329 lines
12 KiB
JavaScript
329 lines
12 KiB
JavaScript
// entropy-scanner.mjs — Detects encoded payloads via Shannon entropy analysis
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// Zero dependencies (Node.js builtins only via lib helpers).
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//
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// Rationale: Malicious skills and MCP servers often hide injected instructions,
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// exfiltration endpoints, or obfuscated scripts in high-entropy encoded blobs
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// (base64, hex, AES-encrypted payloads). This scanner flags those blobs for review.
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//
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// References:
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// - OWASP LLM01 (Prompt Injection via encoded payloads)
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// - OWASP LLM03 (Supply Chain — obfuscated dependencies)
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// - ToxicSkills research: evasion via base64-wrapped instructions
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import { readTextFile } from './lib/file-discovery.mjs';
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import { finding, scannerResult } from './lib/output.mjs';
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import { SEVERITY } from './lib/severity.mjs';
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import { shannonEntropy, extractStringLiterals, isBase64Like, isHexBlob, redact } from './lib/string-utils.mjs';
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// ---------------------------------------------------------------------------
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// Constants
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// ---------------------------------------------------------------------------
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/** Entropy thresholds (bits/char). Empirically calibrated against real distributions:
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*
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* Plaintext prose: H ≈ 3.5–4.2 (len 20–50)
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* Structured code/JSON: H ≈ 3.9–4.4 (len 40–80)
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* SQL queries: H ≈ 4.2–4.5 (len 50–100)
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* Base64 len=40: H ≈ 4.4–5.2 (avg 4.8, p90 5.0)
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* Base64 len=64: H ≈ 4.9–5.4 (avg 5.2, p90 5.3)
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* Base64 len=80: H ≈ 5.0–5.6 (avg 5.3, p90 5.5)
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* Base64 len=128: H ≈ 5.4–5.8 (avg 5.6, p90 5.7)
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*
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* Key insight: base64 alphabet is only 65 chars → max theoretical H = log2(65) ≈ 6.02.
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* Random base64 of len 64 achieves H ≈ 5.2 on average. Thresholds must account for
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* the length-dependent entropy ceiling.
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*
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* Conservative design: prefer low false-negative rate (catch real payloads) at the cost
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* of some false positives that the analyst reviews. The false-positive suppression rules
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* above handle the most common benign cases.
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*/
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const THRESHOLDS = {
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// Large random-looking blob: very likely encoded/encrypted payload
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CRITICAL: { entropy: 5.4, minLen: 128 },
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// Medium-sized high-entropy string: likely encoded secret or payload fragment
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HIGH: { entropy: 5.1, minLen: 64 },
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// Shorter elevated-entropy string: suspicious but may be dense data/config
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MEDIUM: { entropy: 4.7, minLen: 40 },
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};
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/** Known hash/checksum filename patterns — false positive suppression. */
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const LOCK_FILE_PATTERN = /(?:package-lock\.json|yarn\.lock|pnpm-lock\.yaml|\.lock)$/i;
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/** Line-level keywords that suggest integrity hashes rather than encoded payloads. */
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const INTEGRITY_KEYWORDS = /\b(?:integrity|checksum|sha256|sha384|sha512|sha1|md5)\b/i;
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/** Integrity hash value prefixes (SRI format). */
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const SRI_PREFIX = /^(?:sha256-|sha384-|sha512-)/;
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/** Known base64 image/font data-URI prefixes. */
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const DATA_URI_PREFIXES = [
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'iVBORw0KGgo', // PNG
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'/9j/', // JPEG
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'R0lGOD', // GIF
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'PHN2Zy', // SVG
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'AAABAA', // ICO
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'T2dnUw', // OGG (audio)
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'AAAAFGZ0', // MP4
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'UklGR', // WebP/RIFF
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'd09G', // WOFF font
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'AAEAAAALAAI', // TTF font
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];
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/** UUID v4 pattern for false positive suppression. */
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const UUID_PATTERN = /^[0-9a-f]{8}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{12}$/i;
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/** Pure lowercase hex that could be a hash digest (not obfuscated code). */
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const HEX_HASH_PATTERN = /^[a-f0-9]{32,128}$/i;
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// ---------------------------------------------------------------------------
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// False-positive suppression helpers
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// ---------------------------------------------------------------------------
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/**
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* Decide whether a candidate string should be suppressed (likely a false positive).
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*
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* @param {string} str - The extracted string literal value
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* @param {string} line - The full source line it came from
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* @param {string} absPath - Absolute file path
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* @returns {boolean} - true if this string should be skipped
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*/
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function isFalsePositive(str, line, absPath) {
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// 1. URLs — entropy is misleading for long query strings / JWTs in URLs
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if (str.startsWith('http://') || str.startsWith('https://')) return true;
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// 2. File/system paths
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if (
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str.startsWith('/') ||
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str.startsWith('./') ||
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str.startsWith('../') ||
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/^[A-Za-z]:[/\\]/.test(str) // Windows drive letter, e.g. C:\
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) return true;
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// 3. Known hash formats in lock/checksum contexts
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if (HEX_HASH_PATTERN.test(str)) {
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if (
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LOCK_FILE_PATTERN.test(absPath) ||
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INTEGRITY_KEYWORDS.test(line)
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) return true;
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}
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// 4. Test/fixture files — intentionally contain example secrets, tokens, etc.
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if (/(?:test|spec|fixture|mock|__test__|__spec__)/i.test(absPath)) return true;
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// 5. UUID patterns
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if (UUID_PATTERN.test(str)) return true;
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// 6. CSS / SVG / font data URIs embedded in source
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if (/data:image\/|data:font\/|data:application\//i.test(line)) return true;
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// 7. Import / require paths — the string is a module specifier, not a payload
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if (
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/^\s*import\s/i.test(line) ||
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/\brequire\s*\(/i.test(line)
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) return true;
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// 8. SRI integrity hash values (sha256-..., sha384-..., sha512-...)
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if (SRI_PREFIX.test(str)) return true;
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// 9. Line-level integrity keyword context (catches SRI in HTML <link> / <script> tags)
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if (INTEGRITY_KEYWORDS.test(line)) return true;
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// 10. Base64 image data-URI content (raw prefix check, separate from the line check above)
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for (const prefix of DATA_URI_PREFIXES) {
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if (str.startsWith(prefix)) return true;
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}
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return false;
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}
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// ---------------------------------------------------------------------------
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// Severity classification
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// ---------------------------------------------------------------------------
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/**
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* Derive severity from entropy and string length.
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* Returns null if below all thresholds.
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*
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* @param {number} H - Shannon entropy
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* @param {number} len - String length
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* @returns {string|null}
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*/
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function classifyEntropy(H, len) {
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if (H >= THRESHOLDS.CRITICAL.entropy && len >= THRESHOLDS.CRITICAL.minLen) {
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return SEVERITY.CRITICAL;
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}
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if (H >= THRESHOLDS.HIGH.entropy && len >= THRESHOLDS.HIGH.minLen) {
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return SEVERITY.HIGH;
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}
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if (H >= THRESHOLDS.MEDIUM.entropy && len >= THRESHOLDS.MEDIUM.minLen) {
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return SEVERITY.MEDIUM;
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}
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return null;
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}
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/**
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* Merge two severities, keeping the higher one.
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* @param {string|null} a
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* @param {string|null} b
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* @returns {string|null}
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*/
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function maxSeverity(a, b) {
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const order = [SEVERITY.CRITICAL, SEVERITY.HIGH, SEVERITY.MEDIUM, SEVERITY.LOW, SEVERITY.INFO];
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const rank = (s) => (s === null ? Infinity : order.indexOf(s));
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return rank(a) <= rank(b) ? a : b;
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}
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// ---------------------------------------------------------------------------
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// Per-file scanning
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// ---------------------------------------------------------------------------
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/**
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* Scan a single file's content for high-entropy strings.
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*
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* @param {string} content - File text content
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* @param {string} absPath - Absolute file path (for suppression checks)
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* @param {string} relPath - Relative path (for finding output)
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* @returns {object[]} - Array of finding objects
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*/
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function scanFileContent(content, absPath, relPath) {
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const findings = [];
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const lines = content.split('\n');
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// De-duplicate: track (line, evidence) pairs to avoid reporting the same
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// string twice when it appears in both extractStringLiterals and assignment
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// value extraction.
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const seen = new Set();
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for (let lineIdx = 0; lineIdx < lines.length; lineIdx++) {
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const line = lines[lineIdx];
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const lineNo = lineIdx + 1;
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// Collect candidates: string literals from the standard extractor
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const literalCandidates = extractStringLiterals(line);
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// Additional extraction: assignment RHS values not caught by quote-matching
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// (e.g., lines like: const TOKEN = "AQIB3j0..." or yaml: key: AQIB3j0...)
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// We re-use the literal extractor which already handles these cases since it
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// scans the full line. No extra pass needed — extractStringLiterals is
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// comprehensive for quoted strings. Unquoted YAML values can appear here:
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const unquotedYamlMatch = line.match(/^\s*\w[\w.-]*\s*:\s*([A-Za-z0-9+/=]{20,})(?:\s*#.*)?$/);
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if (unquotedYamlMatch) {
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literalCandidates.push(unquotedYamlMatch[1]);
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}
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for (const str of literalCandidates) {
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if (!str || str.length < 10) continue;
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// False positive suppression
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if (isFalsePositive(str, line, absPath)) continue;
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const H = shannonEntropy(str);
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let severity = classifyEntropy(H, str.length);
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// Additional detection: base64-like blobs and hex blobs get at least MEDIUM
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// even if entropy alone didn't trigger (very structured encodings can have
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// slightly lower H than random but are still suspicious at length >100/64).
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if (severity === null) {
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if (isBase64Like(str) && str.length > 100) {
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severity = SEVERITY.MEDIUM;
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} else if (isHexBlob(str) && str.length > 64) {
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severity = SEVERITY.MEDIUM;
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}
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} else {
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// Structured encoding can upgrade or confirm severity
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if (isBase64Like(str) && str.length > 100) {
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severity = maxSeverity(severity, SEVERITY.MEDIUM);
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}
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if (isHexBlob(str) && str.length > 64) {
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severity = maxSeverity(severity, SEVERITY.MEDIUM);
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}
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}
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if (severity === null) continue;
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// De-duplicate
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const key = `${lineNo}:${str.slice(0, 16)}`;
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if (seen.has(key)) continue;
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seen.add(key);
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// Determine OWASP mapping:
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// - Very high entropy (>=5.5) with base64 → likely injection payload → LLM01
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// - Encoded hex deps / supply chain obfuscation → LLM03
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// - Default to LLM01 for encoded content that could carry instructions
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const isLikelyPayload = H >= THRESHOLDS.CRITICAL.entropy || isBase64Like(str);
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const owasp = isLikelyPayload ? 'LLM01' : 'LLM03';
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const evidencePreview = redact(str, 8, 4);
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const evidence = `H=${H.toFixed(2)}, len=${str.length}: ${evidencePreview}`;
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findings.push(
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finding({
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scanner: 'ENT',
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severity,
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title: `High-entropy string (H=${H.toFixed(2)}, len=${str.length})`,
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description:
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`A string with unusually high Shannon entropy was detected. ` +
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`High entropy (H>=${THRESHOLDS.MEDIUM.entropy}) in strings of this length ` +
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`is characteristic of base64-encoded payloads, AES-encrypted blobs, ` +
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`hardcoded secrets, or obfuscated instructions embedded in code or config.`,
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file: relPath,
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line: lineNo,
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evidence,
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owasp,
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recommendation:
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'Inspect this high-entropy string — it may contain an encoded payload, ' +
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'hardcoded secret, or obfuscated code',
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})
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);
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}
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}
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return findings;
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}
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// ---------------------------------------------------------------------------
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// Public scanner entry point
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// ---------------------------------------------------------------------------
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/**
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* Scan a target path for high-entropy encoded strings.
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*
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* @param {string} targetPath - Absolute path to scan (file or directory root)
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* @param {{ files: Array<{ absPath: string, relPath: string, ext: string, size: number }> }} discovery
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* - Pre-computed file discovery result from the orchestrator
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* @returns {Promise<object>} - Scanner result envelope
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*/
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export async function scan(targetPath, discovery) {
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const startMs = Date.now();
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const allFindings = [];
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let filesScanned = 0;
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try {
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for (const fileInfo of discovery.files) {
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const content = await readTextFile(fileInfo.absPath);
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// readTextFile returns null for binary files or unreadable paths — skip silently
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if (content === null) continue;
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filesScanned++;
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const fileFindings = scanFileContent(content, fileInfo.absPath, fileInfo.relPath);
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allFindings.push(...fileFindings);
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}
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const durationMs = Date.now() - startMs;
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const status = 'ok';
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return scannerResult('entropy-scanner', status, allFindings, filesScanned, durationMs);
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} catch (err) {
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const durationMs = Date.now() - startMs;
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return scannerResult(
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'entropy-scanner',
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'error',
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allFindings,
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filesScanned,
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durationMs,
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String(err?.message || err)
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);
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}
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}
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