🛡️ Sentinel: [CRITICAL/HIGH] Fix TOCTOU vulnerability in file creation

.jules/sentinel.md

@@ -31,3 +31,8 @@
 **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 - Fix TOCTOU vulnerability in file creation
+**Vulnerability:** High-level Swift file APIs (`FileHandle(forWritingTo:)`, `String.write(to:atomically:)`) follow symlinks by default, leading to Time-of-Check to Time-of-Use (TOCTOU) symlink vulnerabilities.
+**Learning:** High-level APIs are inherently insecure when writing to paths that could be tampered with. The vulnerability can be exploited to overwrite arbitrary files if an attacker replaces a file or directory with a symlink before the application writes to it.
+**Prevention:** Use POSIX `open(2)` with flags like `O_CREAT | O_WRONLY | O_APPEND | O_NOFOLLOW | O_CLOEXEC` to explicitly refuse following symlinks, securely create the file, and 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
@@ -255,18 +256,27 @@ actor CacheCleaner {
     private func logCleanup(category: String, bytesFreed: Int64) {
         let logDir = FileManager.default.homeDirectoryForCurrentUser
             .appendingPathComponent(".cacheout")
-        try? FileManager.default.createDirectory(at: logDir, withIntermediateDirectories: true)
+        try? FileManager.default.createDirectory(at: logDir, withIntermediateDirectories: true, attributes: [.posixPermissions: 0o700])
 
         let logFile = logDir.appendingPathComponent("cleanup.log")
         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()
-            handle.write(entry.data(using: .utf8) ?? Data())
-            handle.closeFile()
-        } else {
-            try? entry.write(to: logFile, atomically: true, encoding: .utf8)
+        _ = logFile.withUnsafeFileSystemRepresentation { pathPtr -> Int32 in
+            guard let pathPtr = pathPtr else { return -1 }
+            let fd = open(pathPtr, O_CREAT | O_WRONLY | O_APPEND | O_NOFOLLOW | O_CLOEXEC, 0o600)
+            if fd != -1 {
+                let handle = FileHandle(fileDescriptor: fd, closeOnDealloc: true)
+                let data = entry.data(using: .utf8) ?? Data()
+                if #available(macOS 10.15.4, *) {
+                    try? handle.write(contentsOf: data)
+                    try? handle.close()
+                } else {
+                    handle.write(data)
+                    handle.closeFile()
+                }
+            }
+            return fd
         }
     }
 }