🛡️ Sentinel: [CRITICAL] Fixed insecure file creation 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.
+## 2026-05-02 - TOCTOU Vulnerability via Default File Creation Permissions
+**Vulnerability:** Files were created with default umask permissions and subsequently locked down using `FileManager.default.setAttributes`, exposing a brief window where unauthorized users could access or replace the file.
+**Learning:** High-level Swift APIs like `Data.write(to:)` rely on global process umask, making them vulnerable to Time-of-Check to Time-of-Use (TOCTOU) attacks when handling sensitive data.
+**Prevention:** Always use POSIX `open()` with `O_CREAT | O_WRONLY | O_EXCL | O_CLOEXEC` flags and explicitly set secure permissions (e.g., 0600) during file creation, then wrap the resulting file descriptor in a `FileHandle`.

Sources/CacheoutHelperLib/SysctlJournal.swift

@@ -307,14 +307,27 @@ public final class SysctlJournal {
         do {
             let data = try PropertyListEncoder().encode(state)
 
-            // Write to temp file (non-atomic — we control the rename ourselves).
-            try data.write(to: tmpURL)
+            // Preemptively remove stale temp files to avoid EEXIST with O_EXCL
+            try? FileManager.default.removeItem(at: tmpURL)
 
-            // Set permissions to 0600 (root-only) on temp file before rename.
-            try FileManager.default.setAttributes(
-                [.posixPermissions: 0o600],
-                ofItemAtPath: tmpURL.path
-            )
+            // Securely create temp file with 0600 permissions to prevent TOCTOU
+            let fd = tmpURL.withUnsafeFileSystemRepresentation { pathPtr -> Int32 in
+                guard let pathPtr = pathPtr else { return -1 }
+                return open(pathPtr, O_CREAT | O_WRONLY | O_EXCL | O_CLOEXEC, 0o600)
+            }
+
+            guard fd != -1 else {
+                throw NSError(domain: NSPOSIXErrorDomain, code: Int(errno), userInfo: nil)
+            }
+
+            let handle = FileHandle(fileDescriptor: fd, closeOnDealloc: true)
+            if #available(macOS 10.15.4, *) {
+                try handle.write(contentsOf: data)
+                try handle.close()
+            } else {
+                handle.write(data)
+                handle.closeFile()
+            }
 
             // Atomic rename(2) — atomicity on APFS/HFS+.
             if rename(tmpURL.path, url.path) != 0 {