🛡️ Sentinel: [CRITICAL] Fix TOCTOU vulnerability in cleanup logging

.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.
+## 2026-05-21 - TOCTOU Symlink Vulnerability in Logging
+**Vulnerability:** Found `FileHandle(forWritingTo:)` and `String.write(to:atomically:)` being used for writing to a log file, which follow symlinks by default.
+**Learning:** High-level Swift file APIs follow symlinks by default, making them vulnerable to Time-of-Check to Time-of-Use (TOCTOU) symlink attacks if the directory is potentially untrusted.
+**Prevention:** To securely create or append to files in potentially untrusted directories, use POSIX `open(2)` with flags such as `O_CREAT | O_WRONLY | O_APPEND | O_NOFOLLOW | O_CLOEXEC` to explicitly refuse symlinks, then wrap the resulting file descriptor in a `FileHandle`.

Sources/Cacheout/Cleaner/CacheCleaner.swift

@@ -34,6 +34,7 @@
 
 import Foundation
 import AppKit
+import Darwin
 
 actor CacheCleaner {
     private let fileManager = FileManager.default
@@ -261,12 +262,19 @@ actor CacheCleaner {
         let size = ByteCountFormatter.sharedFile.string(fromByteCount: bytesFreed)
         let entry = "[\(ISO8601DateFormatter.shared.string(from: Date()))] Cleaned \(category): \(size)\n"
 
-        if let handle = try? FileHandle(forWritingTo: logFile) {
-            handle.seekToEndOfFile()
+        let fd = logFile.withUnsafeFileSystemRepresentation { pathPtr -> Int32 in
+            guard let pathPtr = pathPtr else { return -1 }
+            return open(pathPtr, O_CREAT | O_WRONLY | O_APPEND | O_NOFOLLOW | O_CLOEXEC, 0o600)
+        }
+
+        if fd != -1 {
+            let handle = FileHandle(fileDescriptor: fd, closeOnDealloc: true)
             handle.write(entry.data(using: .utf8) ?? Data())
-            handle.closeFile()
-        } else {
-            try? entry.write(to: logFile, atomically: true, encoding: .utf8)
+            if #available(macOS 10.15.4, *) {
+                try? handle.close()
+            } else {
+                handle.closeFile()
+            }
         }
     }
 }