The b64 extension of PyMdown Extensions (up to version 10.21.3) improperly limits pathname resolution when inlining images referenced by <img src="..."> tags. It joins the src attribute onto a configured base_path using os.path.normpath but does not verify that the resulting path remains within base_path. This allows path traversal via '../' sequences or absolute paths to read files outside the intended directory, provided the files have allowed image extensions (.png, .jpg, .jpeg, .gif, .svg). The contents of these files are base64-encoded into the rendered Markdown output, leading to disclosure of arbitrary file contents accessible by the process. This vulnerability is tracked as CVE-2026-61632 and is classified as CWE-22 (Improper Limitation of a Pathname to a Restricted Directory).

An attacker who can supply or control Markdown content rendered with pymdownx.b64 enabled can read arbitrary files with certain image extensions accessible to the process outside the intended base directory. This results in information disclosure limited to readable image-extension files. There is no impact on integrity or availability. The CVSS 3.1 base score is 5.3 (medium severity), reflecting network attack vector, no privileges required, no user interaction, and limited confidentiality impact.

Upgrade PyMdown Extensions to version 11.0 or later, where this path traversal vulnerability in the b64 extension has been fixed. Until then, avoid enabling the b64 extension on untrusted Markdown input or restrict input sources to trusted users only. Patch status is not explicitly stated in the vendor advisory, but the fix is included starting with version 11.0.