| Package | Affected versions | Patched versions |
|---|---|---|
| pymdown-extensions | ≥ 10.0.1, < 10.21.3 | 10.21.3 |
| pymdown extensions | ≥ 10.0.1, < 10.21.3 | 10.21.3 |
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.
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:
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.
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 followed by any non-separator character — for example , , .
Running one of these products? Scan your site to see whether it exposes a vulnerable version.
Scan my site free/x/docs/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.
Minimal local PoC, non-destructive:
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<-- "../docs_secret/leak.txt"\n',
extensions=["pymdownx.snippets"],
extension_configs={
"pymdownx.snippets": {
"base_path": [base],
"restrict_base_path": True,
"check_paths": True,
}
},
)
print(out) # -> <p>TOP_SECRET_FROM_SIBLING_DIR</p>
finally:
shutil.rmtree(work)
Default restrict_base_path: True is sufficient — no non-default option is required.
Minimal change — require the separator after the base prefix:
- 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.
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<-- "../nested_sibling_evil/leak.txt" raises SnippetMissingError.
AssertionError: SnippetMissingError not raised, sibling file is silently read).Full suite: python -m pytest tests/ -q → 738 passed (737 baseline + 1 new regression test). No regressions.
>= 10.0.1, <= 10.21.2