765bc74f52
E2E verification against content-heavy repo (`content-claude-code`) revealed 413 entropy findings (8 HIGH / 405 MEDIUM) from markdown image CDN URLs in JSON content indexes — e.g., ``. These are legitimate content-repo artifacts, not credentials. The 40-char hash segment in the CDN URL trips Shannon entropy (H=5.29 over 300 chars), and rule 13 (inline <svg>) doesn't match since there's no literal `<svg>` tag — the `.svg` is just a URL path suffix. Added rule 18 `MARKDOWN_IMAGE = /!\[[^\]]*\]\(\s*https?:\/\//` — matches `` / ``. Line-level (not string-level) so URL is not over-specific. E2E impact on `content-claude-code`: - Before: BLOCK / 65 / 8H 437M 0L - After: WARNING / 56 / 3H 427M 0L Hyperframes unchanged: BLOCK / 80 / 1C 4H 92M — real CRITICAL SQL-injection and HIGH findings still detected. Tests: 2 new (positive + negative fixture) bringing entropy-context to 26, total suite 1485 → 1487. Docs updated to "rules 11-18" and "8 new line-suppression rules". Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
263 lines
12 KiB
Markdown
263 lines
12 KiB
Markdown
# Security Hardening Guide
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This guide documents the environment variables, sandboxing mechanisms, and hook
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modes available in `llm-security`, and how to align them with the capabilities of
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Opus 4.7 and Claude Code 2.1.112.
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The guide is opinionated: it describes the configurations the plugin authors run
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in production. Deviations are fine, but the defaults here are the tested path.
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---
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## 1. Environment variables
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### 1.1 Harness-level (Claude Code)
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| Variable | Values | Effect |
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|----------|--------|--------|
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| `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. |
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| `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. |
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| `CLAUDE_CODE_SCRIPT_CAPS` | JSON blob | Declares maximum capabilities Claude Code can grant scripts it spawns. Use to lock down hook and command execution. |
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### 1.2 Plugin-specific hook modes
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| Variable | Default | Modes |
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|----------|---------|-------|
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| `LLM_SECURITY_INJECTION_MODE` | `block` | `block` — exit 2 on critical/high injection patterns. `warn` — advisory via systemMessage. `off` — disables scan. |
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| `LLM_SECURITY_TRIFECTA_MODE` | `warn` | `block` — exit 2 when lethal trifecta (untrusted input + sensitive data + exfiltration sink) detected. `warn` — advisory. `off` — disables. |
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| `LLM_SECURITY_PRECOMPACT_MODE` | `warn` | `block` — exit 2 on findings during PreCompact. `warn` — advisory via systemMessage. `off` — disables scan. |
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| `LLM_SECURITY_PRECOMPACT_MAX_BYTES` | `512000` | Tail size in bytes read from transcript for scanning. Higher values increase coverage at the cost of latency. |
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| `LLM_SECURITY_UPDATE_CHECK` | `on` | `off` disables the daily update-check HTTP call. |
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| `LLM_SECURITY_AUDIT_*` | unset | Audit trail configuration (destination, format, etc.) for SIEM-ready JSONL output. |
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Apply env vars via shell profile, `.envrc`, or the host MDM. Do not write them
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into the repository.
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---
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## 2. Sandboxing
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### 2.1 macOS — `sandbox-exec`
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`scanners/lib/git-clone.mjs` wraps remote clones in a `sandbox-exec` profile that
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restricts file writes to the specific temp directory. This defends against
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malicious `.gitattributes` filter/smudge drivers. The plugin uses this path by
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default on Darwin.
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### 2.2 Linux — `bubblewrap` (bwrap)
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On Linux, the same flow uses `bwrap` to accomplish equivalent isolation. Works on
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Fedora and Arch without configuration. Ubuntu 24.04+ may require a permissive
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AppArmor profile (administrator privileges); fallback is git-config flags only,
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with a WARN logged in the clone audit trail.
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### 2.3 Windows
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Windows has no equivalent OS sandbox available in default installs. The plugin
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falls back to hardened git-config flags (`core.hooksPath=/dev/null`,
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`core.symlinks=false`, disabled LFS drivers, `protocol.file.allow=never`,
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`transfer.fsckObjects=true`) and environment isolation
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(`GIT_CONFIG_NOSYSTEM=1`, `GIT_CONFIG_GLOBAL=/dev/null`, `GIT_ATTR_NOSYSTEM=1`).
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A WARN is logged so the caller can weigh the residual risk.
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### 2.4 PID-namespace considerations
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On Linux hosts with user namespaces disabled (some hardened kernels), `bwrap`
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may fail to create the PID namespace. Prefer running scans from a normal user
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shell; avoid root, which disables user-namespace confinement.
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---
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## 3. Hook modes in practice
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### 3.1 Start in warn mode
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Every new integration of `llm-security` should begin with all modes set to
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`warn`. This yields advisories without breaking workflow, and lets the team
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calibrate false-positive rates against their actual repositories.
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### 3.2 Promote to block after baselining
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After a baseline period (typically 1-2 weeks), flip each mode to `block` in this
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order: `LLM_SECURITY_INJECTION_MODE`, `LLM_SECURITY_TRIFECTA_MODE`,
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`LLM_SECURITY_PRECOMPACT_MODE`. The injection hook is first because false
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positives there are the most visible; blocking comes last because the others
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build confidence.
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### 3.3 Off mode is a deliberate choice
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Use `off` only when you explicitly need to disable a layer (e.g., during
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performance profiling). Prefer `warn` in all other cases — the signal is still
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recorded in the audit trail.
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---
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## 4. Bash normalization (T1-T6) as defense-in-depth
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`scanners/lib/bash-normalize.mjs` collapses six known bash obfuscation
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techniques before the denylist gate runs. These are **defense-in-depth** layers
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that complement the Claude Code 2.1.98+ harness-level fixes, not a replacement.
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| Layer | Technique | Example | Normalization |
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|-------|-----------|---------|---------------|
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| T1 | Empty quotes | `rm''-rf /` | strip `''` / `""` between tokens |
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| T2 | `${}` expansion | `r${x}m -rf /` | drop `${VAR}` where VAR is unset in scan context |
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| T3 | Backslash continuation | `rm\<newline>-rf /` | collapse backslash-newline pairs |
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| T4 | Tab/whitespace splitting | `rm\t-rf /` | normalize whitespace to single space |
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| T5 | `${IFS}` word-splitting | `rm${IFS}-rf${IFS}/` | replace `${IFS}` with space |
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| T6 | ANSI-C hex quoting | `$'\x72\x6d' -rf /` | decode `$'\xHH'` to ASCII byte |
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See `CLAUDE.md` §Defense Philosophy for the broader framing.
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---
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## 5. Alignment with Opus 4.7 (system card references)
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### 5.1 Agent safety evaluations (§5.2.1)
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The Opus 4.7 system card §5.2.1 documents agentic safety evaluations and notes
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that multi-layer defenses outperform single-layer defenses against adaptive
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attacks. `llm-security` implements this posture: prompt-scan + pathguard +
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trifecta-guard + pre-compact-scan operate in depth. A single layer failing does
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not compromise the defense.
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### 5.2 Instruction following and hierarchy (§6.3.1.1)
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The Opus 4.7 system card §6.3.1.1 describes tighter adherence to the declared
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instruction hierarchy and more literal interpretation of agent instructions.
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Consequently:
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- Stacked imperatives (e.g., "NEVER do X / MUST NOT do X") are less useful than
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tool-level enforcement. Prefer `tools:` frontmatter to restrict capabilities
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at the platform level, so the agent simply does not have the unsafe tool.
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- Agent instructions should mark speculation as speculation, and cite evidence
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(path, line number) rather than generalizing from one observation. See the
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"Step 0 Generaliseringsgrense" note added to `skill-scanner-agent.md` and
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`mcp-scanner-agent.md`.
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- Parallel Read calls are preferred for independent file reads, documented in
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the same Step 0 notes. This reduces latency and aligns with the model's
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improved parallel-tool-use behavior.
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### 5.3 Known limitations (system card §6.3)
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Prompt injection is structurally unsolvable in the current architecture. The
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system card acknowledges this; so does `CLAUDE.md` §Defense Philosophy. The
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hardening described here reduces the attack surface and raises the cost of
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attacks but does not eliminate them.
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---
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## 6. Calibration & false positives (v7.0.0+)
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Security scanners live or die by their signal-to-noise ratio. A scanner that
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cries "extreme" on every project destroys its own credibility — users learn
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to ignore findings, and genuine threats slip past. v7.0.0 ships three
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calibration layers to keep that from happening.
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### 6.1 Risk-score v2 formula
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The v1 formula was a sum-and-cap: `critical*25 + high*10 + medium*4 + low*1`,
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capped at 100. Every non-trivial scan collapsed to 100/Extreme regardless of
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actual distribution. A codebase with 2 mediums and 100 lows scored the same
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as a codebase with 5 criticals.
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v2 (`scanners/lib/severity.mjs`) is severity-dominated and log-scaled within
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tier:
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| Finding mix | Score range | Band |
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|-------------|-------------|------|
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| Critical present | 70–95 (1=80, 2=86, 4=90, 10=95) | Critical/Extreme |
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| High only | 40–65 (1=48, 5=60, 17=65) | High |
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| Medium only | 15–35 (1=20, 5=28, 50=33) | Medium |
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| Low only | 1–11 (1=4, 10=11) | Low |
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| None | 0 | Low |
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Verdict cutoffs (`BLOCK ≥65`, `WARNING ≥15`) are locked to the `riskBand()`
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boundaries so you can't get a "BLOCK / Medium band" contradiction. The legacy
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formula is kept as `riskScoreV1()` for reference only.
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**CI impact:** Pipelines with `--fail-on high` keep working (the severity
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gate is unaffected). Pipelines with score-based thresholds need recalibration
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— old `score >= 21` corresponds roughly to new `score >= 15`.
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### 6.2 Context-aware entropy scanner
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The entropy scanner flags high-Shannon-entropy strings as possible
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credentials. On codebases heavy with shader code, bundled JS, CSS-in-JS or
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SQL it produced astronomical false-positive rates. v7.0.0 adds three
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suppression layers:
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1. **File-extension skip** — whole files with these extensions are never
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inspected for entropy findings: `.glsl, .frag, .vert, .shader, .wgsl,
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.css, .scss, .sass, .less, .svg` + compound `.min.js, .min.css, .map`. A
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skip counter (`calibration.files_skipped_by_extension`) is reported in the
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scanner envelope.
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2. **Line-level rules 11–18** — applied when a line contains any of: GLSL
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keywords (`uniform`, `vec3`, `texture2D`…), CSS-in-JS templates
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(`styled.…`), inline `<svg>` markup, ffmpeg `filter_complex` syntax,
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browser `User-Agent` strings, SQL DDL on a dedicated line
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(`^\s*(SELECT|INSERT|…)`), `throw new Error(\`…\`)` templates, or
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markdown image syntax with external URL (`` — common
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in JSON content indexes / article metadata).
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3. **Per-project policy override** — `.llm-security/policy.json` `entropy`
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section supports:
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```json
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{
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"entropy": {
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"thresholds": {
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"critical": { "entropy": 5.4, "minLen": 128 },
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"high": { "entropy": 5.1, "minLen": 64 },
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"medium": { "entropy": 4.7, "minLen": 40 }
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},
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"suppress_extensions": [".custom"],
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"suppress_line_patterns": ["MY_VENDOR_MARKER"],
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"suppress_paths": ["vendored/", "generated/"]
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}
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}
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```
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The synthesizer agent reports calibration prominently if >80 % of files were
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skipped (signals a policy so aggressive the scan is effectively bypassed)
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and omits it silently if <5 % were skipped.
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### 6.3 Typosquat allowlist
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The DEP scanner flags Levenshtein-close package names against a top-N list
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to catch typosquats (`lod-ash`, `expres`). On real codebases this tripped on
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short-name tools like `knip`, `nx`, `tsx`, `uv`, `ruff`. v7.0.0 extends
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`knowledge/typosquat-allowlist.json` with 22 npm + 5 PyPI entries for modern
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tools.
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### 6.4 Tuning workflow
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1. Run `/security deep-scan` on a representative codebase.
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2. Read `calibration.files_skipped_by_extension` and `files_skipped_by_path`
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from the envelope — are they reasonable?
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3. Review the top 10 findings. For each false positive, pick the narrowest
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suppression that catches it:
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- Whole extension noisy → `suppress_extensions`
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- One line pattern recurring → `suppress_line_patterns`
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- Whole directory vendored → `suppress_paths`
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4. Raise thresholds only as a last resort — you're hiding real signal.
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5. Re-scan and verify verdict/band/score make sense relative to the finding
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set.
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---
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## 7. Recommended baseline for production
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1. Set `CLAUDE_CODE_EFFORT_LEVEL=xhigh` for audit and planning sessions.
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2. Set `ENABLE_PROMPT_CACHING_1H=1` globally — reduces cost, does not weaken
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scanning.
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3. All three plugin hook modes: start at `warn`, promote to `block` after
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baselining.
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4. Keep sandbox wrappers enabled (default on macOS / Linux).
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5. Periodically run `/security posture` (16-category scorecard) and
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`/security dashboard` (cross-project view) to catch drift.
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6. After first `/security deep-scan`, run the §6.4 tuning workflow once to
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calibrate the noise floor for your codebase.
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---
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**Last updated:** 2026-04-19 for v7.0.0.
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