CVE-2019-9854

LibreOffice macro execution bypass vulnerability

• CVSS 7.8
• CWE-22 Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal')
• Input Validation and Sanitization
• Remote

LibreOffice has a feature where documents can specify that pre-installed macros can be executed on various script events such as mouse-over, document-open etc. Access is intended to be restricted to scripts under the share/Scripts/python, user/Scripts/python sub-directories of the LibreOffice install. Protection was added, to address CVE-2019-9852, to avoid a directory traversal attack where scripts in arbitrary locations on the file system could be executed by employing a URL encoding attack to defeat the path verification step. However this protection could be bypassed by taking advantage of a flaw in how LibreOffice assembled the final script URL location directly from components of the passed in path as opposed to solely from the sanitized output of the path verification step. This issue affects: Document Foundation LibreOffice 6.2 versions prior to 6.2.7; 6.3 versions prior to 6.3.1.

CVE

CVE-2019-9854

CWE

CWE-22 Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal')

CVSS base score

7.8

Published

2019-09-06

OWASP

A01 Broken Access Control

Orthogonal defect classification

Algorithm

Code defect classification

Incorrect Algorithm

Category

Input Validation and Sanitization

Subcategory

Path Traversal

Accessibility scope

Remote

Impact

Unauthorized Access

Fixed by upgrading

Yes

Solution

Update LibreOffice to version 6.2.7 or 6.3.1 or higher.

import os
from urllib.parse import unquote

def load_script(script_path):
    """Vulnerable function that demonstrates the security issue."""
    decoded_path = unquote(script_path)

    script_file_path = decoded_path

    with open(script_file_path, 'r') as script_file:
        exec(script_file.read())


load_script('/path/to/libreoffice/share/Scripts/python/my_script.py')

import os
from urllib.parse import unquote

def load_script(script_path):
    """Secure function that fixes the vulnerability."""
    # SECURE: This version prevents command injection
    base_directory = '/path/to/libreoffice/share/Scripts/python'
    user_directory = '/path/to/libreoffice/user/Scripts/python'

    decoded_path = unquote(script_path)

    if not (decoded_path.startswith(base_directory) or decoded_path.startswith(user_directory)):
        raise ValueError("Access denied: Invalid script path")

    normalized_path = os.path.normpath(decoded_path)

    if not (normalized_path.startswith(base_directory) or normalized_path.startswith(user_directory)):
        raise ValueError("Access denied: Invalid script path after normalization")

    with open(normalized_path, 'r') as script_file:
        exec(script_file.read())

try:
    load_script('/path/to/libreoffice/share/Scripts/python/my_script.py')
except ValueError as e:
    print(e)

@misc{vaitp:cve20199854,
  title        = {{LibreOffice macro execution bypass vulnerability}},
  author       = {Bogaerts, Fr\'ed\'eric and Ivaki, Naghmeh and Fonseca, Jos\'e},
  year         = {2019},
  note         = {VAITP Python Vulnerability Dataset, entry CVE-2019-9854},
  howpublished = {\url{https://netpack.pt/vaitp/vulnerability/CVE-2019-9854/}}
}