diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 816970cd..582cd3e4 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -42,7 +42,7 @@ jobs:
         python-version: ["3.12", "3.13", "3.14"]
 
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
 
       - name: Set up uv
         # Pinned to a full commit SHA (third-party action); comment tracks the tag.
@@ -68,7 +68,7 @@ jobs:
     runs-on: ubuntu-latest
     if: github.event_name == 'pull_request'
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
         with:
           fetch-depth: 0
 
diff --git a/Dockerfile b/Dockerfile
index 592e2eee..e185f882 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -1,4 +1,4 @@
-FROM python:3.12-slim-bookworm AS builder
+FROM python:3.12-slim-bookworm@sha256:8a7e7cc04fd3e2bd787f7f24e22d5d119aa590d429b50c95dfe12b3abe52f48b AS builder
 
 WORKDIR /app
 COPY pyproject.toml README.md ./
@@ -6,7 +6,7 @@ COPY src/ src/
 RUN python -m venv .venv
 RUN .venv/bin/pip install --no-cache-dir .
 
-FROM python:3.12-slim-bookworm
+FROM python:3.12-slim-bookworm@sha256:8a7e7cc04fd3e2bd787f7f24e22d5d119aa590d429b50c95dfe12b3abe52f48b
 
 RUN apt-get update \
     && apt-get install --no-install-recommends -y git ca-certificates \