Microsoft Tightens Driver Package Signing Rules, Requiring INF Referencing for Windows Update

Microsoft is fundamentally changing how driver packages are processed through its Hardware Dev Center (HDC), shifting from a permissive signing approach to an explicit reference requirement model. This policy modification directly impacts OEMs, IHVs, and driver developers distributing drivers through Windows Update, mandating technical adjustments to maintain signature validation.

Policy Shift Explained

Previously, HDC automatically signed every file within submitted driver packages (in .cab or HLKx formats), regardless of whether the files were referenced in the driver's INF configuration. Under the new policy:

• Files lacking INF references (e.g., SourceDisksFiles or CatalogFile entries) will be returned unsigned
• Windows Update-published driver bundles will exclusively contain INF-referenced files
• Unreferenced files (helper tools, debug utilities, etc.) become ineligible for Microsoft signing

This change enhances package integrity by eliminating unreferenced artifacts that bypassed validation checks. It reduces security risks from unsigned components in distributed drivers and ensures Windows Update deliveries strictly match INF declarations.

Implementation Timeline

Microsoft is executing a phased rollout:

1. Logging-Only Mode (Effective February 23):
   Submissions process normally, but HDC generates logs flagging unreferenced files via both UI and API outputs. Partners receive warnings without functional impact. The February 2026 HLK refresh adds package validation warnings during creation (HLK release notes).

2. Enforcement Mode (Date TBA):
   Unreferenced files are returned unsigned. Driver packages published to Windows Update will exclude these files entirely. Microsoft will announce the enforcement date after reviewing partner feedback.

Impact Analysis

Aspect	Previous Policy	New Policy
Signing Scope	All package files signed	Only INF-referenced files signed
Windows Update Content	All submitted files included	Only INF-referenced files included
Security Posture	Potential unsigned artifacts	Strict file-to-INF alignment
Developer Overhead	Low (automatic signing)	Requires INF audits and updates

Critical Action Items for Partners

1. Audit Packages: Identify unreferenced files using INF SourceDisksFiles documentation (Microsoft Learn)
2. Validate with HLK: Use the latest Hardware Lab Kit to detect unreferenced files before submission (Download HLK)
3. Update Automation: Modify build pipelines to prevent inclusion of non-INF files
4. Handle Runtime Dependencies: Firmware blobs or configuration files must be INF-referenced or distributed separately

Operational Nuances

• During enforcement, submissions containing unreferenced files won't be rejected; such files will simply remain unsigned
• Nested CAB files remain unaffected (protected by catalog signing)
• Symbol files are exempted and don't require INF references
• Partners can monitor compliance via HDC logs or HLK warnings

Microsoft cites improved supply chain security as the primary motivation, preventing unreferenced files from bypassing signature validation. Partners maintaining helper utilities or diagnostic tools within driver packages face the most significant adaptation burden. Those requiring assistance should submit Partner Center support cases referencing "HDC Unreferenced Files Policy" (Support Portal).

This policy realignment signifies Microsoft's broader shift toward stricter software supply chain controls, mirroring industry trends toward explicit component declarations. Partners should treat this as an opportunity to streamline driver packages and eliminate redundant artifacts.