fix(mdm): prevent windows client certificate lifetime truncation (#52601)

[amasen02]

Related issue: Resolves #52601

Summary of Changes

This PR resolves issue #52601 by fixing the Windows MDM client certificate backdating calculation in populateClientCert and aligning the renewal period constants:

1. Eliminate 180-day backdating truncation (server/mdm/microsoft/wstep.go):

   • populateClientCert previously computed NotBefore by subtracting PolicyCertRenewalPeriodInSecs (180 days) from time.Now(), and added a hardcoded 365-day duration to get NotAfter, leaving the newly-issued certificate with only ~185 days of remaining validity and immediately opening the Windows renewal window at enrollment.
   • Fixed by deriving NotBefore from issuance time with a 10-minute clock-skew allowance (now.Add(-10 * time.Minute)), matching the SCEP / token PKI pattern used across Fleet.
   • Derived NotAfter directly from syncml.PolicyCertValidityPeriodInSecs (now.Add(validityPeriod)), ensuring the issued certificate validity stays strictly synchronized with the policy advertised in GetPolicies.

2. Align renewal period constant (server/mdm/microsoft/syncml/syncml.go):

   • Updated WstepCertRenewalPeriodInDays from "365" to "180" to match PolicyCertRenewalPeriodInSecs = "15552000" (180 days).

3. Automated Unit Tests (server/mdm/microsoft/wstep_test.go):

   • Added TestPopulateClientCertValidityAndNotBefore verifying that NotBefore reflects current issuance time with the 10m leeway, NotAfter provides the full 365-day validity window without truncation, and WstepCertRenewalPeriodInDays agrees with PolicyCertRenewalPeriodInSecs.

Checklist for submitter

• Changes file added for user-visible changes in changes/: changes/52601-windows-mdm-client-cert-validity
• Input data is properly validated
• Timeouts are implemented and retries are limited to avoid infinite loops

Testing

• Added/updated automated tests (TestPopulateClientCertValidityAndNotBefore in server/mdm/microsoft/wstep_test.go)
• QA'd all new/changed functionality manually

Summary by CodeRabbit

• Bug Fixes
  • Corrected Windows MDM client certificate validity dates.
  • Certificates now use a one-year validity period from issuance, with a 10-minute allowance for device clock differences.
  • Aligned certificate renewal timing to a 180-day period.
  • Improved certificate timing to reduce validity issues caused by device clock differences.

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• server/mdm/microsoft/wstep_test.go

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Walkthrough

The Windows MDM provisioning renewal period is now 180 days. populateClientCert derives certificate validity from PolicyCertValidityPeriodInSecs, sets NotBefore 10 minutes before issuance, and sets NotAfter from the validity period. A regression test verifies the certificate dates, effective validity, and renewal period.

Merge Risk: ⚪ Minimal · up to f474f

Windows MDM client certificates now retain the advertised validity period while allowing limited clock skew, and renewal begins at the aligned 180-day interval. No current merge-blocking risk is identified.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 75.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 4 functions across 3 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely describes the primary change: preventing Windows MDM client certificate lifetime truncation. It also includes the issue number.
Description check	✅ Passed	The description identifies the related issue, explains the implementation changes, and documents automated and manual testing. It is sufficiently complete for this bug fix; omitted checklist sections …
Linked Issues check	✅ Passed	The changes satisfy issue #52601. They set NotBefore to issuance time minus 10 minutes, derive NotAfter from PolicyCertValidityPeriodInSecs, align WstepCertRenewalPeriodInDays with the 180-day renewal…
Out of Scope Changes check	✅ Passed	All changes are directly related to issue #52601. The implementation update, renewal-period constant change, change log entry, and unit tests support the stated certificate lifetime and renewal-window…

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Actionable comments posted: 1

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Inline comments:
In `@server/mdm/microsoft/wstep_test.go`:
- Around line 399-400: Update the test around populateClientCert to parse
PolicyCertValidityPeriodInSecs and PolicyCertRenewalPeriodInSecs with strconv,
deriving the expected certificate validity window and renewal days from those
policy values instead of hard-coded durations or "180". Compare the generated
certificate dates and WstepCertRenewalPeriodInDays against the parsed policy
values while preserving the existing tolerance bounds.

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📥 Commits

Reviewing files that changed from the base of the PR and between 359a6ce and 1baae61.

📒 Files selected for processing (4)

• changes/52601-windows-mdm-client-cert-validity
• server/mdm/microsoft/syncml/syncml.go
• server/mdm/microsoft/wstep.go
• server/mdm/microsoft/wstep_test.go

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[codecov]

Codecov Report

❌ Patch coverage is 85.71429% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 75.91%. Comparing base (359a6ce) to head (f474fd5).

Files with missing lines	Patch %	Lines
server/mdm/microsoft/wstep.go	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #52620      +/-   ##
==========================================
- Coverage   75.91%   75.91%   -0.01%     
==========================================
  Files        4102     4102              
  Lines      247984   247985       +1     
  Branches    14295    14295              
==========================================
- Hits       188262   188260       -2     
- Misses      59545    59548       +3     
  Partials      177      177              

Flag	Coverage Δ
backend	77.59% <85.71%> (-0.01%)	⬇️

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