From 23c8a8fc92f5814be9b5c35251dbbd6f1f68dabf Mon Sep 17 00:00:00 2001
From: "google-labs-jules[bot]"
 <161369871+google-labs-jules[bot]@users.noreply.github.com>
Date: Mon, 4 May 2026 02:26:59 +0000
Subject: [PATCH] =?UTF-8?q?=F0=9F=9B=A1=EF=B8=8F=20Sentinel:=20[CRITICAL]?=
 =?UTF-8?q?=20Fix=20symlink=20attack=20vulnerability=20in=20atomic=20file?=
 =?UTF-8?q?=20writes?=
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

Co-authored-by: himattm <6266621+himattm@users.noreply.github.com>
---
 .jules/sentinel.md              |  4 ++++
 internal/burnrate/burnrate.go   | 23 +++++++++++++++++++++--
 internal/sparkline/sparkline.go | 28 +++++++++++++++++++++++++---
 3 files changed, 50 insertions(+), 5 deletions(-)
 create mode 100644 .jules/sentinel.md

diff --git a/.jules/sentinel.md b/.jules/sentinel.md
new file mode 100644
index 0000000..267536d
--- /dev/null
+++ b/.jules/sentinel.md
@@ -0,0 +1,4 @@
+## 2026-05-04 - Fix Predictable Temporary File Names for Atomic Writes
+**Vulnerability:** Predictable temporary file names used during atomic file writes (`path + ".tmp"`) are susceptible to symlink attacks, allowing an attacker to overwrite arbitrary files if they have write access to the temporary directory.
+**Learning:** Atomic file writes often rely on temporary files, but using predictable names allows malicious actors to pre-create symlinks pointing to sensitive files.
+**Prevention:** Always use `os.CreateTemp` to generate unpredictable temporary file names, create them in the same directory as the target file to avoid cross-device rename issues, and securely set permissions before writing.
diff --git a/internal/burnrate/burnrate.go b/internal/burnrate/burnrate.go
index 8cb680c..b558b0f 100644
--- a/internal/burnrate/burnrate.go
+++ b/internal/burnrate/burnrate.go
@@ -64,10 +64,29 @@ 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 {
+	tmpFile, err := os.CreateTemp(filepath.Dir(path), filepath.Base(path)+".*.tmp")
+	if err != nil {
+		return nil, false, err
+	}
+	tmpPath := tmpFile.Name()
+
+	if err := tmpFile.Chmod(0644); err != nil {
+		tmpFile.Close()
+		os.Remove(tmpPath)
+		return nil, false, err
+	}
+
+	if _, err := tmpFile.Write(data); err != nil {
+		tmpFile.Close()
+		os.Remove(tmpPath)
 		return nil, false, err
 	}
+
+	if err := tmpFile.Close(); err != nil {
+		os.Remove(tmpPath)
+		return nil, false, err
+	}
+
 	if err := os.Rename(tmpPath, path); err != nil {
 		os.Remove(tmpPath)
 		return nil, false, err
diff --git a/internal/sparkline/sparkline.go b/internal/sparkline/sparkline.go
index f4c755c..c674d47 100644
--- a/internal/sparkline/sparkline.go
+++ b/internal/sparkline/sparkline.go
@@ -138,11 +138,33 @@ func Save(sessionID, metric string, b *Buffer) {
 	if err != nil {
 		return
 	}
-	tmp := path + ".tmp"
-	if err := os.WriteFile(tmp, data, 0644); err != nil {
+	tmpFile, err := os.CreateTemp(filepath.Dir(path), filepath.Base(path)+".*.tmp")
+	if err != nil {
+		return
+	}
+	tmpPath := tmpFile.Name()
+
+	if err := tmpFile.Chmod(0644); err != nil {
+		tmpFile.Close()
+		os.Remove(tmpPath)
+		return
+	}
+
+	if _, err := tmpFile.Write(data); err != nil {
+		tmpFile.Close()
+		os.Remove(tmpPath)
+		return
+	}
+
+	if err := tmpFile.Close(); err != nil {
+		os.Remove(tmpPath)
+		return
+	}
+
+	if err := os.Rename(tmpPath, path); err != nil {
+		os.Remove(tmpPath)
 		return
 	}
-	os.Rename(tmp, path)
 }
 
 // PushAndSave is a convenience that loads, pushes, saves, and returns the buffer