Harden release workflows against tag injection and credential leakage

[Mpdreamz]

Summary

Security audit of the release-related workflows (create-major-tag.yml, required-labels.yml) revealed several gaps compared to the recently hardened docs-build and docs-deploy workflows. This PR brings them up to the same security standard.

Changes

create-major-tag.yml — 4 fixes

1. Strict semver tag validation (HIGH)

The workflow previously extracted a "major version" from GITHUB_REF via awk with no format validation. Since release: published fires for any release (including manually-created ones), an attacker with release-create permission could publish a release with an arbitrary tag name. The extracted value would then be force-pushed as a tag, potentially overwriting legitimate major version tags like v1 that downstream consumers depend on.

Now rejects any tag not matching ^[0-9]+\.[0-9]+\.[0-9]+$ before performing any git operations.

2. persist-credentials: false on checkout (HIGH)

The checkout step was storing contents: write-scoped credentials in .git/config, making them available to all subsequent run: steps. If any step were compromised or the workflow extended carelessly, those credentials could push arbitrary code to any branch. Now uses explicit token-based remote URL (set and immediately cleared) only in the step that needs it — matching the pattern used in changelog/submit.

3. Least-privilege permissions (MEDIUM)

contents: write was set at the workflow level, meaning any future job added to this file would silently inherit write access. Moved to contents: read at workflow level with contents: write granted only to the job that needs it.

4. Concurrency guard (MEDIUM)

Two releases published in quick succession (e.g. 2.0.0 and 2.1.0) would both extract MAJOR_VERSION=2 and race on git push -f for the v2 tag. Added a serializing concurrency group with cancel-in-progress: false so tag updates are applied in order.

required-labels.yml — 2 fixes

5. Hardened pull_request_target checkout (MEDIUM)

This workflow runs on pull_request_target (base-repo context with elevated trust). While the default checkout is the base branch (safe today), the lack of explicit constraints made it a footgun — a future edit adding ref: ${{ github.event.pull_request.head.sha }} would immediately become a critical code-execution vulnerability from fork PRs.

Now uses sparse-checkout limited to the single file it needs (.github/release-drafter.yml) and persist-credentials: false to eliminate credential exposure.

6. Delimiter-based GITHUB_OUTPUT (LOW-MEDIUM)

The yq output was written to GITHUB_OUTPUT using inline key=value format. If the parsed YAML ever produced output containing newlines, it could inject additional output variables. Now uses a random hex delimiter boundary, which is the recommended pattern for multi-line or untrusted output values.

Test plan

• Merge a PR with a release label → required-labels check should still pass and correctly extract labels from release-drafter.yml
• Publish a release with a valid semver tag (e.g. 99.0.0) → create-major-tag should create v99
• Verify that publishing a release with an invalid tag (e.g. foo-bar) is rejected by the validation step

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