SC-18(3): Prevent Downloading and Execution

SC-18(3): prevent downloading and execution requirement means you must technically block users and systems from downloading and running the specific types of mobile code your system defines as prohibited (the control’s organization-defined parameter). Operationalize it by defining the prohibited mobile code, enforcing blocks at the browser/endpoint/network layers, and retaining repeatable evidence that the blocks work. ¹

Key takeaways:

• You must define what “mobile code” you prohibit, then enforce blocks on both download and execution paths. ¹
• Auditors will look for technical enforcement plus proof the control works in real user environments, not a policy statement. ²
• The fastest path is: define scope → implement allowlisting/denylisting and execution controls → test → automate evidence collection.

SC-18 in NIST SP 800-53 addresses “mobile code,” which is a common delivery mechanism for malware and unwanted functionality. Enhancement (3) narrows the expectation to a concrete outcome: prevent both the download and the execution of whatever classes of mobile code you decide are not allowed in your environment. The catch is that the control text explicitly relies on an organization-defined parameter, so you can’t implement it correctly until you define that parameter and tie it to enforceable technical controls. ¹

For a Compliance Officer, CCO, or GRC lead, the work is less about debating definitions and more about turning SC-18(3) into: (1) an explicit “what is blocked” decision, (2) standard configurations in the tools that already govern endpoints, browsers, email, and egress traffic, and (3) evidence that survives assessment scrutiny. This page gives requirement-level implementation guidance you can hand to control owners and validate quickly, with audit-ready artifacts and a practical execution plan aligned to NIST SP 800-53 Rev. 5. ²

Regulatory text

Control excerpt: “Prevent the download and execution of {{ insert: param, sc-18.03_odp }}.” ¹

What the operator must do:

1. Define the organization-defined parameter (ODP) for SC-18(3): specify the mobile code types, sources, or execution contexts you prohibit (for example: “unsigned ActiveX controls,” “unsigned Java applets,” “scripts from untrusted domains,” “unapproved browser extensions,” “macro-enabled content from the internet”). Then 2) implement technical measures that block both download and execution of that prohibited set, across in-scope endpoints and systems. ¹

Plain-English interpretation (what this requirement really asks)

You need a defensible, testable rule that says: “These forms of mobile code are not allowed here,” and you must enforce that rule so users can’t fetch them and can’t run them even if they arrive through another channel. “Download” controls reduce entry. “Execution” controls reduce impact when something slips through (email, USB, sync tools, cached content). Keep the focus on enforceable outcomes: blocked actions, logged events, and validated configurations.

Who it applies to

Entity types: Federal information systems and contractor systems handling federal data, where NIST SP 800-53 is the governing control baseline or a contractual requirement. ²

Operational context where SC-18(3) matters most:

• User endpoints (managed laptops/desktops, VDI) where browsers, scripting engines, and office apps can execute code.
• Servers with administrative consoles or web-based management planes.
• High-risk ingress paths: web browsing, email clients, file-sharing, developer tooling, and remote access jump hosts.
• Third-party-provided software and plug-ins if they introduce executable content or dynamic code modules.

If you have multiple enclaves (corporate IT, regulated environment, OT, lab), expect different ODPs by enclave. Document the rationale and scope boundaries.

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

Step 1: Set the SC-18(3) ODP (define what is prohibited)

Create a short, explicit specification. Keep it enforceable.

ODP decision table (example format you can adopt):

Mobile code category	Allowed?	Conditions	Enforcement point
Browser extensions	Only approved	Signed + from approved store + allowlisted IDs	Browser management (MDM/GPO)
Office macros	Restricted	Block internet-origin macros; allow signed internal macros	Endpoint policy + Office config
Scripts (JS/VB/PS)	Restricted	Block execution from user-writable paths; allow signed scripts	EDR/app control
Active content in PDFs	Restricted	Disable JavaScript in PDF reader except approved	App config/endpoint

Your security engineering team should propose the initial ODP. You, as GRC, should make sure it is: (a) scoped, (b) testable, (c) mapped to real tools, and (d) approved by the system owner.

Step 2: Map enforcement to technical controls (download + execution)

Implement at least two layers where practical, because “download” and “execution” failures happen in different places.

Common enforcement patterns:

• Browser and web controls (download prevention): category blocking, file-type controls, domain allow/deny lists, blocking unapproved extensions, and restricting drive-by downloads.
• Email and content gateways (download prevention): strip active content, block macro-enabled attachments from untrusted sources, quarantine encrypted archives if you can’t inspect them.
• Endpoint protection and application control (execution prevention): application allowlisting/denylisting, script control, macro control, blocking execution from user-writable directories, preventing child-process creation from Office apps where appropriate.
• Network egress controls (download prevention): block known-bad or unapproved code repositories, restrict direct-to-internet from servers, enforce secure web gateway routing.
• Privileged access workstations / jump hosts (execution prevention): stricter ODP for admin contexts; prohibit all non-essential mobile code.

Step 3: Establish exceptions and compensating controls

You will have legitimate business cases (developers, IT automation, finance macros). Don’t pretend they don’t exist.

Define an exception workflow:

• Request includes business justification, affected users/assets, duration, and proposed guardrails.
• Required compensating controls: stronger monitoring, isolated environment, signed code requirements, separate sandbox/VDI, or restricted network access.
• Approval authority: system owner + security.
• Expiration and periodic review.

This is where teams often fail audits: exceptions exist, but they are informal and untracked.

Step 4: Test the control (prove prevention works)

Build a repeatable test plan that validates:

• A prohibited mobile code artifact cannot be downloaded from a controlled test location.
• If introduced by alternate means (email attachment, local copy), it cannot execute.
• Events are logged with enough detail to support incident response and compliance evidence.

Keep tests safe. Use benign test files that mimic the prohibited characteristics (for example, a signed/unsigned macro sample created for testing) and run them in a controlled environment.

Step 5: Operationalize monitoring and metrics (no invented numbers)

Define operational checks that confirm the control stays in place:

• Configuration drift monitoring for browser policies and endpoint controls.
• Alerts for blocked execution attempts of prohibited mobile code.
• Periodic sampling of endpoints to validate policy application.

Step 6: Assign ownership and evidence cadence

SC-18(3) fails in practice when nobody owns the ODP and the evidence is ad hoc.

Minimum RACI:

• Control owner: Security Engineering or Endpoint Security.
• Accountable system owner: Information System Owner.
• GRC: validates mapping, evidence, and exceptions.
• IT ops: deploys policies and fixes drift.

If you use Daydream for control operations, map SC-18(3) to a named owner, a documented procedure, and recurring evidence artifacts so assessments become a collection exercise, not a scramble. ¹

Required evidence and artifacts to retain

Keep evidence tied to the two verbs in the requirement: prevent download and prevent execution.

Evidence pack checklist:

• ODP definition for SC-18(3) (what is prohibited), with approval record. ¹
• Technical configuration exports:
  • Browser policies (extension allowlist/denylist, download restrictions).
  • Endpoint/app control policies (allowlisting/denylisting, script rules, macro settings).
  • Secure web gateway / proxy rules (file types, categories, SSL inspection stance if applicable).
• Scope list: in-scope endpoints and systems, plus any segmented enclaves with different ODPs.
• Exception register with approvals, compensating controls, and expiration.
• Test results: dated screenshots/log extracts showing blocked download attempts and blocked execution attempts.
• Change tickets for initial deployment and subsequent policy updates.
• Monitoring evidence: alert samples, SIEM rules, or dashboard exports showing detections for blocked activity.

Common exam/audit questions and hangups

Expect assessors to press on four areas:

1. “What exactly is sc-18.03_odp in your environment?”
   If you can’t state the prohibited set clearly, you will struggle to show compliance. ¹

2. “Show me technical enforcement, not a policy.”
   They will ask for config evidence and demonstrations from an actual managed endpoint.

3. “How do you prevent execution if something arrives out-of-band?”
   Download prevention alone is not enough for this enhancement.

4. “How do exceptions work?”
   Auditors often find uncontrolled “power user” groups where the rule does not apply in practice.

Frequent implementation mistakes (and how to avoid them)

• Mistake: Writing an ODP that is not enforceable.
  Avoid “block malicious mobile code.” Define concrete attributes: file types, signing requirements, sources, stores, execution paths.

• Mistake: Only controlling web downloads.
  Add endpoint execution controls (app control, macro/script restrictions). SC-18(3) explicitly includes execution. ¹

• Mistake: Assuming EDR equals prevention.
  Many EDR tools detect and respond; they may not block by default. Verify your mode and document it.

• Mistake: No artifact repeatability.
  If evidence depends on a single engineer, you will fail continuity. Use a standard evidence checklist and collect on a recurring schedule.

• Mistake: Exceptions without expiry.
  Put end dates on exceptions and re-validate compensating controls.

Enforcement context and risk implications

No public enforcement cases were provided in the source catalog for this requirement, so you should treat SC-18(3) primarily as an assessment and authorization risk within NIST-driven programs rather than a standalone enforcement headline. The operational risk is straightforward: allowing untrusted code to download and execute increases the likelihood of malware execution, credential theft, and unauthorized changes, especially on user endpoints and administrative workstations.

Practical 30/60/90-day execution plan

Day 0–30: Define and design

• Confirm system scope and boundary (which endpoints, which enclaves).
• Draft the SC-18(3) ODP with security engineering; get system owner approval. ¹
• Map enforcement points to existing tools (browser management, endpoint controls, web gateway, email security).
• Create the exception workflow and register template.
• Write a short test plan with pass/fail criteria tied to “download” and “execution.”

Day 31–60: Implement and validate

• Deploy browser policies and endpoint execution controls to a pilot group.
• Implement web gateway/file-type rules aligned to the ODP.
• Run tests; capture evidence (logs + screenshots + config exports).
• Fix gaps found in pilot (policy conflicts, business apps, developer workflows).
• Train help desk and IT ops on common break/fix scenarios and exception routing.

Day 61–90: Scale and operationalize

• Roll out to full in-scope population with change control.
• Turn on monitoring and alert routing for blocked attempts.
• Perform a second validation test on a random sample of endpoints.
• Establish recurring evidence collection and exception review.
• In Daydream, track SC-18(3) as a control with an owner, procedure, and scheduled evidence tasks so you can answer assessors with current artifacts instead of point-in-time screenshots. ¹

Frequently Asked Questions

What counts as “mobile code” for SC-18(3)?

The control text leaves the prohibited set to your organization-defined parameter, so “mobile code” must be defined in your ODP in enforceable terms. Treat it as executable active content delivered through common channels (browser, documents, plug-ins, scripts) and scope it to what your tools can block. ¹

Do we need both download prevention and execution prevention?

Yes. The requirement explicitly includes both verbs, and assessments typically expect evidence for each. Implement web/email controls for downloads and endpoint/app controls for execution. ¹

Can we meet SC-18(3) with policy and user training alone?

No. Training supports the program, but SC-18(3) requires prevention, which means technical enforcement with proof it works. Keep policy as supporting documentation, not the control itself. ²

How do we handle developers who need to run scripts and download tools?

Use a scoped exception path or a separate, more permissive developer environment with compensating controls (signing, isolation, monitoring). Document the exception, its guardrails, and its expiration so it remains auditable.

What evidence is fastest to produce for an assessor?

Provide (1) the SC-18(3) ODP, (2) exported configurations from the enforcing tools, and (3) a dated test showing blocked download and blocked execution attempts. Tie each artifact directly to the prohibited items listed in the ODP. ¹

How granular should the ODP be?

Granular enough that you can point to the exact technical setting that enforces each rule. If your ODP has statements you can’t map to a config, revise it until every line is enforceable.

Footnotes

1. NIST SP 800-53 Rev. 5 OSCAL JSON

2. NIST SP 800-53 Rev. 5