A recent vulnerability has been identified in GitLab Community Edition (CE) and Enterprise Edition (EE) affecting all versions starting from 16.11 up to, but not including, 16.11.2. This vulnerability, identified as CVE-2023-6688, exposes GitLab servers to potential regular expression-based Denial of Service (ReDoS) attacks if the server has Google Chat Messages integration enabled. This blog post aims to provide a detailed description of the issue, code snippets to showcase the problem, links to the original references, and a detailed explanation of how the exploit works. Before diving into the exploit details, let's first understand the terminology used:

- GitLab CE/EE: GitLab is a web-based DevOps tool that provides a platform to manage repositories, build and deploy applications, and collaborate on issues and merge requests. CE stands for Community Edition and EE stands for Enterprise Edition.
- Google Chat Messages integration: Google Chat Messages integration is an add-on feature available in GitLab. It allows users to send notifications and updates related to their GitLab projects to specific Google Chat rooms.
- ReDoS: Regular expression Denial of Service (ReDoS) is a type of DoS attack that targets applications that use poorly optimized regular expressions for parsing strings provided by users.

Code Snippet

The vulnerable piece of code lies within the processing logic for Google Chat Messages integration. Below is a simplified example of the problematic code:

import re

def is_valid_user_input(user_input):
    regex_pattern = r'(.*\.\d{1,5}-*\d+)\s*\z' 
    match = re.match(regex_pattern, user_input)

    if not match:
        return False

    return True

def process_user_input(user_input):
    if not is_valid_user_input(user_input):
      raise ValueError("Invalid user input")

    # Other processing logic here

This code example demonstrates a simple function is_valid_user_input() used to check if a given user input is valid based on a regular expression pattern. However, this regular expression pattern is poorly optimized and poses a significant risk for ReDoS attacks. This means that an attacker can carefully craft a user input string to keep the regex engine busy for an extended period, ultimately causing a DoS situation on the GitLab server.

Exploit Details

To exploit this vulnerability, an attacker would first need to identify a target GitLab server running a vulnerable version (between 16.11 and 16.11.1) with Google Chat Messages integration enabled.

Next, the attacker would craft a malicious payload, such as a long string that would cause the regex engine to take an excessive amount of time to process the inputs. A potential payload is a string such as "a" repeated multiple times, followed by a ".", several digits, and then a hyphen. An example payload is as follows:

aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa.12345-

This payload will take a significant amount of time to process by the regex engine because of the inefficient regex pattern used in the is_valid_user_input() function. As a result, the GitLab server would experience severe slowdowns or potentially be rendered completely unusable for all users.

Original References

- GitLab Security Release: https://about.gitlab.com/releases/2023/04/01/gitlab-ce-ee-16_11_2-security-release/
- CVE Details: http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2023-6688

To mitigate this risk

1. Ensure that you are running GitLab CE/EE version 16.11.2 or later, as this version includes the relevant security fixes.
2. If you're using an affected version, disable Google Chat Messages integration until you can update to a secure version.

Regularly check for security updates and patches and apply them as soon as possible.

In the long term, developers, open source maintainers, and other users should be mindful of potential risks related to regex patterns and review their code for vulnerable usage regularly. Regular expression optimizations can greatly reduce the impact of ReDoS attacks and significantly improve the overall security posture of applications and platforms that rely on regex for input validation.

Timeline

Published on: 05/14/2024 14:35:33 UTC
Last modified on: 05/14/2024 16:13:02 UTC