CVE-2023-30861 Explained : Impact and Mitigation

This article provides insights into CVE-2023-30861, a vulnerability in Flask that could lead to the disclosure of the permanent session cookie due to a missing Vary: Cookie header.

Understanding CVE-2023-30861

Flask is a lightweight WSGI web application framework. This vulnerability arises when specific conditions are met, allowing a response intended for one client to be cached and shared with other clients, potentially exposing session cookies.

What is CVE-2023-30861?

Flask, when hosted behind a caching proxy that does not handle cookies properly, may disclose session cookies to unintended clients. The severity of this issue depends on various factors related to session handling within the application and proxy behavior.

The Impact of CVE-2023-30861

The risk associated with CVE-2023-30861 relies on the interplay of multiple conditions, including the application's use of sessions, the proxy's cookie caching behavior, and the version of Flask being utilized.

Technical Details of CVE-2023-30861

This section delves into the specifics of the vulnerability, affected systems, versions, and the mechanism through which exploitation can occur.

Vulnerability Description

The vulnerability in Flask arises from the failure to include the

Vary: Cookie

header when refreshing sessions without accessing or modifying them. This omission can lead to the inadvertent sharing of session cookies among clients.

Affected Systems and Versions

Flask versions prior to 2.2.5 and between 2.3.0 to 2.3.2 are susceptible to this vulnerability, potentially exposing sensitive session information due to incorrect handling of cookies.

Exploitation Mechanism

Exploiting CVE-2023-30861 requires specific conditions to be met: hosting behind a caching proxy that retains cookies, setting

session.permanent = True

, and not accessing or modifying the session during requests.

Mitigation and Prevention

Outlined below are steps to mitigate the risks associated with CVE-2023-30861 and prevent potential exploitation.

Immediate Steps to Take

Users of Flask should immediately update to versions 2.2.5 or 2.3.2 to address the vulnerability and ensure the proper handling of session cookies to prevent unauthorized access.

Long-Term Security Practices

Implement best practices for session management, including regular security audits, monitoring, and staying informed about security advisories related to Flask and its dependencies.

Patching and Updates

Regularly check for security updates and patches released by Flask maintainers. Stay proactive in applying these updates to maintain a secure web application environment.