• Turn on sender authentication — SPF (Sender Policy Framework), DKIM (DomainKeys Identified Mail), and DMARC (Domain-based Message Authentication, Reporting, and Conformance) — to spot forged senders.
• Block risky attachment types and known-bad senders, and consider DNS (Domain Name System) filtering to cut off malicious domains.
• Keep the filter and its rules updated as new releases and signatures come out, under your change process.

Framework mapping

• CIS Controls v8 — Control 9 — Email and Web Browser Protections

How to move it toward Implemented

• Install and enable a spam filter (for example spamassassin with spamd, or rspamd) and wire it into Postfix at the entry and exit points.
• Keep its rules current — run sa-update on a schedule — and publish SPF, DKIM, and DMARC records for the domain.
• Send a test message through and save the mail log showing the filter scoring and acting on it.
• Attach that mail-log excerpt and rule-update output as hardening evidence on the asset, naming SI-8 in the Requirement field — that moves it from ‘To assess’ toward ‘Completed’.