From a7e51e804635a3bc1df72483124a05fc0623130e Mon Sep 17 00:00:00 2001
From: Jamkris <dltmdgus1412@gmail.com>
Date: Thu, 14 May 2026 11:21:41 +0900
Subject: [PATCH] fix(hooks): close subshell bypass in
 pre-bash-dev-server-block
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

Before this commit the dev-server-block hook ran the leading-command
and dev-pattern check only against the top-level segments returned by
`splitShellSegments`, which doesn't split on `$(...)`, backticks, or
plain `(...)`. That left the policy bypassable by wrapping a dev
command in any of those constructs:

  $(npm run dev)
  `npm run dev`
  echo $(npm run dev)
  (npm run dev)

Each verified by piping a synthetic PreToolUse payload into the hook
on this branch: every form above returned exit 0 (allow) where a plain
`npm run dev` correctly returned exit 2 (block).

Fix: expand the check space before running the leading-command rule.
A small BFS walks the raw command, harvesting bodies from
`extractCommandSubstitutions` (`$(...)` and backticks) and from
`extractSubshellGroups` (plain `(...)`), then splits each harvested
body through `splitShellSegments` and feeds the result into the
existing `isBlockedDevSegment` check.

This preserves every existing allow case (`tmux new-session -d -s dev
"npm run dev"`, quoted-string mentions like `git commit -m "npm run
dev fix"`, `echo hi`) because the leading-command rule is unchanged —
only the set of segments it runs against grew.

Known limitation, not fixed here: `eval "$(echo npm run dev)"` still
slips through because the substitution body's leading command is
`echo`, and statically modeling echo's output to recover the executed
command is out of scope. The same class affects `gateguard-fact-force`
(via `eval "$(echo rm -rf /)"` etc.) and is best addressed in both
hooks together as a follow-up rather than as a one-off here.
---
 scripts/hooks/pre-bash-dev-server-block.js | 58 ++++++++++++++++++----
 1 file changed, 47 insertions(+), 11 deletions(-)

diff --git a/scripts/hooks/pre-bash-dev-server-block.js b/scripts/hooks/pre-bash-dev-server-block.js
index 9c0861b8..d3eea48b 100755
--- a/scripts/hooks/pre-bash-dev-server-block.js
+++ b/scripts/hooks/pre-bash-dev-server-block.js
@@ -4,6 +4,10 @@
 const MAX_STDIN = 1024 * 1024;
 const path = require('path');
 const { splitShellSegments } = require('../lib/shell-split');
+const {
+  extractCommandSubstitutions,
+  extractSubshellGroups
+} = require('../lib/shell-substitution');
 
 const DEV_COMMAND_WORDS = new Set([
   'npm',
@@ -154,23 +158,55 @@ process.stdin.on('data', chunk => {
   }
 });
 
+const TMUX_LAUNCHER = /^\s*tmux\s+(new|new-session|new-window|split-window)\b/;
+const DEV_PATTERN = /\b(npm\s+run\s+dev|pnpm(?:\s+run)?\s+dev|yarn\s+dev|bun\s+run\s+dev)\b/;
+
+/**
+ * Collect every command-line segment we should evaluate. Returns the top-level
+ * segments first, then segments harvested from `$(...)` / backtick command
+ * substitutions and plain `(...)` subshell groups, recursively.
+ *
+ * Without this expansion the leading-command and dev-pattern check below only
+ * sees the outermost command, so wrappers like `$(npm run dev)` and
+ * `(npm run dev)` (which still spawn a dev server) sneak past.
+ */
+function collectCheckSegments(cmd) {
+  const segments = [...splitShellSegments(cmd)];
+  const queue = [cmd];
+  const seen = new Set();
+
+  while (queue.length) {
+    const current = queue.shift();
+    if (seen.has(current)) continue;
+    seen.add(current);
+
+    for (const body of extractCommandSubstitutions(current)) {
+      for (const seg of splitShellSegments(body)) segments.push(seg);
+      queue.push(body);
+    }
+    for (const body of extractSubshellGroups(current)) {
+      for (const seg of splitShellSegments(body)) segments.push(seg);
+      queue.push(body);
+    }
+  }
+
+  return segments;
+}
+
+function isBlockedDevSegment(segment) {
+  const commandWord = getLeadingCommandWord(segment);
+  if (!commandWord || !DEV_COMMAND_WORDS.has(commandWord)) return false;
+  return DEV_PATTERN.test(segment) && !TMUX_LAUNCHER.test(segment);
+}
+
 process.stdin.on('end', () => {
   try {
     const input = JSON.parse(raw);
     const cmd = String(input.tool_input?.command || '');
 
     if (process.platform !== 'win32') {
-      const segments = splitShellSegments(cmd);
-      const tmuxLauncher = /^\s*tmux\s+(new|new-session|new-window|split-window)\b/;
-      const devPattern = /\b(npm\s+run\s+dev|pnpm(?:\s+run)?\s+dev|yarn\s+dev|bun\s+run\s+dev)\b/;
-
-      const hasBlockedDev = segments.some(segment => {
-        const commandWord = getLeadingCommandWord(segment);
-        if (!commandWord || !DEV_COMMAND_WORDS.has(commandWord)) {
-          return false;
-        }
-        return devPattern.test(segment) && !tmuxLauncher.test(segment);
-      });
+      const segments = collectCheckSegments(cmd);
+      const hasBlockedDev = segments.some(isBlockedDevSegment);
 
       if (hasBlockedDev) {
         console.error('[Hook] BLOCKED: Dev server must run in tmux for log access');