CVE-2026-66881

## Summary Relative Path Traversal vulnerability in livebook-dev livebook allows an attacker-authored notebook to write a file with attacker-controlled content to an arbitrary path. A `.livemd` notebook can declare `file_entries` metadata, each entry carrying a `name`. Every path that creates a file entry through the user interface validates that name with `Livebook.Notebook.validate_file_entry_name/2`, which requires a flat filename of alphanumerics, dashes, underscores and dots, ending in an extension. The import path does not: `Livebook.LiveMarkdown.Import.file_entry_metadata_to_attrs/1` in `lib/livebook/live_markdown/import.ex` takes the name verbatim from the notebook source. For a URL-type file entry, `Livebook.Session.file_entry_cache_file/2` in `lib/livebook/session.ex` resolves that name beneath the session's temporary directory without checking that the result stays inside it, and `Livebook.FileSystem.Utils.resolve_unix_like_path/2` collapses parent-directory segments while clamping only at the filesystem root. When the entry's content is requested and no cached copy exists, Livebook fetches the entry's URL and writes the response body to the resolved path, creating parent directories as needed. The attacker therefore controls both the destination and the contents of the written file, which may land anywhere the Livebook process can write. The same missing containment check is present in `Livebook.Session.to_attachment_file_entry/2`. A victim who opens an attacker-supplied notebook and causes the entry to be fetched triggers the write within their own authenticated session; the attacker needs no account on the target instance. URL-type entries are also not placed under notebook stamping quarantine on import, so no warning is shown. This issue affects livebook: from 0.11.0 before 0.18.7 and from 0.19.0 before 0.19.9.