Safeguard 9.6: Block Unnecessary File Types

Compliance leaders usually meet this requirement after a real incident: a malicious attachment, a drive-by download, or an unexpected script execution from a “harmless” file type. Safeguard 9.6 pushes you to reduce that attack surface by blocking file types your organization does not require for business operations. The win is simple: fewer pathways for malware, fewer tools for living-off-the-land techniques, and fewer “why was this allowed?” findings in audits and customer diligence.

The hard part is not the idea. It’s operations. Different teams genuinely need different file types (finance macros, engineering packages, security tooling, design binaries). Blocking at only one layer (for example, email gateway) leaves other ingestion paths open (chat uploads, cloud drives, browser downloads, removable media, third-party file transfer). Blocking too aggressively without exceptions drives shadow IT and constant emergency requests.

This page translates the safeguard into an operator-run control: define scope, set ownership, implement technical enforcement, document exception rules, and build an evidence bundle that stands up in internal audit, external audit, and customer reviews. Source references are limited to the CIS materials provided. ¹

Regulatory text

Excerpt (framework expectation): “CIS Controls v8 safeguard 9.6 implementation expectation (Block Unnecessary File Types).” ¹

What the operator must do:
You must (1) identify file types that are unnecessary for normal business, (2) technically block them where files enter and execute in your environment, and (3) manage exceptions so approved business needs do not turn into permanent holes. The control is considered operational only if blocks are enforced continuously and you can prove the current configuration, who approved it, and how changes are governed. ¹

Plain-English interpretation

Block file types that commonly carry active content (scripts, installers, macro-enabled documents, disk images, archives, and other executable or easily weaponized formats) unless a defined business function requires them. If a business function requires them, allow them only in the narrowest possible way: specific users, specific systems, specific delivery paths, and with compensating controls (scanning, sandboxing, code signing, restricted execution). ²

Who it applies to (entity and operational context)

Applies to: Enterprises and technology organizations implementing CIS Controls v8. ¹

Operationally, it applies wherever your organization:

• Receives files: email, web downloads, contact forms, customer portals, ticketing attachments, chat/collaboration uploads, cloud storage sharing, third-party file transfer.
• Stores and syncs files: file shares, SharePoint/OneDrive/Google Drive equivalents, endpoint sync clients, VDI profiles.
• Runs files: endpoints (Windows/macOS/Linux), servers, CI/CD runners, admin workstations, jump boxes.

Teams you will need involved:

• Security engineering (email/web security, endpoint controls)
• IT endpoint management (MDM, GPO, device hardening)
• Collaboration tooling owners (M365/Google Workspace, Slack/Teams)
• Application owners (portals accepting uploads)
• Legal/compliance (exception governance, retention)
• HR/IT service desk (request and approval workflow)

What you actually need to do (step-by-step)

1) Create a “control card” so ownership and operation are unambiguous

Document the control like you would any audit-tested requirement:

• Objective: Block unnecessary file types across ingress and execution paths.
• Owner: Named role (not a team alias).
• Scope: Corporate endpoints, managed servers, email, web, collaboration tools, and defined high-risk segments.
• Cadence: Configuration enforced continuously; health check performed on a recurring schedule.
• Trigger events: New app onboarding, merger/acquisition, email security change, collaboration tool change, exception request.
• Exception rules: Who can approve, expiry expectation, compensating controls, and review cycle.

This maps directly to sustained operation expectations auditors and customers look for: clear ownership, cadence, and evidence. ²

2) Build the file-type policy using an allowlist-first approach

A practical method:

• Inventory business workflows that depend on “active” file types. Examples: finance spreadsheets with macros, engineering installers, security tools, design binaries.
• Define an allowlist by environment zone.
  • Standard user endpoints: strict allowlist
  • Privileged/admin workstations: tighter controls but may allow additional admin formats
  • Dev/build environments: allow more types, but isolate and monitor
• Define a default-deny list for high-risk file types. Keep it policy-level (categories), then map to extensions/MIME types in tooling. Your policy should explicitly call out that both extension and content type checks matter, because attackers rename files.

Deliverable: a controlled document (standard) that lists:

• Allowed categories by user group/system type
• Blocked categories by default
• Approved intake channels (where a file type may be received)
• Required scanning/sandboxing for allowed “risky but needed” types

3) Enforce blocks at the main choke points

You need layered enforcement. Pick the controls you already run, then close obvious gaps.

A. Email security controls

• Block prohibited attachments by file type.
• Quarantine or detonate (sandbox) higher-risk allowed types.
• Apply stricter rules to external senders.

B. Web gateway / secure web access

• Block downloads for prohibited types.
• Enforce content-type inspection where supported.

C. Endpoint execution controls

• Prevent execution of prohibited types (application control/allowlisting).
• Block script interpreters or restrict them to admin/dev contexts where required.
• Ensure removable media policies align (or you will reintroduce the same risk via USB).

D. Collaboration and cloud storage

• Restrict uploads for prohibited types where the platform supports it.
• Apply DLP or malware scanning on upload/download.
• Pay attention to external sharing settings; a block that only applies internally misses a major route.

E. Application-layer upload controls (customer portals, forms, ticketing)

• Validate file type by MIME and content sniffing, not only extension.
• Enforce size and archive inspection (archives can hide blocked types).
• Log rejected uploads with enough metadata to investigate abuse.

4) Establish an exception process that is fast but controlled

Most programs fail here: the business needs a way to get work done, and security needs a way to say “yes, narrowly.”

Define:

• Request intake: ticket with business justification, file types, users/systems, duration, and channel(s).
• Approval: control owner plus system owner; require compensating controls for higher-risk types.
• Implementation: time-bounded configuration change (policy exception scoped to user group/device group/app).
• Review and closure: verify the exception is removed or renewed with justification.

5) Run recurring control health checks and remediate gaps to closure

Your health check should answer:

• Are blocks still enabled in each enforcement layer?
• Did any new tools or upload endpoints bypass policy?
• Do exception entries match approved tickets and are they still necessary?
• Are there drift and configuration changes outside change control?

Track findings to validated closure with owners and due dates. This is where many teams lose audit credibility. ²

Required evidence and artifacts to retain

Retain evidence that proves design, implementation, and ongoing operation:

Minimum evidence bundle (practical)

• Control card / runbook: objective, owner, cadence, trigger events, exception rules. ²
• File-type policy standard: allowlist/blocklist categories and scope.
• System configurations (point-in-time exports or screenshots) for:
  • Email attachment rules
  • Web download/file-type policies
  • Endpoint application control / script restrictions
  • Collaboration platform file restrictions (where available)
  • App upload validation rules (config snippets or code review references)
• Exception tickets: request, approval, implementation notes, compensating controls, expiry/review decision.
• Health check records: checklist output, drift findings, remediation tickets, closure evidence. ²
• Change control records: approvals for policy updates and major rule changes.
• Logging proof: sample logs showing blocked events (sanitized) and where logs are retained.

Retention: align to your organization’s audit and security logging retention standards; document where artifacts live (GRC repository, ticketing system, SIEM). ²

Common exam/audit questions and hangups

Auditors and customer assessors tend to press on these points:

1. “Show me what file types are blocked and where.”
   They want a map of enforcement points and current configs, not a policy statement.

2. “How do you prevent bypass via renaming extensions?”
   Be ready to explain MIME/content inspection at gateways and execution control at endpoints.

3. “How are exceptions approved, scoped, and removed?”
   A permanent exception list with no review is a red flag.

4. “How do you know the control still works?”
   Provide health checks and evidence of remediation closure. ²

5. “Does this apply to third-party pathways?”
   If third parties can upload files into your environment (support portals, shared drives, collaboration), scope must include those workflows.

Frequent implementation mistakes and how to avoid them

Mistake	Why it fails	How to avoid it
Blocking only at email	Files enter via chat, cloud drives, browsers, portals	Document all ingress routes and assign an owner per route
Relying on extension-only matching	Attackers rename files	Use tools that check MIME/content and add endpoint execution control
No exception governance	Business pushes back; teams route around controls	Make exceptions fast, scoped, approved, and reviewable
Over-broad exceptions	“Allow .iso for everyone” becomes a standing gap	Scope by user group, device group, and channel; require compensating controls
No evidence bundle	You cannot prove ongoing operation	Define the minimum evidence bundle and retention location up front 2
One-time rollout	Config drift, tool changes, and new apps bypass controls	Schedule recurring health checks and track remediation to closure 2

Enforcement context and risk implications

No public enforcement cases were provided in the source catalog for this requirement, so this page does not cite specific actions.

From a risk perspective, failing Safeguard 9.6 typically increases:

• Malware delivery success through attachments and downloads
• Script-based execution and user execution of installers
• Exposure created by collaboration tools and unmanaged upload surfaces
• Audit findings for “control not operating” when blocks exist only on paper ²

Practical 30/60/90-day execution plan

First 30 days (stabilize scope and governance)

• Assign a single control owner and publish the control card (objective, scope, exceptions, health checks). ²
• Identify your top ingestion paths: email, browser downloads, collaboration uploads, and your main customer/third-party upload portals.
• Draft the initial allowlist/blocklist policy by business function and environment zone.
• Stand up an exception workflow in your ticketing system with required fields and approvers.

Days 31–60 (implement layered enforcement)

• Implement or tighten email attachment blocking and quarantine rules.
• Implement web download restrictions for prohibited types.
• Implement endpoint execution controls for prohibited script/executable categories where feasible.
• Apply collaboration platform restrictions and malware scanning settings where available.
• Update application upload validation for the highest-risk external-facing portals.
• Start collecting baseline “blocked event” logs and configuration exports for the evidence bundle.

Days 61–90 (prove operation and close gaps)

• Run the first control health check across all enforcement points; open remediation items and drive them to closure. ²
• Review all exceptions for scope, compensating controls, and continued need.
• Add “new application intake” questions: does the app accept uploads, allow downloads, or sync files to endpoints?
• Package a ready-to-audit evidence set: control card, policy, configs, exception samples, and health check results.

How Daydream fits (without adding process overhead)

Most teams struggle to show, on demand, who owns this requirement, how often it runs, and which evidence proves it operates. Daydream works well as the system of record for the control card, the minimum evidence bundle, and recurring health-check attestations, so you can answer diligence requests without rebuilding proof from scratch. ²

Frequently Asked Questions

Do we need to block file types everywhere, or is email blocking enough?

Email blocking alone leaves open other ingestion paths like collaboration uploads, cloud drives, and web downloads. Treat Safeguard 9.6 as layered enforcement across intake and execution points. ²

How do we handle departments that require macro-enabled or script-based files?

Use scoped exceptions tied to specific user groups and devices, and require compensating controls such as sandboxing and restricted execution. Document the approval and review it on a defined cadence. ²

What’s the audit-ready “minimum” evidence for this safeguard?

Keep a control card/runbook, the file-type standard, configuration exports from each enforcement layer, a sample of exception approvals, and health-check outputs with remediation closure. That bundle proves both design and operation. ²

Our collaboration tool can’t block certain extensions. Are we stuck?

Treat that as a control gap and add compensating controls: malware scanning on upload/download, stricter external sharing rules, and endpoint execution blocks so the file cannot run even if it arrives. Document the decision and the compensating measures. ²

How do we prevent users from renaming a blocked file type to bypass controls?

Pair gateway rules with content-type inspection where supported, then enforce endpoint execution controls so renamed files still cannot execute. Also ensure upload portals validate MIME/content, not just filename extensions. ²

Does this requirement apply to third parties sending us files?

Yes, if third parties can transmit files into your environment through email, portals, shared drives, or collaboration tools. Include those pathways in scope and retain evidence that the same restrictions apply or that compensating controls exist. ²

Footnotes

1. CIS Controls v8; CIS Controls Navigator v8

2. CIS Controls v8