🛡️ Sentinel: [HIGH] Fix TOCTOU vulnerability in config permission enforcement

.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.
+## 2024-05-23 - Fix TOCTOU vulnerability in file permission enforcement
+**Vulnerability:** Enforcing file permissions using `chmod()` on a potentially user-controlled file path introduces a Time-of-Check to Time-of-Use (TOCTOU) symlink vulnerability.
+**Learning:** `chmod()` operates on paths, allowing a malicious actor to swap the file for a symlink between when the path was verified and when the permissions were modified, potentially altering permissions of arbitrary files.
+**Prevention:** Avoid `chmod()`. Safely obtain a file descriptor using `open()` with the `O_NOFOLLOW` flag to prevent following symlinks, and then apply permissions atomically using `fchmod()`.

Sources/Cacheout/Headless/DaemonMode.swift

@@ -966,8 +966,17 @@ 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 { cPath in
+            guard let cPath = cPath else { return }
+            let fd = open(cPath, O_RDONLY | O_NOFOLLOW)
+            if fd >= 0 {
+                fchmod(fd, 0o600)
+                close(fd)
+            } else {
+                logger.warning("Failed to secure config file permissions (maybe a symlink?): errno \(errno)")
+            }
+        }
 
         // Read file
         guard let data = FileManager.default.contents(atPath: path) else {