A critical security vulnerability, identified as CVE-2023-3909, has been discovered in GitLab CE/EE, affecting a range of GitLab versions across multiple releases. This post will provide an in-depth look at the vulnerability, its potential impact, and the necessary steps to mitigate its effects and prevent future exploitation.

Vulnerability Description

The issue lies within the improper handling of timeout input in the gitlab-ci.yml file, which allows an attacker to trigger a Regular Expression Denial of Service (ReDoS) attack. By inserting a large string into the timeout input, an attacker can cause the application to consume excessive computing resources, ultimately leading to a decrease in performance or a complete denial of service.

Here's an example of what a malicious gitlab-ci.yml file might look like

  # Injecting a large string as timeout input
  minutes: "10000000000000000000000000000000000000000000000000... (continues)"

For more information on CVE-2023-3909, you can refer to the following original sources

1. GitLab Security Advisory
2. NVD - CVE-2023-3909

Exploit Details

A successful ReDoS attack can cause severe performance issues or denial of service for the affected GitLab instance. An attacker can exploit this vulnerability by creating or updating a gitlab-ci.yml file with a large string in the timeout input, causing the application to spend excessive time processing the input and potentially making it unresponsive to incoming requests.

Additionally, this vulnerability can be exploited by any user who has permissions to create or modify a GitLab project, making it especially critical for GitLab instances hosting multiple projects with numerous users.

Mitigation Steps

To address this vulnerability, GitLab has released patches for the affected versions. It is highly recommended to upgrade your GitLab instance to the appropriate patched version:

For GitLab 16.5, upgrade to 16.5.1

Please refer to the GitLab Upgrade Documentation for detailed instructions on how to perform the upgrade process.


In conclusion, CVE-2023-3909 is a critical vulnerability due to its potential impact on GitLab server performance and the ease with which it can be exploited by users with project access. It is crucial to apply the provided patches to your GitLab instance and follow best practices for configuring user permissions to minimize the risk of exploitation. By doing so, you can protect your GitLab instance from potential denial of service attacks and maintain the integrity and performance of your server.


Published on: 11/06/2023 13:15:09 UTC
Last modified on: 11/14/2023 18:01:26 UTC