diff --git a/README.md b/README.md
index 153154a..ab1a50a 100644
--- a/README.md
+++ b/README.md
@@ -3,9 +3,11 @@
 Composite Gitea Action that builds a container image with buildx and optionally
 runs a smoke test. **Does not push** — pair with `action/image-push` to publish.
 
-Splitting build from push lets a PR workflow run `image-build` (no secrets, no
-side effects) for validation while `main` runs the full build → push → deploy
-chain.
+Splitting build from push lets a PR workflow run `image-build` without push or
+deploy side effects while `main` runs the full build → push → deploy chain. A
+PR build that pulls a private base image still needs a registry token limited to
+the `read:package` capability; public-base builds need no token. The action logs
+in as `ci-bot`, so the token must be issued to that account.
 
 ## Usage
 
@@ -21,6 +23,22 @@ The image is built and tagged as `<image>:<github.run_number>` in the runner's
 local Docker daemon. Subsequent steps (e.g. `action/image-push`) can reference
 the same tag.
 
+### Private bases in PRs
+
+Pass `token` only when the PR head is trusted, and limit `ci-bot` package access
+to the private base images that the build requires. Never expose an
+organization-wide package reader to a contributor-controlled Dockerfile: it can
+pull and disclose any package that the account can read.
+
+```yaml
+with:
+  token: ${{ secrets.PACKAGE_READ_TOKEN }} # caller-chosen secret name
+```
+
+The token must be issued to `ci-bot` with `read:package` capability. Tokens from
+other accounts fail because the action's registry username is fixed. Omit the
+input for public bases.
+
 ## Inputs
 
 | Name | Required | Default | Description |
@@ -31,6 +49,7 @@ the same tag.
 | `build-args` | no | — | Multiline `KEY=VALUE` build args. Visible in `docker history` — never put secrets here. |
 | `secrets` | no | — | Multiline `id=VALUE` BuildKit secrets (`--secret`). For tokens the build needs (e.g. a ci-bot token to `go mod download` a private module) that must not leak into layers. Reference with `RUN --mount=type=secret,id=<id>`. |
 | `smoke-test` | no | — | Shell command run after build. `$IMAGE` is set to `<image>:<run_number>`. Non-zero exit fails the action. |
+| `token` | no | — | `ci-bot` access token with `read:package` capability. Required to pull a private base image; omit for public bases. |
 
 ## Outputs
 
diff --git a/action.yaml b/action.yaml
index 5d865dc..d23d4be 100644
--- a/action.yaml
+++ b/action.yaml
@@ -37,10 +37,13 @@ inputs:
     default: ''
   token:
     description: |
-      ci-bot token (CI_BOT_TOKEN) for `docker login code.fritzlab.net`. Required
-      when the Dockerfile's FROM is a PRIVATE fritzlab image (e.g. FROM
-      code.fritzlab.net/fritzlab/base) — the org is `limited`, so buildx can't pull
-      it anonymously. Omit for public-base builds (e.g. base itself = FROM debian).
+      ci-bot access token with `read:package` capability for
+      `docker login code.fritzlab.net`. The login username is fixed to `ci-bot`,
+      so a token issued to another account will fail. Required when the
+      Dockerfile's FROM is a PRIVATE fritzlab image (e.g. FROM
+      code.fritzlab.net/fritzlab/base) — the org is `limited`, so buildx can't
+      pull it anonymously. Omit for public-base builds (e.g. base itself = FROM
+      debian).
     required: false
     default: ''
 outputs: