diff --git a/.jules/sentinel.md b/.jules/sentinel.md
index 4b53157..c07d5d4 100644
--- a/.jules/sentinel.md
+++ b/.jules/sentinel.md
@@ -3,3 +3,8 @@
 **Vulnerability:** In `apps/zettel/src/tools/transcribe.ts`, the `binaryAvailable` function used `sh -c command -v ${bin}` which was susceptible to command injection if the `bin` argument (derived from `process.env.WHISPER_BIN`) contained shell metacharacters.
 **Learning:** Even when reading from environment variables, interpolating strings into shell execution wrappers (`sh -c`) can expose systems to command injection vulnerabilities. `execFileSync` without `shell: true` should be preferred.
 **Prevention:** Avoid using shell interpolation to evaluate environment variables or command-line arguments. Instead, use an argument array to pass executable names securely. For checking binary existence on `$PATH`, `execFileSync("which", [bin])` safely prevents shell execution by relying directly on the `which` executable and standard IO bindings.
+
+## 2026-09-04 - [Path Traversal bypass via path.basename in POSIX env]
+**Vulnerability:** Path traversal in file upload endpoints handling `file.name` via `path.basename`.
+**Learning:** In Node.js running on POSIX systems (Linux/macOS), `path.basename` only strips `/`, but an attacker can provide a Windows-style path traversal payload (e.g. `..\..\etc\passwd`). Node's `path.basename` considers the entire string as the file name, meaning `path.join` will parse it and traverse directories.
+**Prevention:** Thoroughly sanitize file names using strict regex matching (e.g., `file.name.replace(/[^a-zA-Z0-9.-]/g, "_")`) instead of relying on `path.basename` across mixed OS environments.
diff --git a/apps/zettel/src/index.ts b/apps/zettel/src/index.ts
index 332788e..b318650 100644
--- a/apps/zettel/src/index.ts
+++ b/apps/zettel/src/index.ts
@@ -265,9 +265,10 @@ const routes = api
       const arrayBuffer = await file.arrayBuffer();
       const data = Buffer.from(arrayBuffer);
 
+      const safeFileName = file.name.replace(/[^a-zA-Z0-9.-]/g, "_");
       const tmpPath = path.join(
         os.tmpdir(),
-        `zettel-audio-${Date.now()}-${path.basename(file.name)}`,
+        `zettel-audio-${Date.now()}-${safeFileName}`,
       );
       fs.writeFileSync(tmpPath, data);