# PR#18: bwrap starts with an empty environment

> **WARNING:** This document describes only the change applied in this PR.
> It may already be outdated once the next PR is merged.
> Historic PR documentation is not maintained along with new PRs; treat it as a snapshot.

## Related Links

- [werkdock PR#17](https://git.javagil.de/mi/werkdock/pulls/17): bwrap reads the sandbox's environment from fd 4.
- [werkator PR#166](https://git.javagil.de/mi/werkator/pulls/166):
  Werkator passes the values of a build's variables to werkdock through its environment.

## Problem

Since werkator PR#166, werkdock's own environment holds the values of a build's variables, e.g. its secrets,
and a branch chooses some of their names.
bwrap inherits that environment, and bwrap runs on the host.
A host program that reads a variable a branch chose can be steered by the branch:
the Codex review of werkator PR#166 showed `BASH_ENV` reaching a credential helper of the docker client that way.
bwrap needs no variable at all, since PR#17 hands it the sandbox's environment through fd 4.

## Non-Goals

- No change of what the sandbox sees.
- No change of the environment of `werkdock supervise`, which is werkdock itself.

## Specification

### Feature: no host program reads the caller's variables

#### Scenario#18.01: bwrap gets nothing of werkdock's environment

… so that a variable a build chose cannot steer a program on the host.

- **Given** a variable in werkdock's own environment
- **When** werkdock starts a sandbox
- **Then** bwrap's environment is empty
  - **and** the sandbox still sees its own variables

##### Verified by

- [TestStartSetsTheEnvironmentOffTheCommandLine](../../internal/engine/bwrap_test.go)

## Solution

- `Bwrap.Start` sets `cmd.Env` to an empty list; bwrap needs no variable, its arguments and fd 4 carry everything.
- `werkdock run` starts no other host program:
  it does not pull, and `run -d` starts `werkdock supervise`, which is werkdock itself.
  So werkdock is the only host program that reads the caller's variables, and it reads only its own names.
- Red/green: without the empty list, the test finds 5,349 bytes in bwrap's `/proc/<pid>/environ`.
- Version 0.13.3 with a section in `CHANGELOG.md`; `docs/risk-analysis.md` names the measure.

## Open Questions

None.

## Additional Changes

None.