CVE-2026-61632

Path traversal in PyMdown's b64 extension allows arbitrary file disclosure.

• CVSS 5.3
• 22
• Input Validation and Sanitization
• Remote

PyMdown Extensions is a set of extensions for the Python-Markdown markdown project. In versions up to and including 10.21.3, the b64 extension is vulnerable to a path traversal that discloses arbitrary files: it inlines images referenced by <img src="…"> by joining the src onto the configured base_path with os.path.normpath and opening the result directly, without verifying that the resolved path stays inside base_path. As a result, an src containing ../ sequences or an absolute path reads a file outside base_path as long as it has an allowed image extension (.png, .jpg, .jpeg, .gif, .svg), and the file's contents are then base64-encoded into the rendered output, disclosing them. An application that renders untrusted Markdown with pymdownx.b64 enabled can therefore leak the contents of image-extension files readable by the process to whoever controls the Markdown or views the output, a targeted file-read bounded by the extension check. This issue has been fixed in version 11.0.

CVE

CVE-2026-61632

CWE

22

CVSS base score

5.3

Published

2026-08-06

OWASP

A01 Broken Access Control

Orthogonal defect classification

Checking

Code defect classification

Missing Check

Category

Input Validation and Sanitization

Subcategory

Path Traversal

Accessibility scope

Remote

Impact

Information Disclosure

Affected component

PyMdown Exte

Fixed by upgrading

Yes

Solution

Upgrade PyMdown Extensions to version 11.0 or later.

import os
import base64
from xml.etree import ElementTree

class B64Extension:
    def __init__(self, base_path='.', allowed_exts=None):
        self.base_path = os.path.abspath(base_path)
        if allowed_exts is None:
            self.allowed_exts = {'.png', '.jpg', '.jpeg', '.gif', '.svg'}
        else:
            self.allowed_exts = allowed_exts

    def run(self, root: ElementTree.Element):
        """Iterate through the `root` and inline images."""
        for el in root.iter():
            if el.tag == 'img':
                src = el.get('src')
                if not src or src.startswith('data:'):
                    continue

                ext = os.path.splitext(src)[1].lower()
                if ext not in self.allowed_exts:
                    continue

                # VULNERABLE: The path is normalized but not validated to be within the base_path, allowing directory traversal.
                image_path = os.path.normpath(os.path.join(self.base_path, src))

                try:
                    with open(image_path, "rb") as f:
                        encoded = base64.b64encode(f.read()).decode('ascii')
                        el.set('src', f"data:image/{ext[1:]};base64,{encoded}")
                except OSError:
                    # Could not open the file, leave the src as is.
                    pass

import os
import base64
from xml.etree import ElementTree

class B64Extension:
    def __init__(self, base_path='.', allowed_exts=None):
        self.base_path = os.path.abspath(base_path)
        if allowed_exts is None:
            self.allowed_exts = {'.png', '.jpg', '.jpeg', '.gif', '.svg'}
        else:
            self.allowed_exts = allowed_exts

    def run(self, root: ElementTree.Element):
        """Iterate through the `root` and inline images."""
        for el in root.iter():
            if el.tag == 'img':
                src = el.get('src')
                if not src or src.startswith('data:'):
                    continue

                ext = os.path.splitext(src)[1].lower()
                if ext not in self.allowed_exts:
                    continue

                image_path = os.path.normpath(os.path.join(self.base_path, src))
                
                # FIX: Ensure the resolved path is within the configured base path directory.
                if not os.path.abspath(image_path).startswith(self.base_path + os.sep):
                    continue

                try:
                    with open(image_path, "rb") as f:
                        encoded = base64.b64encode(f.read()).decode('ascii')
                        el.set('src', f"data:image/{ext[1:]};base64,{encoded}")
                except OSError:
                    # Could not open the file, leave the src as is.
                    pass

<img src="../../../../../../../etc/secrets.svg">

@misc{vaitp:cve202661632,
  title        = {{Path traversal in PyMdown's b64 extension allows arbitrary file disclosure.}},
  author       = {Bogaerts, Fr\'ed\'eric and Ivaki, Naghmeh and Fonseca, Jos\'e},
  year         = {2026},
  note         = {VAITP Python Vulnerability Dataset, entry CVE-2026-61632},
  howpublished = {\url{https://netpack.pt/vaitp/vulnerability/CVE-2026-61632/}}
}