Open_Ultra_SAST/docs/threat-model.md

Threat model & hardening

OpenUltraSAST analyzes untrusted code. This document states what it trusts, what it does not, and the controls that keep a scan safe to run in CI and safe to share.

Trust boundaries

• Scanned code is untrusted input. Quick and standard modes never import, build or run the target: quick reads source as text, and standard additionally parses it into a code property graph with Joern (joern-parse, a parser, not a build) and may send code excerpts to the configured model. Only deep mode executes target-derived code: its REGRESS stage loads promoted candidates inside the Docker sandbox below, and it runs only when docker info succeeds (otherwise the stage is skipped and recorded as sandbox_unavailable).
• The ruleset and CWE policy are trusted, governed data. They change only through reviewed commits or the bounded self-improvement loop (ousast improve), which can flip rule status and tune score constants but can never edit pattern text or the authoritative 0–5 severity. See architecture.md.
• Provider credentials are the operator's. The core install (PyYAML only) makes no network calls on its own. A local scan calls a model only in standard or deep mode, and only when a key is available: the model layer asks the residual question the graph cannot settle, and [models] hunter enables the tool hunter. LLM calls go to DeepSeek (DEEPSEEK_API_KEY); OpenRouter (OPENROUTER_API_KEY) serves embeddings. Keys come from the environment or a gitignored .env, which never overrides an exported variable. The pre-push check calls a model only with an explicit --model-config. Agentic work runs on the plane (ousast plane run): ousast reads the variable a task's Model names (secretKey.key) from the operator's environment or .env and sends the key only in that task's start request, through Agent Substrate's router to that task's actor. It is not baked into an image or task template, and it is never logged or written to artifacts.

Controls

Secret redaction

Scan artifacts can quote source that contains live credentials. redaction.py masks recognizable secret shapes (provider API keys, AWS/GitHub/Slack/Google tokens, bearer tokens, URL-embedded credentials, PEM private keys, and key = value assignments for sensitive names) before traces (trace/events.jsonl) and the markdown report are written. On by default; disable with [hardening] redact_secrets = false.

Plane egress

Each plane task runs in its own actor behind Agent Substrate's egress gateway, which denies by default. The task's EgressPolicy (src/openultrasast/plane/egress.py) allows exactly three things: plain HTTP to the artifact receiver, and TLS passthrough to the Git hosts of the task's bound Workspaces and to the hosts its bound Model declares (openultrasast.io/egress-hosts). There are no wildcards, no IP addresses and no TLS interception. The policy is deleted with the actor.

Cost & CI budgets

• Agentic spend is bounded per plane task by the Run's budget: {usd, calls}. The metered client refuses the next call once either ceiling is reached (the task is unfinished and resumes on a rerun with a larger budget). A usd budget on a Model without prices is refused rather than metered as zero, and an account or authentication error fails the task and starts nothing further. The standard-mode tool hunter is bounded by the per-tier hunter budgets.
• Output size is bounded by [hardening] max_findings (0 = unlimited). Truncation is severity-ordered and disclosed as a budget degradation in the manifest.

Provider reliability

LLM/embedding calls retry transient failures (HTTP 429/5xx, connection errors, timeouts) with exponential backoff; non-transient errors (e.g. 4xx, malformed JSON) fail fast.

Visible degradation & determinism

When an optional engine is missing (Joern: cpg_unavailable; Docker: sandbox_unavailable), the scan falls back to its deterministic equivalent and records a degradations entry in the manifest — it is never silently downgraded. Without a provider key the model layer asks no question and reports only what the graph entailed. Runs are reproducible: fixed config, artifact manifests, prompt hashes, and model identifiers are recorded.

MCP surface

ousast mcp exposes only the ten narrow project tools. No tool runs arbitrary shell, Docker, or internal hunter tools, and no tool accepts a free-form command argument.

Sandbox (deep mode)

The REGRESS stage (regress/, sandbox/runner.py) runs each snippet with docker run and never mounts the host Docker socket:

Control	Setting
network	--network none, always; a host-network argument is refused
source	the target bind-mounted read-only at /workspace; root filesystem --read-only
scratch	a tmpfs at /scratch for the snippet
user	non-root 65534:65534, --cap-drop ALL, no-new-privileges
memory	[sandbox] memory_mb, default 2048 MB
pids	[sandbox] pids_limit, default 512
timeout	[sandbox] timeout_seconds, default 300 s

Before anything runs, a structural deny-list (regress/safety.py) rejects a snippet that mentions sockets, curl, host networking or writes under /workspace.