fix(scanners): preserve single-quoted regions through bash-normalize pipeline

Masks non-empty '...' content before T5/T2-T4 run so literal strings such
as `echo '${IFS}'` are not rewritten. Empty '' pairs are stripped first
so c''u''rl -> curl evasion keeps resolving. ANSI-C $'...' is decoded
before masking.

Caught by the false-positive probe added in Step 3 of ultraplan-v6.2.0.

plugins/llm-security/scanners/lib/bash-normalize.mjs

@@ -17,68 +17,71 @@
 //   T5 — IFS word-splitting:        rm${IFS}-rf${IFS}/ -> rm -rf /
 //   T6 — ANSI-C hex quoting:        $'\x72\x6d' -rf / -> rm -rf /
 //
-// T5 and T6 run before T1-T4 so their outputs feed the rest of the pipeline
-// in canonical form. Both preserve single-quoted literals (false-positive
-// probe: `echo '${IFS}'` stays untouched).
+// Execution order:
+//   1. Strip empty single-quote pairs (T1) so c''u''rl -> curl before masking.
+//   2. Decode ANSI-C hex inside $'...' (T6) before masking.
+//   3. Mask remaining non-empty single-quoted regions. T3's ${...} sweep and
+//      other transforms cannot rewrite their content, preserving literals
+//      (false-positive probe: `echo '${IFS}'` stays untouched).
+//   4. Run T5 (IFS) and T2/T3/T4 on the masked string.
+//   5. Unmask.
+
+const MASK = '\x00';
 
 /**
- * T5 — strip IFS-based word splitting outside single-quoted regions.
+ * Decode ANSI-C hex quoting inside `$'...'` contexts.
  *
- * Patterns matched: ${IFS}, ${IFS:0:1}, $IFS. Each replaced with a single
- * space. Content inside '...' is preserved via placeholder masking so the
- * literal string `'${IFS}'` never expands.
+ * Shell treats $'\x72\x6d' as the bytes r and m. We decode only \xHH escape
+ * sequences inside the $'...' wrapper. The $'...' construct itself is
+ * replaced with its decoded bytes (matching shell evaluation).
  */
-function normalizeIFS(cmd) {
+function decodeAnsiCHex(cmd) {
+  return cmd.replace(/\$'([^']*)'/g, (_, content) =>
+    content.replace(/\\x([0-9a-fA-F]{2})/g, (_m, hex) =>
+      String.fromCharCode(parseInt(hex, 16)),
+    ),
+  );
+}
+
+/**
+ * Mask non-empty single-quoted regions with placeholders. Empty '' is NOT
+ * masked — T1 already stripped them in the previous pass.
+ */
+function maskSingleQuoted(cmd) {
   const placeholders = [];
-  const MARK = '\x00';
-  const masked = cmd.replace(/'[^']*'/g, (match) => {
+  const masked = cmd.replace(/'[^']+'/g, (match) => {
     placeholders.push(match);
-    return `${MARK}${placeholders.length - 1}${MARK}`;
+    return `${MASK}${placeholders.length - 1}${MASK}`;
   });
-  const normalized = masked
-    .replace(/\$\{IFS:0:1\}/g, ' ')
-    .replace(/\$\{IFS\}/g, ' ')
-    .replace(/\$IFS\b/g, ' ');
-  return normalized.replace(
-    new RegExp(`${MARK}(\\d+)${MARK}`, 'g'),
+  return { masked, placeholders };
+}
+
+function unmaskSingleQuoted(str, placeholders) {
+  return str.replace(
+    new RegExp(`${MASK}(\\d+)${MASK}`, 'g'),
     (_, idx) => placeholders[parseInt(idx, 10)],
   );
 }
 
-/**
- * T6 — decode ANSI-C hex quoting inside `$'...'` contexts only.
- *
- * Shell treats $'\x72\x6d' as the bytes r and m. Attackers use this to
- * hide command names from regex gates. We decode only the \xHH escape
- * sequences inside the $'...' wrapper. Regular single-quoted strings
- * '...' are not touched.
- */
-function normalizeAnsiCHex(cmd) {
-  return cmd.replace(/\$'([^']*)'/g, (match, content) => {
-    return content.replace(/\\x([0-9a-fA-F]{2})/g, (_, hex) =>
-      String.fromCharCode(parseInt(hex, 16)),
-    );
-  });
-}
-
 /**
  * Normalize bash parameter expansion and quoting evasion in a command string.
  *
- * Strips (T1-T6):
- *   - T1 Empty single quotes: '' (e.g., w''get -> wget)
- *   - T2 Empty double quotes: "" (e.g., r""m -> rm)
- *   - T3 Single-char parameter expansion: ${x} -> x (evasion: attacker sets x=x)
- *   - T3 Multi-char parameter expansion: ${ANYTHING} -> '' (unknown value)
+ * Strips / rewrites (T1-T6):
+ *   - T1 Empty single quotes: ''                      (e.g., w''get -> wget)
+ *   - T2 Empty double quotes: ""                      (e.g., r""m -> rm)
+ *   - T3 Single-char parameter expansion: ${x} -> x   (c${u}rl -> curl)
+ *   - T3 Multi-char parameter expansion:  ${FOO} -> '' (unknown value)
  *   - T4 Backslash escapes between word chars, iteratively (c\u\r\l -> curl)
- *   - T5 IFS word-splitting: ${IFS}, ${IFS:0:1}, $IFS -> ' '
+ *   - T5 IFS word-splitting: ${IFS} / ${IFS:0:1} / $IFS -> ' '
  *   - T6 ANSI-C hex quoting inside $'...' -> decoded bytes
  *   - Backtick subshell with empty/whitespace content
  *
- * Does NOT strip:
+ * Does NOT rewrite:
  *   - Quotes around arguments (only targets empty quotes that split command names)
- *   - $VAR without braces outside IFS (not an evasion pattern)
+ *   - $VAR without braces (non-IFS; not an evasion pattern)
  *   - Backslashes before non-word chars (\n, \t, etc.)
- *   - Content inside single-quoted regions (T5 preserves them; `echo '${IFS}'` untouched)
+ *   - Content inside non-empty single-quoted regions
+ *     (false-positive probe: `echo '${IFS}'` stays untouched)
  *
  * @param {string} cmd - Raw command string
  * @returns {string} Normalized command string
@@ -86,15 +89,25 @@ function normalizeAnsiCHex(cmd) {
 export function normalizeBashExpansion(cmd) {
   if (!cmd || typeof cmd !== 'string') return cmd || '';
 
-  // T5 + T6 run first so their outputs feed the rest of the pipeline in
-  // canonical form. Order inside T5/T6 is internal; externally we label
-  // the full pipeline T1-T6.
-  let result = normalizeIFS(cmd);
-  result = normalizeAnsiCHex(result);
+  // T1 — strip empty single-quote pairs first so adjacent-empty-quote evasion
+  // (c''u''rl -> curl) resolves before single-quote masking runs.
+  let result = cmd.replace(/''/g, '');
+
+  // T6 — decode ANSI-C hex inside $'...' before masking treats it as a literal.
+  result = decodeAnsiCHex(result);
+
+  // Mask remaining non-empty single-quoted regions.
+  const { masked, placeholders } = maskSingleQuoted(result);
+  result = masked;
+
+  // T5 — IFS word-splitting. Runs before T2/T3/T4 so the canonical spaces
+  // it emits feed into subsequent transforms.
+  result = result
+    .replace(/\$\{IFS:0:1\}/g, ' ')
+    .replace(/\$\{IFS\}/g, ' ')
+    .replace(/\$IFS\b/g, ' ');
 
   result = result
-    // T1 Strip empty single quotes: w''get -> wget
-    .replace(/''/g, '')
     // T2 Strip empty double quotes: r""m -> rm
     .replace(/""/g, '')
     // T3 Single-char ${x} -> x (evasion: c${u}rl -> curl, assumes x=x)
@@ -104,12 +117,12 @@ export function normalizeBashExpansion(cmd) {
     // Strip backtick subshell with empty/whitespace content
     .replace(/`\s*`/g, '');
 
-  // T4 Iteratively strip backslash between word chars (c\u\r\l needs 2 passes)
+  // T4 — iteratively strip backslash between word chars (c\u\r\l needs 2 passes)
   let prev;
   do {
     prev = result;
     result = result.replace(/(\w)\\(\w)/g, '$1$2');
   } while (result !== prev);
 
-  return result;
+  return unmaskSingleQuoted(result, placeholders);
 }