Crawl4AI: Arbitrary file write (path traversal) in crawler downloads can lead to RCE

Summary

When the crawler saves a downloaded file, the destination filename was taken from attacker-influenced input and joined to the downloads directory with no confinement. A filename containing an absolute path (e.g. /etc/cron.d/evil) or ../ traversal escaped the downloads directory, giving an arbitrary file write with attacker-controlled contents. Because the written bytes are attacker-controlled, this escalates to remote code execution (overwriting a shell rc-file, ~/.ssh/authorized_keys, a cron entry, or a Python module on the import path).

Affected paths

Two download sinks in crawl4ai/async_crawler_strategy.py:

• HTTP crawler (AsyncHTTPCrawlerStrategy): the filename is parsed from the response Content-Disposition header by _extract_filename() and written via aiofiles.open(filepath, 'wb'). Reachable directly via the SDK, and via the unauthenticated Docker /crawl endpoint when an HTTPCrawlerConfig is supplied.
• Browser crawler (AsyncPlaywrightCrawlerStrategy): the download's suggested_filename (controllable by the visited page) is joined to downloads_path and written via download.save_as().

The HTTP-strategy sink is reachable pre-auth on the default Docker deployment; both are reachable for SDK users simply by crawling an attacker-controlled URL. The default Playwright crawl path that does not trigger a download is unaffected.

Impact

Arbitrary file write with attacker-controlled content as the user running the crawler, escalating to remote code execution.

Fix

Both sinks now resolve the destination through a single hardened helper (_safe_download_filepath) that reduces the attacker-influenced name to a bare basename (dropping absolute paths and .. components) and re-checks, via realpath, that the resolved path stays inside the downloads root (defeating symlink/TOCTOU escapes). A traversal attempt is rejected; normal downloads are unchanged.

Workarounds

• Upgrade to the patched version (0.9.0).
• Run the crawler as an unprivileged user with a dedicated, isolated downloads directory on a volume with no sensitive paths writable.
• Enable authentication (CRAWL4AI_API_TOKEN) on the Docker server.

Credits

Y4tacker - reported the Content-Disposition path traversal in the HTTP crawler with a clear PoC and a basename + realpath-containment fix recommendation.