Threat model: running anvil with untrusted users

anvil is built as a single-tenant, owner-operated forge: the operator and the people with accounts are assumed to trust each other (a person, a family, a small team). This document records what would have to be true before opening registration (or repo write access) to people you don't trust, ranked by severity. It is the output of the security-audit pass; keep it updated as items land.

Supported stance: single-tenant / owner-operated. Untrusted multi-tenancy is not supported until at least items 1–4 below are closed. Agent sessions (§7) raise the bar further and are off by default.

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1. CI: arbitrary code execution by design

Anyone who can push to a repo with a .anvil/ci.yml runs arbitrary code on your hardware. That is the point of CI, so the question is only how well the blast radius is contained.

Broker model (implemented). anvil itself is the only Docker client. The job container gets:

• no Docker socket, no bind mounts, no volumes — the checkout is uploaded into the container as a tar via the Docker API, so the job can never reach anvil's data directory or the host filesystem;
• --cap-drop=ALL and no-new-privileges unconditionally;
• pids / memory(+swap) / cpu caps (ci.pids_limit, ci.memory_mb, ci.cpus; defaults 512 / 2048 MiB / 2);
• a wall-clock timeout (ci.timeout_secs, default 30 minutes) after which the container is force-removed;
• optionally no network (ci.network = false) and a non-root user (ci.run_as = "1000:1000") — most real builds need network and many base images assume root, so these default to permissive;
• an image allowlist (ci.allowed_images) — empty allows any image, which is fine single-tenant; set it before letting strangers push.

Secrets in a pipeline. A run can request repository secrets (see secrets.md), which arrive as environment variables inside that same container. Anyone who can push to the repo can therefore read every secret it declares, by editing the pipeline; log masking stops accidents, not intent. The compensating control is that anvil cannot decrypt them at all unless the owner has unlocked the repo, so the exposure window is bounded by the unlock TTL rather than being permanent.

Deliberately not done: read-only rootfs (the workspace lives in the container filesystem precisely so no volume is ever attached; builds also write $HOME caches), and egress filtering (network is all-or-nothing).

Residual risk / stronger tier. Containers share the host kernel; a kernel or runc escape defeats all of the above. For genuinely hostile tenants run the jobs under gVisor/Kata/Firecracker (a runtime flag on the broker — the "isolated workers" idea), and add per-user CI-minute and disk quotas (image pulls consume host disk). Until then, CI for untrusted users should stay off.

2. Stored XSS via served content

Repo browsing renders escaped text (Maud auto-escapes; highlighting emits sanitized HTML), so hostile file content does not execute in the forge origin today.

Pages hosting is the exception by design: it serves attacker-authored HTML/JS. On a single-origin deployment, a pages site runs in the same origin as the forge UI — its JS could read forge pages and drive authenticated requests in a visitor's session. Mitigations in place: session cookies are HttpOnly (no token theft) and all mutating routes require the CSRF token. But same-origin JS can still read the token off a fetched page, so for untrusted users pages must move to a separate origin (e.g. *.pages.example.com), as GitHub does. The same applies to any future "raw blob" endpoint: serve text/plain + nosniff + a restrictive CSP, or a separate origin.

3. Git resource exhaustion

A hostile pusher can send decompression bombs (tiny pack, enormous objects), deep delta chains, or millions of refs; a hostile cloner can request expensive packs repeatedly. Needed before untrusted use: per-repo and per-user storage quotas, an upload size cap on receive-pack, timeouts/memory bounds on pack ingestion and pack generation, and a cap on advertised refs. (The CI tar materializer also loads a full checkout into memory — bounded today only by push quotas not existing.)

4. Open registration anti-abuse

Registration is currently operator-controlled (CLI), which is the real mitigation. Opening it requires: email verification, rate limiting on signup / login / repo creation, a CAPTCHA or proof-of-work, per-user quotas (repos, storage, CI minutes), and an admin ban/cleanup path. Reserved usernames and the /-/ route namespace already prevent route-shadowing squats.

5. Authorization granularity

Access today is owner-or-admin, repo public-or-private. Fine single-tenant; multi-user collaboration needs collaborator roles (read/write/admin per repo), per-repo deploy keys, and scoped tokens instead of full-account SSH keys. The deploy webhook is already scoped to exactly one configured repo.

6. Webhook SSRF (future)

User-configurable webhooks don't exist yet (the deploy webhook URL is operator-set in the config file, not user data). When they land: resolve and block private/link-local/metadata ranges (and re-check on redirect), pin DNS, cap response sizes and time, and never reflect response bodies to users.

7. Agent sessions: a model reading repo content as instructions

Off by default (agent.enabled), and it should stay off unless you trust everyone who can push. See agent-sessions.md for the design.

This is not a variant of §1. CI runs code the pusher wrote: hostile, but authored. An agent session runs a model that reads repository content, file names, issue text and TODO items and may treat any of it as instruction. A prompt injected into a README is therefore a path to arbitrary tool use inside the session — and the session has network access it cannot do without.

What carries over from §1. The container is created through the same broker: no Docker socket, no bind mounts, no volumes, --cap-drop=ALL, no-new-privileges, pids/memory/cpu caps. Sessions additionally run as an unprivileged user (agent, uid 1000) rather than root, which CI does not. The workspace and the agent's credentials both arrive as tar uploads through the Docker API, so nothing on anvil's filesystem is reachable.

What is new.

• Network is mandatory. ci.network = false has no counterpart here: the session must reach the model API. Egress filtering does not exist, so an injected instruction can exfiltrate anything in the workspace.
• --dangerously-skip-permissions is passed deliberately. The container is the security boundary; a permission prompt inside a container the agent already owns end-to-end buys nothing. The consequence is that containment is entirely the container's job — there is no second line of defence inside it.
• Credentials sit in the container. agent.credentials_dir is copied into every session, so any code the agent runs — including code the repository supplied — can read the model credentials. Scope that account accordingly.
• A push credential is coming and is not yet scoped. M1 seeds files only and hands out no credential at all. When the container gets a real clone it will need one, and restricting it to refs/heads/agent/* requires a ref filter in receive-pack that does not exist. Until it does, a session credential could write any branch, main included.
• No rate limiting. agent.max_concurrent bounds how many run at once, not how many are started. Once push/issue triggers land, automated pushes could queue sessions indefinitely; restricting who can push is the only control today.

Before untrusted use: all of §§1–4, plus per-user session quotas, egress filtering or an allowlisted proxy, the ref-scoped push credential, and a credential per repository rather than one instance-wide.

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Already right (keep it that way)

• Private repos 404 for non-readers — no existence leak (resolve_repo).
• Reserved usernames + /-/ namespace for app routes.
• CI checkout is tar-uploaded, never bind-mounted; job containers get no socket; sandbox defaults are on (see §1).
• CD webhook gated to a single configured repo + shared-secret header.
• Passwords: argon2. Sessions: HttpOnly + SameSite=Lax + Secure (auto when base_url is https). CSRF: HMAC synchronizer token, constant-time compare, on all mutating forms; htmx requests carry it via hx-headers.
• SSH auth by exact public-key match; unknown keys rejected.