🛡️ Sentinel: [CRITICAL] Fix symlink vulnerability in atomic writes

.jules/sentinel.md

@@ -0,0 +1,7 @@
+## 2025-02-18 - Fix Predictable Temporary File Symlink Vulnerability
+
+**Vulnerability:** The code previously used string concatenation (e.g., `path + ".tmp"`) with `os.WriteFile` and `os.Rename` for atomic file writes in shared directories like `os.TempDir()`. This pattern is vulnerable to symlink attacks, where an attacker could pre-create a symlink with the predictable name pointing to an arbitrary file they want to overwrite.
+
+**Learning:** Predictable temporary file names in shared directories (like `/tmp`) are inherently insecure for atomic writes. Even if the content being written isn't malicious, the *location* being written to can be controlled via symlinks, leading to arbitrary file overwrite or corruption issues. Furthermore, creating a file with `os.CreateTemp` ensures the OS opens it with `O_EXCL`, preventing any prior symlink from being followed. Finally, on Windows, file handles must be explicitly closed before the `os.Rename` step, or it will fail.
+
+**Prevention:** For atomic writes in shared directories, *always* use `os.CreateTemp` with an unpredictable pattern (like `prism-*.tmp`). Ensure the temporary file is created in the same directory as the target file (using `filepath.Dir()`) to avoid cross-device rename errors. Explicitly `Chmod` to match required permissions, handle the error gracefully by closing and removing the temporary file, and explicitly close the file descriptor before calling `os.Rename`.

internal/burnrate/burnrate.go

@@ -64,10 +64,28 @@ func LoadOrCreateSnapshotAt(sessionID string, currentCost float64, now time.Time
 		return nil, false, err
 	}
 
-	tmpPath := path + ".tmp"
-	if err := os.WriteFile(tmpPath, data, 0644); err != nil {
+	// Use os.CreateTemp to avoid symlink vulnerability with predictable names
+	tmpFile, err := os.CreateTemp(filepath.Dir(path), "prism-burn-*.tmp")
+	if err != nil {
+		return nil, false, err
+	}
+	tmpPath := tmpFile.Name()
+
+	if _, err := tmpFile.Write(data); err != nil {
+		tmpFile.Close()
+		os.Remove(tmpPath)
+		return nil, false, err
+	}
+	// explicit close is important especially on Windows before rename
+	if err := tmpFile.Close(); err != nil {
+		os.Remove(tmpPath)
+		return nil, false, err
+	}
+	if err := os.Chmod(tmpPath, 0644); err != nil {
+		os.Remove(tmpPath)
 		return nil, false, err
 	}
+
 	if err := os.Rename(tmpPath, path); err != nil {
 		os.Remove(tmpPath)
 		return nil, false, err

internal/sparkline/sparkline.go

@@ -138,10 +138,31 @@ func Save(sessionID, metric string, b *Buffer) {
 	if err != nil {
 		return
 	}
-	tmp := path + ".tmp"
-	if err := os.WriteFile(tmp, data, 0644); err != nil {
+
+	// Use os.CreateTemp to avoid symlink vulnerability with predictable names
+	tmpFile, err := os.CreateTemp(filepath.Dir(path), "prism-spark-*.tmp")
+	if err != nil {
+		return
+	}
+	tmp := tmpFile.Name()
+
+	if _, err := tmpFile.Write(data); err != nil {
+		tmpFile.Close()
+		os.Remove(tmp)
+		return
+	}
+
+	// explicit close is important especially on Windows before rename
+	if err := tmpFile.Close(); err != nil {
+		os.Remove(tmp)
+		return
+	}
+
+	if err := os.Chmod(tmp, 0644); err != nil {
+		os.Remove(tmp)
 		return
 	}
+
 	os.Rename(tmp, path)
 }