diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 02f2d89..cb2410d 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -92,7 +92,8 @@ jobs:
           --cov-report=xml
       - name: Upload coverage
         if: matrix.python-version == '3.12'
-        uses: codecov/codecov-action@v6
+        # codecov/codecov-action pinned to full commit SHA — v6.0.0
+        uses: codecov/codecov-action@57e3a136b779b570ffcdbf80b3bdc90e7fab3de2
         with:
           files: coverage.xml
           fail_ci_if_error: false
diff --git a/.github/workflows/docker-publish.yml b/.github/workflows/docker-publish.yml
index 613206e..56d6a18 100644
--- a/.github/workflows/docker-publish.yml
+++ b/.github/workflows/docker-publish.yml
@@ -36,10 +36,12 @@ jobs:
           VERSION="${GITHUB_REF_NAME#v}"
           echo "version=$VERSION" >> "$GITHUB_OUTPUT"
 
-      - uses: docker/setup-buildx-action@v4
+      # docker/setup-buildx-action pinned to full commit SHA — v4.0.0
+      - uses: docker/setup-buildx-action@4d04d5d9486b7bd6fa91e7baf45bbb4f8b9deedd
 
       - name: Log in to GHCR
-        uses: docker/login-action@v4
+        # docker/login-action pinned to full commit SHA — v4.1.0
+        uses: docker/login-action@4907a6ddec9925e35a0a9e82d7399ccc52663121
         with:
           registry: ghcr.io
           username: ${{ github.actor }}
@@ -47,13 +49,15 @@ jobs:
 
       - name: Log in to Docker Hub
         if: vars.DOCKERHUB_USERNAME != ''
-        uses: docker/login-action@v4
+        # docker/login-action pinned to full commit SHA — v4.1.0
+        uses: docker/login-action@4907a6ddec9925e35a0a9e82d7399ccc52663121
         with:
           username: ${{ vars.DOCKERHUB_USERNAME }}
           password: ${{ secrets.DOCKERHUB_TOKEN }}
 
       - name: Build and push
-        uses: docker/build-push-action@v7
+        # docker/build-push-action pinned to full commit SHA — v7.1.0
+        uses: docker/build-push-action@bcafcacb16a39f128d818304e6c9c0c18556b85f
         with:
           context: .
           push: true
diff --git a/.github/workflows/docker-smoke.yml b/.github/workflows/docker-smoke.yml
index e0039ec..cacf96b 100644
--- a/.github/workflows/docker-smoke.yml
+++ b/.github/workflows/docker-smoke.yml
@@ -48,10 +48,12 @@ jobs:
       - uses: actions/checkout@v6
 
       - name: Set up Docker Buildx
-        uses: docker/setup-buildx-action@v3
+        # docker/setup-buildx-action pinned to full commit SHA — v3.12.0
+        uses: docker/setup-buildx-action@8d2750c68a42422c14e847fe6c8ac0403b4cbd6f
 
       - name: Build image (no push)
-        uses: docker/build-push-action@v6
+        # docker/build-push-action pinned to full commit SHA — v6.19.2
+        uses: docker/build-push-action@10e90e3645eae34f1e60eeb005ba3a3d33f178e8
         with:
           context: .
           push: false
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 884cc42..68614d5 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -8,6 +8,17 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Security
+- **Third-party GitHub Actions pinned to full commit SHAs** (instead of floating `@vN` major-version tags). A moving major-version tag on a third-party action is an unreviewed supply-chain channel — the upstream owner can move the tag to any commit at any time, including after a repo compromise. Pinning to the commit SHA freezes the code under review. Affected references in `.github/workflows/`:
+  - `codecov/codecov-action@v6` → `@57e3a136…` (v6.0.0) in `ci.yml`
+  - `docker/setup-buildx-action@v3` → `@8d2750c6…` (v3.12.0) in `docker-smoke.yml`
+  - `docker/build-push-action@v6` → `@10e90e36…` (v6.19.2) in `docker-smoke.yml`
+  - `docker/setup-buildx-action@v4` → `@4d04d5d9…` (v4.0.0) in `docker-publish.yml`
+  - `docker/login-action@v4` → `@4907a6dd…` (v4.1.0) in `docker-publish.yml` (two invocations)
+  - `docker/build-push-action@v7` → `@bcafcacb…` (v7.1.0) in `docker-publish.yml`
+
+  Each pinned line carries a trailing `# <action> pinned to full commit SHA — <version>` comment so the human-readable version stays visible and Dependabot's `github-actions` ecosystem can still propose updates. First-party `actions/*` (checkout, setup-python) remain tag-pinned — GitHub's own actions are in a separate trust boundary. Closes medium-severity gap #5 from the 2026-04-21 contribution-safety audit (`audit-contribution-safety-2026-04-21`).
+
 ### Added
 - **`.github/PULL_REQUEST_TEMPLATE.md`** — standard template for incoming PRs. Sections: linked-issue (engagement-first), summary, scope, AI-assistance disclosure checkboxes, `## QA` block (prerequisites + MCP-tool-driven manual tests), and a contributor checklist (CHANGELOG entry, README/data-dictionary updates, no-secrets affirmation, local CI, CLA signature). Mirrors the shape already expected by `CLAUDE.md` and `CONTRIBUTING.md` §"Pull request guidelines" — the template just makes it discoverable at PR-authoring time instead of requiring contributors to find the conventions themselves.
 - **README `## Contributing` section** — one paragraph between "How it's built" and "License" pointing at `CONTRIBUTING.md` for the PR flow and `SECURITY.md` for vulnerability reports. Previously neither was linked from the README, so contributors had to discover them by file listing.