Privacy Guard

---
name: privacy-guard
description: Prevents private infrastructure details (node hostnames, internal project names, local usernames and personal emails, absolute home paths, private and VPN IP ranges) from leaking into public repositories through commits, PRs, docs or release artifacts. Use when working in a public or soon-to-be-public repo, before commits or releases, when writing deployment docs for an OSS project, or when the user asks to "set up the privacy guard", "install privacy-guard", "check for leaks", "protect this public repo". Bootstraps a pre-commit hook backed by a gitignored local denylist plus gitleaks.
argument-hint: "[setup <repo-path> | check | update-denylist]"
---

# privacy-guard

Guard against accidentally publishing details of your private infrastructure in public
repositories (or in repositories that may become public later). Two legs: behavioral rules
for the Claude session, always on, and a per-repo technical gate: a pre-commit hook backed
by a gitignored local denylist, plus gitleaks.

## Threat model

The public repository is the boundary. Anything that describes your private infrastructure
must not cross it, in any form: committed files, commit messages, branch names, PR and
issue bodies, release notes, published artifacts, screenshots.

**Sensitive, never in a public repo:**
- node hostnames and internal aliases (workstations, VPS, client machines)
- names of internal projects and service instances that were never published
- local usernames, personal and work email addresses
- personal domains and URLs of internal services
- absolute paths of home directories and internal mounts
- private-network IP ranges and VPN address space (for example the Tailscale CGNAT range
  `100.64.0.0/10`)
- cookie files, tokens, credentials (also covered by gitleaks)

**Not sensitive, fine in public docs:**
- public products and technologies named generically (Tailscale, Docker, n8n)
- the maintainer's public GitHub username
- deployment patterns described in generic form ("a Docker host reachable over your private
  network / behind a VPN")

The concrete list of sensitive tokens is yours and stays private. This skill ships
`references/denylist-template.txt`, a placeholder-only starting point: fill it in a private
location (a private dotfiles repo, a private notes repo) and treat that filled copy as the
seed you propagate from. Never commit the filled version to a public repo.

## Behavioral rules (Claude session in a public repo)

1. Never write internal tokens into committed files. Private deployment details are
   documented in a private repo or in a gitignored `.local/` directory, never in the public
   repo's docs.
2. The user mentioning internal names in conversation does NOT authorize writing them into
   the repo: the conversation is private, the repo is not.
3. Before every commit: re-read the diff looking for denylist tokens. The hook is the safety
   net, not the first check.
4. This applies outside files too: commit messages, branch names, PR and issue titles and
   bodies, CHANGELOG entries, published artifacts.
5. If a sensitive token has already reached published git history: tell the user immediately
   (rotation, history rewrite with BFG or filter-repo are their calls), do not just remove it
   from the tip.

## Per-repo setup (`setup <repo-path>`)

From the root of the target repo:

1. Copy `references/check_privacy.sh` to `scripts/check_privacy.sh` and make it executable.
   It stays a **copy**: fix it here and recopy, never edit it there. See *Keeping copies
   current* below for why that rule needs a check behind it.
2. Create `.local/privacy-denylist.txt` from your private seed (or from
   `references/denylist-template.txt` on a first run), adapting the patterns to this repo's
   context.
3. Make sure `.gitignore` contains `.local/`.
4. Add the blocks from `references/pre-commit-snippet.yaml` to `.pre-commit-config.yaml`
   (gitleaks + privacy-denylist).
5. Run `pre-commit install`, adding `--hook-type pre-push` if the repo uses push hooks.
6. Verify: create a temporary file containing one token from the denylist, **stage it**, and
   check that `scripts/check_privacy.sh <file>` exits with code 1. An unfilled denylist (only
   comments) makes the hook a silent no-op, so this step is what tells you the guard is
   actually armed. Check the other direction too — a file with no token must exit 0 — or you
   have only shown that the script can fail, not that it discriminates.

   > **Put a token in that file, not a pattern.** The denylist holds extended regexes, and
   > most of them are anchored (`\bexample\b`). Writing that text verbatim does not match
   > itself: the file then contains literal backslashes, the pattern finds nothing, and the
   > script exits 0 — which reads as "the guard is not armed" when in fact the test was
   > wrong. Strip the anchors and use the bare string the pattern is meant to catch. Measured
   > on 2026-08-09 while arming three repos: 6 of 29 patterns happened to match their own
   > text, so picking the first line of the denylist gave a false "not armed" verdict.
   >
   > Staging matters for the same reason: `check_privacy.sh` reads `git diff --cached`, so an
   > unstaged file has no added lines and the script reports that nothing was checked.

Design property: the denylist is NOT committed, because publishing the list would reveal the
very tokens it protects. As a consequence the hook is a no-op for external contributors and
in CI. Generic secrets (keys, tokens) stay covered by gitleaks, which runs everywhere.

### Variant: repos with versioned git hooks

If the target repo sets `core.hooksPath` (a shared `.githooks/` directory committed to the
repo), `pre-commit install` refuses to run, by design: *"Cowardly refusing to install hooks
with `core.hooksPath` set"*. Do not unset it to make room for the framework, that would
disable the hooks the repo already relies on. Instead, replace steps 4 and 5 with a call to
`check_privacy.sh` from the existing hook, over the staged file list:

```sh
files=()
while IFS= read -r -d '' f; do files+=("$f"); done \
    < <(git diff --cached --name-only -z --diff-filter=d)
if [ ${#files[@]} -gt 0 ]; then
    bash scripts/check_privacy.sh "${files[@]}" || exit 1
fi
```

The `-z`/`read -d ''` pair keeps filenames with spaces intact, and `|| exit 1` is what makes
the hook actually block: swallowing the script's exit code leaves a guard that reports and
lets the commit through anyway.

Two consequences worth stating to the user. Without the framework nothing invokes gitleaks
per commit, so run it in CI instead. And the framework's stash step no longer changes what
this check sees: it reads `git diff --cached`, so it looks at the staged blobs either way.
The flip side is a real loss and should be said out loud: a token living only in an unstaged
edit is no longer reported, where the old whole-file form did notice it. It is not being
committed, so a pre-commit guard staying quiet about it is defensible, but it is a narrower
net than before.

## Denylist maintenance (`update-denylist`)

When a new sensitive token appears (a new node, a new instance, a new domain):
1. update your private seed, the single source of truth;
2. propagate it to the `.local/privacy-denylist.txt` of every public repo active on the node.

## Keeping copies current (`check-sync.sh`)

`check_privacy.sh` is copied into each repo on purpose: the repo must stand on its own for
external contributors and CI, which do not have this plugin installed. The price of copying
is drift, and drift here is **invisible** — a copy a month behind looks exactly like a
current one. Observed: one copy had been missing a fix for a month while also carrying a
local improvement nobody had brought upstream, and nothing could have surfaced either.

Two things make it visible. Each copy carries its **version and provenance** in the header,
so a copy can be placed. And:

```sh
references/check-sync.sh REPO...     # 0 all current, 1 any divergence, 2 usage
```

It asks **git** which copies a repo ships (`ls-files`), not the filesystem and not a
hardcoded path: that answers the real question ("what does this repo distribute") and it
answers it wherever the copy lives. Four verdicts, and the last is the one that matters:

- `OK` — byte-identical to the canonical copy.
- `BEHIND` — older version, or no version line at all (a copy predating the line itself).
- `DIVERGED` — **same version, different content**. The worst case, because the version
  claims to be current and is not. Take the change upstream and recopy; leave it, and the
  next sync reverts it in silence.
- `NONE` — the repo ships no copy. Said out loud rather than passed over, because silence
  would read as "current".

Run it when you touch this skill, and when a repo's guard behaves unlike the documentation.
It compares only what a repo *ships*; a copy that exists but is untracked is not the guard
that reaches anyone else.

## Known limits

- The guard is client-side: it protects commits made from a node that has the denylist.
- `check-sync.sh` compares against the copy shipped with *this* plugin checkout, and there
  are **three** copies in play that do not align on their own: the source repo, the
  marketplace clone, and the installed plugin under
  `~/.claude/plugins/cache/<marketplace>/<plugin>/<version>/`. On a node whose plugin cache
  is stale it will confidently report repos as current against an old canonical. Measured
  on two nodes: the marketplace clone was five days and 28 commits behind while the
  installed plugin was two minor versions back. Refresh both before trusting a clean run:

  ```sh
  claude plugin marketplace update <marketplace>
  claude plugin update <plugin>@<marketplace>    # restart to apply
  ```
- It scans the lines a commit ADDS, not whole files: a token already committed keeps
  passing until someone removes it. That is deliberate (scanning whole files wedged every
  later edit to a file that legitimately names its own denylist tokens, and left
  `--no-verify` as the only exit), but it means the guard stops NEW leaks and does not
  audit history. For an audit, grep the tracked tree against the denylist by hand.
- It does not cover manual pastes into the GitHub web UI. Only the behavioral rules do.
- Word-boundary patterns (`\bfoo\b`) can produce false positives inside hashes and IDs. The
  hook prints the matched lines, so judge case by case.