SA 17(4) Informal Correspondence

---
name: "SA-17(4)_informal-correspondence"
description: "Require the developer of the system, system component, or system service to: Produce, as an integral part of the development process, an informal desc"
category: "configuration"
version: "5.2.0"
author: "cyberstrike-official"
tags:
  - nist
  - sp800-53
  - rev5
  - sa-17-4
  - sa
  - enhancement
tech_stack:
  - any
cwe_ids:
  - CWE-16
chains_with:
  - AC-3
  - AC-4
  - AC-25
  - SA-4
  - SA-5
prerequisites:
  - SA-17
severity_boost:
  AC-3: "Chain with AC-3 for comprehensive security coverage"
  AC-4: "Chain with AC-4 for comprehensive security coverage"
  AC-25: "Chain with AC-25 for comprehensive security coverage"
---

# SA-17(4) Informal Correspondence

> **Enhancement of:** SA-17

## High-Level Description

**Family:** System and Services Acquisition (SA)
**Framework:** NIST SP 800-53 Rev 5

Correspondence is an important part of the assurance gained through modeling. It demonstrates that the implementation is an accurate transformation of the model, and that additional code or implementation detail has no impact on the behaviors or policies being modeled. Consistency between the descriptive top-level specification (i.e., high-level/low-level design) and the formal policy model is generally not amenable to being fully proven. Therefore, a combination of formal and informal methods may be needed to show such consistency. Hardware, software, and firmware mechanisms strictly internal to security-relevant hardware, software, and firmware include mapping registers and direct memory input and output.

## What to Check

- [ ] Verify SA-17(4) Informal Correspondence is documented in SSP
- [ ] Validate all 5 control requirements are implemented
- [ ] Confirm control is operating effectively
- [ ] Review evidence of continuous monitoring for SA-17(4)
- [ ] Verify enhancement builds upon base control SA-17

## How to Test

### Step 1: Review Documentation

Examine the System Security Plan (SSP) and related artifacts for SA-17(4) implementation details. Verify the organization has documented how this control is satisfied.

### Step 2: Validate Implementation

```
# For cloud environments, use cloud-audit-mcp tools
# For on-premises, review system configurations directly

# Example: Check if account management policies exist
grep -r "account.management\|access.control" /etc/security/ 2>/dev/null
```

### Step 3: Test Operating Effectiveness

Verify the control is actively functioning, not just documented. Check logs, configurations, and operational evidence.

## Tools

| Tool          | Purpose                           | Usage |
| ------------- | --------------------------------- | ----- |
| Manual Review | Documentation and interview-based | N/A   |

## Remediation Guide

### Control Statement

Require the developer of the system, system component, or system service to:
Produce, as an integral part of the development process, an informal descriptive top-level specification that specifies the interfaces to security-relevant hardware, software, and firmware in terms of exceptions, error messages, and effects;
Show via [organization-defined] that the descriptive top-level specification is consistent with the formal policy model;
Show via informal demonstration, that the descriptive top-level specification completely covers the interfaces to security-relevant hardware, software, and firmware;
Show that the descriptive top-level specification is an accurate description of the interfaces to security-relevant hardware, software, and firmware; and
Describe the security-relevant hardware, software, and firmware mechanisms not addressed in the descriptive top-level specification but strictly internal to the security-relevant hardware, software, and firmware.

### Implementation Guidance

Correspondence is an important part of the assurance gained through modeling. It demonstrates that the implementation is an accurate transformation of the model, and that additional code or implementation detail has no impact on the behaviors or policies being modeled. Consistency between the descriptive top-level specification (i.e., high-level/low-level design) and the formal policy model is generally not amenable to being fully proven. Therefore, a combination of formal and informal methods may be needed to show such consistency. Hardware, software, and firmware mechanisms strictly internal to security-relevant hardware, software, and firmware include mapping registers and direct memory input and output.

## Risk Assessment

| Finding                                          | Severity | Impact                                     |
| ------------------------------------------------ | -------- | ------------------------------------------ |
| SA-17(4) Informal Correspondence not implemented | Medium   | System and Services Acquisition            |
| SA-17(4) partially implemented                   | Low      | Incomplete System and Services Acquisition |

## CWE Categories

| CWE ID | Title         |
| ------ | ------------- |
| CWE-16 | Configuration |

## References

- [NIST SP 800-53 Rev 5 - SA-17(4)](https://csrc.nist.gov/projects/cprt/catalog#/cprt/framework/version/SP_800_53_5_1_1/home?element=sa-17.4)
- [NIST SP 800-53A Rev 5 (Assessment Procedures)](https://csrc.nist.gov/pubs/sp/800/53/a/r5/final)
- [NIST SP 800-53 Rev 5 Full Catalog](https://csrc.nist.gov/pubs/sp/800/53/r5/upd1/final)

## Checklist

- [ ] Control documented in SSP
- [ ] Implementation evidence collected
- [ ] Operating effectiveness validated
- [ ] Continuous monitoring in place
- [ ] Related controls (AC-3, AC-4, AC-25, SA-4, SA-5) reviewed