docs: add security-hardening-guide with Opus 4.7 + Claude Code 2.1.112 env-vars

plugins/llm-security/docs/security-hardening-guide.md

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+# Security Hardening Guide
+
+This guide documents the environment variables, sandboxing mechanisms, and hook
+modes available in `llm-security`, and how to align them with the capabilities of
+Opus 4.7 and Claude Code 2.1.112.
+
+The guide is opinionated: it describes the configurations the plugin authors run
+in production. Deviations are fine, but the defaults here are the tested path.
+
+---
+
+## 1. Environment variables
+
+### 1.1 Harness-level (Claude Code)
+
+| Variable | Values | Effect |
+|----------|--------|--------|
+| `CLAUDE_CODE_EFFORT_LEVEL` | `low` \| `medium` \| `high` \| `xhigh` | Tunes how aggressively the model spends compute per turn. `xhigh` is recommended for security-sensitive planning and audits. |
+| `ENABLE_PROMPT_CACHING_1H` | `1` \| unset | Enables 1-hour prompt cache TTL. Reduces cost and latency for repeated context; cache hits do not weaken scanning. |
+| `CLAUDE_CODE_SCRIPT_CAPS` | JSON blob | Declares maximum capabilities Claude Code can grant scripts it spawns. Use to lock down hook and command execution. |
+
+### 1.2 Plugin-specific hook modes
+
+| Variable | Default | Modes |
+|----------|---------|-------|
+| `LLM_SECURITY_INJECTION_MODE` | `block` | `block` — exit 2 on critical/high injection patterns. `warn` — advisory via systemMessage. `off` — disables scan. |
+| `LLM_SECURITY_TRIFECTA_MODE` | `warn` | `block` — exit 2 when lethal trifecta (untrusted input + sensitive data + exfiltration sink) detected. `warn` — advisory. `off` — disables. |
+| `LLM_SECURITY_PRECOMPACT_MODE` | `warn` | `block` — exit 2 on findings during PreCompact. `warn` — advisory via systemMessage. `off` — disables scan. |
+| `LLM_SECURITY_PRECOMPACT_MAX_BYTES` | `512000` | Tail size in bytes read from transcript for scanning. Higher values increase coverage at the cost of latency. |
+| `LLM_SECURITY_UPDATE_CHECK` | `on` | `off` disables the daily update-check HTTP call. |
+| `LLM_SECURITY_AUDIT_*` | unset | Audit trail configuration (destination, format, etc.) for SIEM-ready JSONL output. |
+
+Apply env vars via shell profile, `.envrc`, or the host MDM. Do not write them
+into the repository.
+
+---
+
+## 2. Sandboxing
+
+### 2.1 macOS — `sandbox-exec`
+
+`scanners/lib/git-clone.mjs` wraps remote clones in a `sandbox-exec` profile that
+restricts file writes to the specific temp directory. This defends against
+malicious `.gitattributes` filter/smudge drivers. The plugin uses this path by
+default on Darwin.
+
+### 2.2 Linux — `bubblewrap` (bwrap)
+
+On Linux, the same flow uses `bwrap` to accomplish equivalent isolation. Works on
+Fedora and Arch without configuration. Ubuntu 24.04+ may require a permissive
+AppArmor profile (administrator privileges); fallback is git-config flags only,
+with a WARN logged in the clone audit trail.
+
+### 2.3 Windows
+
+Windows has no equivalent OS sandbox available in default installs. The plugin
+falls back to hardened git-config flags (`core.hooksPath=/dev/null`,
+`core.symlinks=false`, disabled LFS drivers, `protocol.file.allow=never`,
+`transfer.fsckObjects=true`) and environment isolation
+(`GIT_CONFIG_NOSYSTEM=1`, `GIT_CONFIG_GLOBAL=/dev/null`, `GIT_ATTR_NOSYSTEM=1`).
+A WARN is logged so the caller can weigh the residual risk.
+
+### 2.4 PID-namespace considerations
+
+On Linux hosts with user namespaces disabled (some hardened kernels), `bwrap`
+may fail to create the PID namespace. Prefer running scans from a normal user
+shell; avoid root, which disables user-namespace confinement.
+
+---
+
+## 3. Hook modes in practice
+
+### 3.1 Start in warn mode
+
+Every new integration of `llm-security` should begin with all modes set to
+`warn`. This yields advisories without breaking workflow, and lets the team
+calibrate false-positive rates against their actual repositories.
+
+### 3.2 Promote to block after baselining
+
+After a baseline period (typically 1-2 weeks), flip each mode to `block` in this
+order: `LLM_SECURITY_INJECTION_MODE`, `LLM_SECURITY_TRIFECTA_MODE`,
+`LLM_SECURITY_PRECOMPACT_MODE`. The injection hook is first because false
+positives there are the most visible; blocking comes last because the others
+build confidence.
+
+### 3.3 Off mode is a deliberate choice
+
+Use `off` only when you explicitly need to disable a layer (e.g., during
+performance profiling). Prefer `warn` in all other cases — the signal is still
+recorded in the audit trail.
+
+---
+
+## 4. Bash normalization (T1-T6) as defense-in-depth
+
+`scanners/lib/bash-normalize.mjs` collapses six known bash obfuscation
+techniques before the denylist gate runs. These are **defense-in-depth** layers
+that complement the Claude Code 2.1.98+ harness-level fixes, not a replacement.
+
+| Layer | Technique | Example | Normalization |
+|-------|-----------|---------|---------------|
+| T1 | Empty quotes | `rm''-rf /` | strip `''` / `""` between tokens |
+| T2 | `${}` expansion | `r${x}m -rf /` | drop `${VAR}` where VAR is unset in scan context |
+| T3 | Backslash continuation | `rm\<newline>-rf /` | collapse backslash-newline pairs |
+| T4 | Tab/whitespace splitting | `rm\t-rf /` | normalize whitespace to single space |
+| T5 | `${IFS}` word-splitting | `rm${IFS}-rf${IFS}/` | replace `${IFS}` with space |
+| T6 | ANSI-C hex quoting | `$'\x72\x6d' -rf /` | decode `$'\xHH'` to ASCII byte |
+
+See `CLAUDE.md` §Defense Philosophy for the broader framing.
+
+---
+
+## 5. Alignment with Opus 4.7 (system card references)
+
+### 5.1 Agent safety evaluations (§5.2.1)
+
+The Opus 4.7 system card §5.2.1 documents agentic safety evaluations and notes
+that multi-layer defenses outperform single-layer defenses against adaptive
+attacks. `llm-security` implements this posture: prompt-scan + pathguard +
+trifecta-guard + pre-compact-scan operate in depth. A single layer failing does
+not compromise the defense.
+
+### 5.2 Instruction following and hierarchy (§6.3.1.1)
+
+The Opus 4.7 system card §6.3.1.1 describes tighter adherence to the declared
+instruction hierarchy and more literal interpretation of agent instructions.
+Consequently:
+
+- Stacked imperatives (e.g., "NEVER do X / MUST NOT do X") are less useful than
+  tool-level enforcement. Prefer `tools:` frontmatter to restrict capabilities
+  at the platform level, so the agent simply does not have the unsafe tool.
+- Agent instructions should mark speculation as speculation, and cite evidence
+  (path, line number) rather than generalizing from one observation. See the
+  "Step 0 Generaliseringsgrense" note added to `skill-scanner-agent.md` and
+  `mcp-scanner-agent.md`.
+- Parallel Read calls are preferred for independent file reads, documented in
+  the same Step 0 notes. This reduces latency and aligns with the model's
+  improved parallel-tool-use behavior.
+
+### 5.3 Known limitations (system card §6.3)
+
+Prompt injection is structurally unsolvable in the current architecture. The
+system card acknowledges this; so does `CLAUDE.md` §Defense Philosophy. The
+hardening described here reduces the attack surface and raises the cost of
+attacks but does not eliminate them.
+
+---
+
+## 6. Recommended baseline for production
+
+1. Set `CLAUDE_CODE_EFFORT_LEVEL=xhigh` for audit and planning sessions.
+2. Set `ENABLE_PROMPT_CACHING_1H=1` globally — reduces cost, does not weaken
+   scanning.
+3. All three plugin hook modes: start at `warn`, promote to `block` after
+   baselining.
+4. Keep sandbox wrappers enabled (default on macOS / Linux).
+5. Periodically run `/security posture` (13-category scorecard) and
+   `/security dashboard` (cross-project view) to catch drift.
+
+---
+
+**Last updated:** 2026-04-17 for v6.2.0.