🛡️ Sentinel: [HIGH] Fix TOCTOU vulnerability in config permissions

.jules/sentinel.md

@@ -31,3 +31,7 @@
 **Vulnerability:** External shell command executed in `listLocalSnapshots()` triggered a deadlock when `tmutil` output exceeded 64KB, because stdout and stderr were read synchronously inside the process termination handler.
 **Learning:** In Swift, reading from a process pipe synchronously inside a `terminationHandler` can result in a permanent deadlock if the child blocks writing to a full pipe, preventing it from exiting.
 **Prevention:** Asynchronously drain pipes continuously while the process is running using background queues.
+## 2026-05-25 - TOCTOU Vulnerability via chmod
+**Vulnerability:** Found `chmod(path, 0o600)` being used to enforce file permissions on a configuration path that could be user-controlled.
+**Learning:** `chmod()` is vulnerable to Time-of-Check to Time-of-Use (TOCTOU) attacks because it follows symlinks. If an attacker replaces the target file with a symlink before `chmod()` executes, permissions of an arbitrary file could be altered.
+**Prevention:** To safely apply permissions, obtain a file descriptor using `open()` with the `O_NOFOLLOW` flag to guarantee the target is not a symlink, then apply permissions safely using `fchmod()`. Always use `URL.withUnsafeFileSystemRepresentation` to pass paths to POSIX APIs.

Sources/Cacheout/Headless/DaemonMode.swift

@@ -966,8 +966,15 @@ public actor DaemonMode: StatusSocket.DataSource {
             return
         }
 
-        // Enforce 0600 permissions
-        chmod(path, 0o600)
+        // Enforce 0600 permissions safely to avoid TOCTOU symlink vulnerabilities
+        URL(fileURLWithPath: path).withUnsafeFileSystemRepresentation { ptr in
+            guard let ptr = ptr else { return }
+            let fd = open(ptr, O_RDONLY | O_NOFOLLOW | O_CLOEXEC)
+            if fd >= 0 {
+                fchmod(fd, 0o600)
+                close(fd)
+            }
+        }
 
         // Read file
         guard let data = FileManager.default.contents(atPath: path) else {