nbconvert: Arbitrary file write through path traversal in cell output filenames when exporting to md, latex, rst and asciidoc

### Summary nbconvert <= v7.17.1 has an arbitrary file write vulnerability due to a path traversal in ipynb cell output metadata, more specifically on the filename attribute. Vulnerable converters : `--to markdown`, `--to latex`, `--to rst`, `--to asciidoc`. In `ExtractOutputPreprocessor`, `extractoutput.py:116-119` joins attacker-controlled `path` to `output_files_dir` with no canonicalization check, enabling arbitrary file write. This is a sibling vulnerability of https://github.com/advisories/GHSA-4c99-qj7h-p3vg. ### Impact If a victim runs nbconvert on a malicious ipynb file, it can create some files with arbitrary data in arbitrary locations, i.e. override authorized_keys. Note that running the cell with jupyter overrides the output, so it would "clean up" malicious cells. ### PoC In a ipynb file: ```json "cells": [{ "cell_type": "code", "id": "abc123", "execution_count": 1, "metadata": {}, "source": "# PoC", "outputs": [{ "output_type": "display_data", "metadata": { "filename": "../../../tmp/PWNED.png" # ← traverses outside output_dir }, "data": { # minimal valid PNG (1×1 pixel) "image/png": "iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAADUlEQVR42mNk+M9QDwADhgGAWjR9awAAAABJRU5ErkJggg==" } }] }] ``` ### Patches Fixed in - https://github.com/jupyter/nbconvert/commit/ce4bd99aec31d81d759fa8eeb8643179392ec66f - [7.17.2](https://github.com/jupyter/nbconvert/releases/tag/v7.17.2)