CVE-2026-46338

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:

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.

Reproduction

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.

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<-- "../nested_sibling_evil/leak.txt" raises SnippetMissingError.

• Without fix: test fails (AssertionError: SnippetMissingError not raised, sibling file is silently read).
• With fix: test passes.

Full suite: python -m pytest tests/ -q → 738 passed (737 baseline + 1 new regression test). No regressions.

Affected versions

>= 10.0.1, <= 10.21.2

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.

Input manipulates file paths to reach files outside the intended directory, such as configuration or credential files. Typical impact: unauthorized file read or write outside the intended directory.

CVE-2026-46338 has a CVSS score of 4.3 (Medium). The vector is network-reachable, no privileges required, and user interaction required. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (10.21.3); upgrading removes the vulnerable code path.

Kodem intelligence

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.

1. What is CVE-2026-46338? CVE-2026-46338 is a medium-severity path traversal vulnerability in pymdown-extensions (pip), affecting versions >= 10.0.1, <= 10.21.2. It is fixed in 10.21.3. Input manipulates file paths to reach files outside the intended directory, such as configuration or credential files.
2. How severe is CVE-2026-46338? CVE-2026-46338 has a CVSS score of 4.3 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of pymdown-extensions are affected by CVE-2026-46338? pymdown-extensions (pip) versions >= 10.0.1, <= 10.21.2 is affected.
4. Is there a fix for CVE-2026-46338? Yes. CVE-2026-46338 is fixed in 10.21.3. Upgrade to this version or later.
5. Is CVE-2026-46338 exploitable, and should I be worried? Whether CVE-2026-46338 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-46338 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-46338? Upgrade pymdown-extensions to 10.21.3 or later.