CVE-2023-3900: GitLab CE/EE DoS Vulnerability

CVE-2023-3900 Overview

A Denial of Service vulnerability has been discovered in GitLab CE/EE affecting all versions starting from 16.1 before 16.1.3 and all versions starting from 16.2 before 16.2.2. The vulnerability allows an attacker to cause a Denial of Service condition by providing an invalid start_sha value on the merge requests page, which prevents the Changes tab from loading.

Critical Impact

This vulnerability can disrupt critical development workflows by preventing access to merge request changes, potentially impacting team productivity and code review processes across affected GitLab instances.

Affected Products

• GitLab Community Edition (CE) versions 16.1.0 to 16.1.2
• GitLab Enterprise Edition (EE) versions 16.1.0 to 16.1.2
• GitLab Community Edition (CE) and Enterprise Edition (EE) versions 16.2.0 to 16.2.1

Discovery Timeline

• 2023-08-02 - CVE-2023-3900 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2023-3900

Vulnerability Analysis

This vulnerability is classified under CWE-1287 (Improper Validation of Specified Type of Input). The core issue lies in GitLab's improper handling of the start_sha parameter when processing merge request pages. When a user or attacker supplies an invalid start_sha value, the application fails to properly validate this input before attempting to use it in subsequent operations.

The merge request functionality in GitLab relies on SHA values to identify specific commits for displaying changes between branches. When the start_sha parameter contains malformed or invalid data, the Changes tab fails to render, effectively denying users access to critical merge request information.

Root Cause

The root cause stems from insufficient input validation on the start_sha parameter within the merge request handling logic. GitLab does not adequately verify that the provided SHA value conforms to expected formats or references valid Git objects before processing. This lack of validation allows invalid values to propagate through the system, causing the Changes tab rendering to fail entirely rather than gracefully handling the error condition.

Attack Vector

The vulnerability is exploitable over the network without requiring authentication or user interaction. An attacker can craft requests to merge request pages with malicious start_sha parameter values. Since the attack vector is network-based with low complexity, it presents a relatively accessible target for attackers seeking to disrupt GitLab operations.

The attack does not compromise confidentiality or integrity but directly impacts availability by preventing legitimate users from accessing merge request change information. This can be particularly disruptive in environments with active development workflows dependent on merge request reviews.

For detailed technical information regarding this vulnerability, refer to the GitLab Issue Tracking and the HackerOne Report #2058514.

Detection Methods for CVE-2023-3900

Indicators of Compromise

• Unusual patterns of failed merge request page loads, specifically the Changes tab
• HTTP requests containing malformed or unusually long start_sha parameter values
• Increased error rates in GitLab application logs related to merge request processing
• User reports of inability to view Changes tab on specific merge requests

Detection Strategies

• Monitor GitLab application logs for errors related to start_sha validation failures
• Implement web application firewall (WAF) rules to detect anomalous start_sha parameter values
• Set up alerting for elevated rates of 500 errors on merge request endpoints
• Review access logs for repeated requests to merge request URLs with unusual query parameters

Monitoring Recommendations

• Enable verbose logging for the merge request controller to capture validation errors
• Configure SentinelOne Singularity XDR to monitor GitLab application behavior and alert on DoS patterns
• Establish baseline metrics for merge request page load success rates to detect anomalies
• Implement rate limiting on merge request endpoints to mitigate potential abuse

How to Mitigate CVE-2023-3900

Immediate Actions Required

• Upgrade GitLab CE/EE to version 16.1.3 or later for 16.1.x installations
• Upgrade GitLab CE/EE to version 16.2.2 or later for 16.2.x installations
• Review recent merge request access logs for potential exploitation attempts
• Implement network-level rate limiting on GitLab merge request endpoints

Patch Information

GitLab has addressed this vulnerability in version 16.1.3 for the 16.1.x branch and version 16.2.2 for the 16.2.x branch. Organizations should prioritize upgrading to these patched versions to eliminate the vulnerability. Detailed patch and upgrade information can be found in the GitLab Issue Tracking.

Workarounds

• Implement a reverse proxy or WAF rule to validate start_sha parameters before they reach GitLab
• Configure rate limiting on merge request page endpoints to reduce DoS impact
• Monitor and block suspicious request patterns targeting merge request URLs
• Consider restricting merge request access to authenticated users only while awaiting patching

# Example nginx rate limiting configuration for GitLab merge requests
limit_req_zone $binary_remote_addr zone=gitlab_mr:10m rate=10r/s;

location ~ ^/.*/-/merge_requests/ {
    limit_req zone=gitlab_mr burst=20 nodelay;
    proxy_pass http://gitlab_upstream;
}