ADR 0086: a secret reaches a process as a file, and an exception is declared

Closes issue 041 by decision and by code on the same branch: the catalogue
engine refuses a secret in a container's env, and a secret-carrying env-file
unless the container declares its reason; the controller reads all six of
its credentials from files; design 13 states the rule and how it is checked.

02-DECISIONS/0086-a-secret-reaches-a-process-as-a-file.md

@@ -0,0 +1,75 @@
+---
+topic: building it
+status: accepted
+date: 2026-09-21
+deciders: jochen
+reconstructed: false
+extends: 02-DECISIONS/0085-a-secret-is-a-provision.md
+---
+
+# 86. A secret reaches a process as a file, and an exception is declared
+
+## Context
+
+The mesh seals a secret to the machine that uses it and discards the plaintext; the host unseals it
+into a file at 0600 ([ADR 0048](0048-a-provider-creates-the-credential-the-mesh-minted.md)). Then
+the module hands it to its container through an env-file, and the runtime puts it where anything
+on the machine that can talk to the runtime can read it: `docker inspect` prints it, and
+`/proc/<pid>/environ` holds it for the life of the process
+([issue 041](../04-ISSUES/041-a-sealed-credential-ends-up-in-the-process-environment/00-report.md)).
+
+Two facts made this a decision rather than a fix. **It is a supported shape**: placing
+`${secret:…}` in a file a container reads as its environment is what playbook 06 shows, and 36
+containers in 25 catalogue modules do it, the controller's own among them. And **nothing says which
+secrets are exposed**: a reader cannot tell from a manifest whether a credential is protected from
+`inspect` or not, because the manifest looks the same either way. The vault
+([ADR 0085](0085-a-secret-is-a-provision.md)) rests on the seal this undoes.
+
+## Considered Options
+
+1. **Leave it: the environment is where configuration goes.** Rejected — it gives the sealed value
+   back to a routine operation, and the care taken to seal it is then theatre.
+2. **Refuse every secret in an environment.** Rejected — some software reads its configuration
+   from the environment and nothing else, and a rule the catalogue cannot obey is a rule that gets
+   switched off.
+3. **A secret reaches a process as a file; an environment exception is declared, with a reason,
+   and refused otherwise.** Adopted.
+
+## Decision
+
+**A secret reaches a process as a file.** A module mounts the file the host wrote and points the
+program at it; the mesh's own programs accept a `_FILE` twin for every variable that carries a
+credential, the way the controller's store connections already did.
+
+**A container that reads a secret from its environment says so.** The manifest key
+`secrets-in-environment` on the container carries the reason. It is catalogue-level — the host
+never sees it — and it exists so a reader can tell from the manifest which secrets are exposed
+that way and why.
+
+**Everything else is refused.** A `${secret:…}` placeholder inside a container's `env` is never
+filled and is refused outright. A file carrying a secret that a container names in `env-file` is
+refused unless the container declares the exception.
+
+## Consequences
+
+The property *a sealed credential is readable only where it is used* becomes a manifest-level
+fact: true where no exception is declared, stated where one is. The controller reads all six of
+its credentials from files; the 35 other containers are marked with their reason and convert one
+by one where their software accepts a path, which is per-module work under
+[issue 041](../04-ISSUES/041-a-sealed-credential-ends-up-in-the-process-environment/00-report.md).
+
+What got harder: a module author meets one more refusal, and the reason they write is only as
+honest as they are. What is not decided: the reach of the exposure on a node — which identities can
+talk to the runtime — which decides whether a declared exception is a hardening item or something
+sharper.
+
+## How it is checked
+
+The catalogue engine refuses at composition, before anything reaches a machine, and its tests
+refuse both shapes and prove the reason is stripped from what is sent.
+
+## References
+
+- [issue 041](../04-ISSUES/041-a-sealed-credential-ends-up-in-the-process-environment/00-report.md)
+- [ADR 0048](0048-a-provider-creates-the-credential-the-mesh-minted.md), [ADR 0085](0085-a-secret-is-a-provision.md)
+- [`03-DESIGN/01-to-be/13-credentials-and-their-rotation.md`](../03-DESIGN/01-to-be/13-credentials-and-their-rotation.md)

03-DESIGN/01-to-be/13-credentials-and-their-rotation.md

@@ -5,11 +5,12 @@ code:
   - mesh-controller internal/inventory/secrets.go
   - mesh-controller cmd/mesh-controller/rotate.go
   - mesh-controller examples/postgres-provisioner
-updated: 2026-09-20
+updated: 2026-09-21
 decisions:
   - 02-DECISIONS/0001-mesh-brokers-nodes-host-agents-think.md
   - 02-DECISIONS/0009-modules-and-the-graph.md
   - 02-DECISIONS/0085-a-secret-is-a-provision.md
+  - 02-DECISIONS/0086-a-secret-reaches-a-process-as-a-file.md
 ---
 
 # 13 — Credentials, and moving them
@@ -88,6 +89,19 @@ reads exactly like a credential that was never delivered. That has now happened
 environment needs a person in the middle of the one path that exists so there is not one, and puts
 a superuser password where `docker inspect` prints it.
 
+## A secret reaches a process as a file
+
+The care above ends at the container's door if the module then hands the value to the runtime as
+an environment variable: `docker inspect` prints it, and the process's `/proc` entry holds it for
+anything on the machine that can talk to the runtime. So a secret reaches a process **as a file**
+([ADR 0086](../../02-DECISIONS/0086-a-secret-reaches-a-process-as-a-file.md)): the module mounts
+the file the host wrote and points the program at it, and the mesh's own programs take a `_FILE`
+twin for every variable that carries a credential. Software that reads only its environment is
+not forbidden; it is **declared**, on the container, with a reason, so the manifest says which
+secrets are exposed that way. **Checked** at composition: the catalogue engine refuses a
+`${secret:…}` in a container's `env` outright, and a secret-carrying file named in `env-file`
+unless the container carries the declaration.
+
 ## How it is checked
 
 Not by comparing two files. **Two ends holding a matching string proves they agree, not that either

04-ISSUES/041-a-sealed-credential-ends-up-in-the-process-environment/00-report.md

@@ -1,9 +1,9 @@
 ---
-status: open
+status: resolved
 opened: 2026-09-10
-located-in: []
+located-in: [mesh-controller internal/catalogue, mesh-catalog modules/mesh-controller]
-fixed-by:
+fixed-by: ADR 0086; mesh-controller feat/secret-not-in-environment (envfile twins for the broker settings, catalogue refusal of secrets in env / undeclared env-file); mesh-catalog (the controller reads its six credentials from files; 35 containers declare their exception); mesh-host (the installer delivers any MESH_…_FILE own secret)
-amended-design:
+amended-design: 03-DESIGN/01-to-be/13-credentials-and-their-rotation.md
 ---
 
 # 041 — A credential the mesh took care to seal ends up in the process environment
@@ -65,3 +65,11 @@ environment; 28 catalogue manifests use env-file for a secret, and nothing in th
 engine refuses a `${secret:…}` placeholder in one. The vault work of
 [ADR 0085](../../02-DECISIONS/0085-a-secret-is-a-provision.md) rests on the seal this weakens.
 
+## Resolved 2026-09-21
+
+By [ADR 0086](../../02-DECISIONS/0086-a-secret-reaches-a-process-as-a-file.md): a secret reaches
+a process as a file, an environment exception is declared with a reason, and the catalogue engine
+refuses the undeclared shape. The controller, the instance this was opened on, reads all six of
+its credentials from files. The 35 other containers carry a declared reason; converting each where
+its software accepts a path remains per-module work and is not this issue's.
+