# 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.

The plugin's "defense-in-depth" claim resolves to **three independent detection
layers with documented bypass classes**: (1) the Claude Code harness denylist
(out of plugin scope, evolves with platform); (2) `bash-normalize.mjs` T1-T6
collapse rules; (3) `pre-bash-destructive.mjs` post-normalization pattern match
+ `post-session-guard.mjs` runtime trifecta correlation. Each layer has known
bypasses (see Defense Philosophy in `CLAUDE.md` and `docs/critical-review-2026-04-20.md`
§4 for the evasion arsenal). Stacking layers raises attacker cost; it does not
provide formal worst-case guarantees.

| 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. Calibration & false positives (v7.0.0+)

Security scanners live or die by their signal-to-noise ratio. A scanner that
cries "extreme" on every project destroys its own credibility — users learn
to ignore findings, and genuine threats slip past. v7.0.0 ships three
calibration layers to keep that from happening.

### 6.1 Risk-score v2 formula

The v1 formula was a sum-and-cap: `critical*25 + high*10 + medium*4 + low*1`,
capped at 100. Every non-trivial scan collapsed to 100/Extreme regardless of
actual distribution. A codebase with 2 mediums and 100 lows scored the same
as a codebase with 5 criticals.

v2 (`scanners/lib/severity.mjs`) is severity-dominated and log-scaled within
tier:

| Finding mix | Score range | Band |
|-------------|-------------|------|
| Critical present | 70–95 (1=80, 2=86, 4=90, 10=95) | Critical/Extreme |
| High only | 40–65 (1=48, 5=60, 17=65) | High |
| Medium only | 15–35 (1=20, 5=28, 50=33) | Medium |
| Low only | 1–11 (1=4, 10=11) | Low |
| None | 0 | Low |

Verdict cutoffs (`BLOCK ≥65`, `WARNING ≥15`) are locked to the `riskBand()`
boundaries so you can't get a "BLOCK / Medium band" contradiction. The legacy
formula is kept as `riskScoreV1()` for reference only.

**CI impact:** Pipelines with `--fail-on high` keep working (the severity
gate is unaffected). Pipelines with score-based thresholds need recalibration
— old `score >= 21` corresponds roughly to new `score >= 15`.

### 6.2 Context-aware entropy scanner

The entropy scanner flags high-Shannon-entropy strings as possible
credentials. On codebases heavy with shader code, bundled JS, CSS-in-JS or
SQL it produced astronomical false-positive rates. v7.0.0 adds three
suppression layers:

1. **File-extension skip** — whole files with these extensions are never
   inspected for entropy findings: `.glsl, .frag, .vert, .shader, .wgsl,
   .css, .scss, .sass, .less, .svg` + compound `.min.js, .min.css, .map`. A
   skip counter (`calibration.files_skipped_by_extension`) is reported in the
   scanner envelope.
2. **Line-level rules 11–18** — applied when a line contains any of: GLSL
   keywords (`uniform`, `vec3`, `texture2D`…), CSS-in-JS templates
   (`styled.…`), inline `<svg>` markup, ffmpeg `filter_complex` syntax,
   browser `User-Agent` strings, SQL DDL on a dedicated line
   (`^\s*(SELECT|INSERT|…)`), `throw new Error(\`…\`)` templates, or
   markdown image syntax with external URL (`![alt](https://cdn…)` — common
   in JSON content indexes / article metadata).
3. **Per-project policy override** — `.llm-security/policy.json` `entropy`
   section supports:

```json
{
  "entropy": {
    "thresholds": {
      "critical": { "entropy": 5.4, "minLen": 128 },
      "high":     { "entropy": 5.1, "minLen": 64 },
      "medium":   { "entropy": 4.7, "minLen": 40 }
    },
    "suppress_extensions": [".custom"],
    "suppress_line_patterns": ["MY_VENDOR_MARKER"],
    "suppress_paths": ["vendored/", "generated/"]
  }
}
```

The synthesizer agent reports calibration prominently if >80 % of files were
skipped (signals a policy so aggressive the scan is effectively bypassed)
and omits it silently if <5 % were skipped.

### 6.3 Typosquat allowlist

The DEP scanner flags Levenshtein-close package names against a top-N list
to catch typosquats (`lod-ash`, `expres`). On real codebases this tripped on
short-name tools like `knip`, `nx`, `tsx`, `uv`, `ruff`. v7.0.0 extends
`knowledge/typosquat-allowlist.json` with 22 npm + 5 PyPI entries for modern
tools.

### 6.4 Tuning workflow

1. Run `/security deep-scan` on a representative codebase.
2. Read `calibration.files_skipped_by_extension` and `files_skipped_by_path`
   from the envelope — are they reasonable?
3. Review the top 10 findings. For each false positive, pick the narrowest
   suppression that catches it:
   - Whole extension noisy → `suppress_extensions`
   - One line pattern recurring → `suppress_line_patterns`
   - Whole directory vendored → `suppress_paths`
4. Raise thresholds only as a last resort — you're hiding real signal.
5. Re-scan and verify verdict/band/score make sense relative to the finding
   set.

---

## 7. 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` (16-category scorecard) and
   `/security dashboard` (cross-project view) to catch drift.
6. After first `/security deep-scan`, run the §6.4 tuning workflow once to
   calibrate the noise floor for your codebase.

---

**Last updated:** 2026-04-29 for v7.1.0.

### v7.1.0 calibration note

v7.1.0 is a patch release. No calibration changes; the §6 tuning workflow above is
unchanged. Two hook-level bugs were fixed that affect production posture:

- `pre-write-pathguard.mjs` now blocks multi-segment `.env.*.*.*` paths (previously a
  regex hole let `.env.production.local.backup` through).
- `post-session-guard.mjs` `block` mode now blocks every detected trifecta. Previously
  required a "concentrated MCP" or "sensitive path" qualifier, so distributed
  trifectas were advisory-only even in block mode.

If you run with `LLM_SECURITY_TRIFECTA_MODE=block`, expect the false-block rate to
rise after this upgrade — the previous gate suppressed real trifectas. Re-baseline
the warn-mode noise floor before promoting to block, per §3.
