GHSA-62q4-447f-wv8h Regression in pymdownx.snippets reintroduces sibling-prefix path traversal bypass despite restrict_base_path

• PyPI pymdown-extensions: >= 10.0.1, < 10.21.3 → fixed in 10.21.3

Summary `pymdownx.snippets` has a regression of the CVE-2023-32309 / GHSA-jh85-wwv9-24hv fix. With `restrict_base_path: True` (the default), the current `filename.startswith(base)` containment check does not enforce a directory boundary. As a result, a markdown snippet directive can read files from sibling paths that share the same prefix as `base_path`, such as `docs` vs `docs_internal`. The regression was introduced in PR #2039 / commit `7c13bda5b7793b172efd1abb6712e156a83fe07d`, which replaced the original directory-identity check with a plain string-prefix comparison. Details The regression was introduced in commit `7c13bda5b7793b172efd1abb6712e156a83fe07d` (2023-05-15, #2039 *"Fix regression of snippets nested deeply under specified base path"*), which relaxed the original `os.path.samefile(base, os.path.dirname(filename))` check to a plain `startswith(base)`. `SnippetPreprocessor.get_snippet_path()` in `pymdownx/snippets.py`: ```python if self.restrict_base_path: filename = os.path.abspath(os.path.join(base, path)) # If the absolute path is no longer under the specified base path, reject the file if not filename.startswith(base): continue ``` `base` is `os.path.abspath(b)` and has no trailing separator. `str.startswith(base)` is `True` for any `filename` whose string representation begins with the same characters as `base`, regardless of whether those characters end at a directory boundary. Concrete example: • `base = "/x/docs"` • `path = "../docs_secret/leak.txt"` (inside the markdown snippet directive) • `os.path.join(base, path)` → `"/x/docs/../docs_secret/leak.txt"` • `os.path.abspath(...)` → `"/x/docs_secret/leak.txt"` • `filename.startswith(base)` → `True`, because `"/x/docs_secret/..."` begins with the literal string `"/x/docs"`. All releases from 10.0.1 (2023-05-15) through 10.21.2 (current) are affected. Impact Arbitrary file read within the host the build runs on, bounded by the prefix match. With `base_path = /x/docs` the attacker can read files from any sibling directory whose path begins with the literal string `/x/docs` followed by any non-separator character — for example `/x/docs_internal/`, `/x/docs.bak/`, `/x/docs2/`. The threat model is the same as the original CVE-2023-32309: markdown content processed by the snippets preprocessor in a build pipeline (typical scenario: an MkDocs documentation site built in CI from PR contributions or otherwise less-trusted markdown) can read files outside the configured base. CI builds that publish the generated HTML expose the read file to the public; CI builds with secrets on disk leak those secrets. Reproduction Minimal local PoC, non-destructive: ```python import os, shutil, tempfile, markdown work = tempfile.mkdtemp(prefix="pmx_poc_") try: base = os.path.join(work, "docs") sibling = os.path.join(work, "docs_secret") os.makedirs(base) os.makedirs(sibling) with open(os.path.join(sibling, "leak.txt"), "w") as f: f.write("TOP_SECRET_FROM_SIBLING_DIR\n") out = markdown.markdown( '--8 TOP_SECRET_FROM_SIBLING_DIR finally: shutil.rmtree(work) ``` Default `restrict_base_path: True` is sufficient — no non-default option is required. Suggested fix Minimal change — require the separator after the base prefix: ```diff • if not filename.startswith(base): + # Append `os.sep` so a sibling directory whose name shares a prefix + # (e.g. `/x/docs` vs `/x/docs_evil`) cannot satisfy the check. + if not filename.startswith(base + os.sep): continue ``` This preserves the original intent (allow snippets nested at any depth under `base_path`) while restoring the directory-boundary check. It does not affect the `os.path.isdir(base)` branch where `base` is a file (that branch still uses `os.path.samefile`). Alternative: `os.path.commonpath([base, filename]) == base` is equivalent and slightly more idiomatic, though it raises `ValueError` on different drives on Windows and would need a `try/except`. The `startswith(base + os.sep)` fix is the smaller diff. Note: this fix does not change behaviour for symlinks inside `base_path`. The existing implementation uses `os.path.abspath` (not `os.path.realpath`), so a symlink within `base_path` pointing outside is still followed. That is a separate concern — symlinks require write access to `base_path`, a much higher bar than the current bypass — and matches the behaviour the CVE-2023 fix established. Regression test A regression test class `TestSnippetsSiblingPrefix` was added in `tests/test_extensions/test_snippets.py`. It uses `tests/test_extensions/_snippets/nested` as `base_path` and a new fixture directory `tests/test_extensions/_snippets/nested_sibling_evil/leak.txt`. It asserts that the markdown directive `--8= 10.0.1, <= 10.21.2`