diff --git a/.github/workflows/security.yml b/.github/workflows/security.yml
new file mode 100644
index 0000000..9b9c3ba
--- /dev/null
+++ b/.github/workflows/security.yml
@@ -0,0 +1,57 @@
+name: Security
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+  schedule:
+    - cron: '0 6 * * 1'
+
+permissions:
+  contents: read
+
+jobs:
+  codeql:
+    name: CodeQL (${{ matrix.language }})
+    runs-on: ubuntu-latest
+    timeout-minutes: 30
+    concurrency:
+      group: check-${{ github.workflow }}-${{ github.ref }}-codeql-${{ matrix.language }}
+      cancel-in-progress: true
+    permissions:
+      contents: read
+      security-events: write
+    strategy:
+      fail-fast: false
+      matrix:
+        language: [python, actions]
+    steps:
+      - uses: actions/checkout@v6
+
+      - name: Initialize CodeQL
+        uses: github/codeql-action/init@v4
+        with:
+          languages: ${{ matrix.language }}
+
+      - name: Autobuild
+        uses: github/codeql-action/autobuild@v4
+
+      - name: Analyze
+        uses: github/codeql-action/analyze@v4
+
+  dependency-review:
+    name: Dependency Review
+    if: github.event_name == 'pull_request'
+    runs-on: ubuntu-latest
+    timeout-minutes: 10
+    permissions:
+      contents: read
+      pull-requests: write
+    steps:
+      - uses: actions/checkout@v6
+
+      - name: Review dependency changes
+        uses: actions/dependency-review-action@v5
+        with:
+          fail-on-severity: high
+          comment-summary-in-pr: on-failure
diff --git a/changelog.d/20260809_issue_48_security_scanning.md b/changelog.d/20260809_issue_48_security_scanning.md
new file mode 100644
index 0000000..789c322
--- /dev/null
+++ b/changelog.d/20260809_issue_48_security_scanning.md
@@ -0,0 +1,4 @@
+### Security
+
+- Add CodeQL scanning for Python code and GitHub Actions workflows, plus
+  dependency review for pull requests.
diff --git a/tests/test_workflows.py b/tests/test_workflows.py
index 725b5b5..ab62e58 100644
--- a/tests/test_workflows.py
+++ b/tests/test_workflows.py
@@ -105,6 +105,35 @@ def test_workflow_run_blocks_do_not_interpolate_untrusted_inputs() -> None:
             )
 
 
+def test_security_workflow_scans_code_actions_and_dependencies() -> None:
+    """Security checks must cover pushes, pull requests, and scheduled scans."""
+    workflow = read_workflow("security.yml")
+    codeql_job = workflow_job_block(workflow, "codeql")
+    dependency_job = workflow_job_block(workflow, "dependency-review")
+
+    assert "branches: [main]" in workflow
+    assert "pull_request:" in workflow
+    assert "schedule:" in workflow
+    assert "cron: '0 6 * * 1'" in workflow
+    assert "permissions:\n  contents: read" in workflow
+
+    assert "timeout-minutes: 30" in codeql_job
+    assert "security-events: write" in codeql_job
+    assert "language: [python, actions]" in codeql_job
+    assert "languages: ${{ matrix.language }}" in codeql_job
+    assert "uses: github/codeql-action/init@v4" in codeql_job
+    assert "uses: github/codeql-action/autobuild@v4" in codeql_job
+    assert "uses: github/codeql-action/analyze@v4" in codeql_job
+    assert "cancel-in-progress: true" in codeql_job
+
+    assert "if: github.event_name == 'pull_request'" in dependency_job
+    assert "timeout-minutes: 10" in dependency_job
+    assert "pull-requests: write" in dependency_job
+    assert "uses: actions/dependency-review-action@v5" in dependency_job
+    assert "fail-on-severity: high" in dependency_job
+    assert "comment-summary-in-pr: on-failure" in dependency_job
+
+
 def test_changelog_check_safely_requires_a_fragment() -> None:
     """Source-changing pull requests must fail safely without a fragment."""
     workflow = read_workflow("release.yml")