CVE-2019-9855

LibreOffice Windows 8.3 Path Equivalence Vulnerability

• CVSS 9.8
• CWE-417
• Input Validation and Sanitization
• Remote

LibreOffice is typically bundled with LibreLogo, a programmable turtle vector graphics script, which can execute arbitrary python commands contained with the document it is launched from. LibreOffice also has a feature where documents can specify that pre-installed scripts can be executed on various document script events such as mouse-over, etc. Protection was added to block calling LibreLogo from script event handers. However a Windows 8.3 path equivalence handling flaw left LibreOffice vulnerable under Windows that a document could trigger executing LibreLogo via a Windows filename pseudonym. This issue affects: Document Foundation LibreOffice 6.2 versions prior to 6.2.7; 6.3 versions prior to 6.3.1.

CVE

CVE-2019-9855

CWE

CWE-417

CVSS base score

9.8

Published

2019-09-06

OWASP

A08 Software and Data Integrity Failures

Orthogonal defect classification

Function

Code defect classification

Incorrect Functionality

Category

Input Validation and Sanitization

Subcategory

Path Traversal

Accessibility scope

Remote

Impact

Arbitrary Code Execution

Fixed by upgrading

Yes

Solution

Update LibreOffice to version 6.2.7 or 6.3.1 or higher.

import os

def execute_script(script_name):
    """Vulnerable function that demonstrates the security issue."""
    
    script_path = f"C:\\Program Files\\LibreOffice\\share\\librelogo\\{script_name}.py"
    
    exec(open(script_path).read())

script = "C:\\Program Files\\LibreOffice\\share\\librelogo\\..\\..\\..\\..\\..\\..\\Windows\\System32\\script.py"

execute_script(script)

import os

def check_path(base_path, user_input):
    """Secure function that fixes the vulnerability."""
    user_input_abs = os.path.abspath(user_input)
    base_path_abs = os.path.abspath(base_path)
    
    return os.path.commonpath([base_path_abs]) == os.path.commonpath([base_path_abs, user_input_abs])

def execute_script(script_path):
    """Secure function that fixes the vulnerability."""
    base_path = '/path/to/scripts'
    if check_path(base_path, script_path):
        exec(open(script_path).read())
    else:
        raise ValueError("Attempted to execute a script outside of the safe directory.")

try:
    execute_script('user_input_script.py')
except Exception as e:
    print(f"Error: {e}")

@misc{vaitp:cve20199855,
  title        = {{LibreOffice Windows 8.3 Path Equivalence Vulnerability}},
  author       = {Bogaerts, Fr\'ed\'eric and Ivaki, Naghmeh and Fonseca, Jos\'e},
  year         = {2019},
  note         = {VAITP Python Vulnerability Dataset, entry CVE-2019-9855},
  howpublished = {\url{https://netpack.pt/vaitp/vulnerability/CVE-2019-9855/}}
}