core: fix client-side only validation allowing admin to set blank user password

[BeryJu]

Details

Previously was only relying on client-side validation

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Checklist

• Local tests pass (ak test authentik/)
• The code has been formatted (make lint-fix)

If an API change has been made

• The API schema has been updated (make gen-build)

If changes to the frontend have been made

• The code has been formatted (make web)

If applicable

• The documentation has been updated
• The documentation has been formatted (make docs)

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.78%. Comparing base (665cbd2) to head (6e6566d).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #16467   +/-   ##
=======================================
  Coverage   92.78%   92.78%           
=======================================
  Files         838      838           
  Lines       45295    45304    +9     
=======================================
+ Hits        42025    42035   +10     
+ Misses       3270     3269    -1     

Flag	Coverage Δ
e2e	46.57% <20.00%> (-0.01%)	⬇️
integration	23.53% <20.00%> (+<0.01%)	⬆️
unit	90.92% <100.00%> (+<0.01%)	⬆️

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[authentik-automation]

🍒 Cherry-pick to version-2025.8 created: #16469

[authentik-automation]

⚠️ Cherry-pick to version-2025.6 has conflicts: #16470

[github-actions]

authentik PR Installation instructions

Instructions for docker-compose

Add the following block to your .env file:

AUTHENTIK_IMAGE=ghcr.io/goauthentik/dev-server
AUTHENTIK_TAG=gh-6e6566d811578e0982b11a7617d3a745c5c8e08e
AUTHENTIK_OUTPOSTS__CONTAINER_IMAGE_BASE=ghcr.io/goauthentik/dev-%(type)s:gh-%(build_hash)s

Afterwards, run the upgrade commands from the latest release notes.

Instructions for Kubernetes

Add the following block to your values.yml file:

authentik:
    outposts:
        container_image_base: ghcr.io/goauthentik/dev-%(type)s:gh-%(build_hash)s
global:
    image:
        repository: ghcr.io/goauthentik/dev-server
        tag: gh-6e6566d811578e0982b11a7617d3a745c5c8e08e

Afterwards, run the upgrade commands from the latest release notes.