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docs: document sidebar prompt injection defense across user docs

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README adds a user-facing paragraph on the layered defense with links to
ARCHITECTURE. ARCHITECTURE gains a "Prompt injection defense (sidebar
agent)" subsection under Security model covering the L1-L6 layers, the
Bun-compile import constraint, env knobs, and visibility affordances.
BROWSER.md expands the "Untrusted content" note into a concrete
description of the classifier stack. docs/skills.md adds a defense
sentence to the /open-gstack-browser deep dive.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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Garry Tan
2026-04-20 11:13:15 +08:00
parent 375a317f5f
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@@ -109,6 +109,26 @@ Cookies are the most sensitive data gstack handles. The design:
The browser registry (Comet, Chrome, Arc, Brave, Edge) is hardcoded. Database paths are constructed from known constants, never from user input. Keychain access uses `Bun.spawn()` with explicit argument arrays, not shell string interpolation.
### Prompt injection defense (sidebar agent)
The Chrome sidebar agent has tools (Bash, Read, Glob, Grep, WebFetch) and reads hostile web pages, so it's the part of gstack most exposed to prompt injection. Defense is layered, not single-point.
1. **L1-L3 content security (`browse/src/content-security.ts`).** Runs on every page-content command and every tool output: datamarking, hidden-element strip, ARIA regex, URL blocklist, and a trust-boundary envelope wrapper. Applied at both the server and the agent.
2. **L4 ML classifier — TestSavantAI (`browse/src/security-classifier.ts`).** A 22MB BERT-small ONNX model (int8 quantized) bundled with the agent. Runs locally, no network. Scans every user message and every Read/Glob/Grep/WebFetch tool output before Claude sees it. Opt-in 721MB DeBERTa-v3 ensemble via `GSTACK_SECURITY_ENSEMBLE=deberta`.
3. **L4b transcript classifier.** A Claude Haiku pass that looks at the full conversation shape (user message, tool calls, tool output), not just text. Gated by `LOG_ONLY: 0.40` so most clean traffic skips the paid call.
4. **L5 canary token (`browse/src/security.ts`).** A random token injected into the system prompt at session start. Rolling-buffer detection across `text_delta` and `input_json_delta` streams catches the token if it shows up anywhere in Claude's output, tool arguments, URLs, or file writes. Deterministic BLOCK — if the token leaks, the attacker convinced Claude to reveal the system prompt, and the session ends.
5. **L6 ensemble combiner (`combineVerdict`).** BLOCK requires agreement from two ML classifiers at >= `WARN` (0.60), not a single confident hit. This is the Stack Overflow instruction-writing false-positive mitigation. On tool-output scans, single-layer high confidence BLOCKs directly — the content wasn't user-authored, so the FP concern doesn't apply.
**Critical constraint:** `security-classifier.ts` runs only in the sidebar-agent process, never in the compiled browse binary. `@huggingface/transformers` v4 requires `onnxruntime-node`, which fails `dlopen` from Bun compile's temp extract directory. Only the pure-string pieces (canary inject/check, verdict combiner, attack log, status) are in `security.ts`, which is safe to import from `server.ts`.
**Env knobs:** `GSTACK_SECURITY_OFF=1` is a real kill switch (skips ML scan, canary still injects). Model cache at `~/.gstack/models/testsavant-small/` (112MB, first run) and `~/.gstack/models/deberta-v3-injection/` (721MB, opt-in only). Attack log at `~/.gstack/security/attempts.jsonl` (salted sha256 + domain, rotates at 10MB, 5 generations). Per-device salt at `~/.gstack/security/device-salt` (0600), cached in-process to survive FS-unwritable environments.
**Visibility.** The sidebar header shows a shield icon (green/amber/red) polled via `/sidebar-chat`. A centered banner appears on canary leak or BLOCK verdict with the exact layer scores. `bin/gstack-security-dashboard` aggregates local attempts; `supabase/functions/community-pulse` aggregates opt-in community telemetry across users.
## The ref system
Refs (`@e1`, `@e2`, `@c1`) are how the agent addresses page elements without writing CSS selectors or XPath.