CVE-2023-5933: GitLab Auth Bypass Vulnerability

CVE-2023-5933 Overview

An improper input sanitization vulnerability has been discovered in GitLab CE/EE affecting all versions after 13.7 before 16.6.6, 16.7 prior to 16.7.4, and 16.8 prior to 16.8.1. The vulnerability stems from inadequate sanitization of user name input, which allows authenticated attackers to perform arbitrary API PUT requests. This flaw is classified as a Cross-Site Scripting (XSS) vulnerability (CWE-79, CWE-80) that could enable attackers to execute malicious scripts in the context of other users' sessions.

Critical Impact

Authenticated attackers can exploit improper username sanitization to execute arbitrary API PUT requests, potentially modifying GitLab resources and compromising user session integrity through cross-site scripting attacks.

Affected Products

• GitLab Community Edition (CE) versions 13.7 through 16.6.5
• GitLab Enterprise Edition (EE) versions 13.7 through 16.6.5
• GitLab CE/EE versions 16.7.0 through 16.7.3
• GitLab CE/EE version 16.8.0

Discovery Timeline

• 2024-01-25 - GitLab releases security patch in version 16.8.1
• 2024-01-26 - CVE-2023-5933 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2023-5933

Vulnerability Analysis

This vulnerability exploits a flaw in GitLab's input validation mechanism for user names. When user-supplied name data is processed by the application, insufficient sanitization allows specially crafted input to bypass security controls. The attack requires network access and user interaction, but can be executed by authenticated users with low privileges. The scope is changed, meaning the vulnerable component impacts resources beyond its security scope, affecting both confidentiality and integrity of user data and sessions.

The vulnerability enables attackers to craft malicious payloads within username fields that, when rendered by the application, can execute arbitrary JavaScript in victims' browsers or trigger unauthorized API PUT requests. This represents a stored XSS attack vector where the malicious payload persists in the GitLab database.

Root Cause

The root cause of CVE-2023-5933 lies in improper input sanitization within GitLab's username handling logic. The application fails to adequately validate and sanitize special characters and script content within user name fields before storing or rendering them. This allows attackers to inject malicious HTML or JavaScript content that bypasses output encoding mechanisms, resulting in both basic XSS (CWE-80) and improper neutralization of input during web page generation (CWE-79).

Attack Vector

The attack leverages network-based access requiring authentication with low-level privileges. An attacker creates or modifies their GitLab username to include malicious content such as JavaScript code or API request triggers. When other users view pages containing the attacker's username—such as merge requests, issues, comments, or user profiles—the unsanitized content executes in their browser context. This can result in session hijacking, unauthorized API calls, or data exfiltration.

The vulnerability requires user interaction, meaning a victim must view content containing the malicious username for the attack to succeed. However, given GitLab's collaborative nature where usernames appear frequently throughout the interface, exploitation opportunities are abundant.

Detection Methods for CVE-2023-5933

Indicators of Compromise

• Unusual characters, HTML tags, or JavaScript code in GitLab usernames or display names
• Unexpected API PUT requests originating from user sessions without corresponding user activity
• Browser console errors or script execution warnings when viewing GitLab pages
• Audit log entries showing API modifications that users did not intentionally perform

Detection Strategies

• Monitor GitLab audit logs for unusual API PUT request patterns or modifications to resources by unexpected users
• Implement Content Security Policy (CSP) headers to detect and block inline script execution attempts
• Review user database entries for usernames containing suspicious patterns such as <script>, javascript:, or encoded variants
• Deploy web application firewall (WAF) rules to detect XSS payloads in user input fields

Monitoring Recommendations

• Enable GitLab's comprehensive audit logging and regularly review for anomalous activity
• Configure alerting for API requests that modify sensitive resources outside normal business hours or from unusual IP addresses
• Implement browser-based XSS detection using security headers and monitor CSP violation reports
• Periodically scan the GitLab database for stored XSS payloads in user-controlled fields

How to Mitigate CVE-2023-5933

Immediate Actions Required

• Upgrade GitLab CE/EE to version 16.8.1, 16.7.4, or 16.6.6 immediately to remediate the vulnerability
• Audit existing usernames in the GitLab database for suspicious content and sanitize or remove malicious entries
• Review GitLab audit logs for evidence of exploitation or unauthorized API modifications
• Implement additional input validation at the web application firewall level as a defense-in-depth measure

Patch Information

GitLab has released security patches addressing this vulnerability in version 16.8.1 (released January 25, 2024). Organizations should upgrade to one of the following patched versions based on their current deployment:

• Version 16.8.1 or later for systems on the 16.8 branch
• Version 16.7.4 or later for systems on the 16.7 branch
• Version 16.6.6 or later for systems on the 16.6 branch or earlier

For detailed patch information, refer to the GitLab Security Release. Additional technical details are available in GitLab Issue #430236 and the HackerOne Report #2225710.

Workarounds

• Implement strict Content Security Policy (CSP) headers to prevent inline script execution as a temporary mitigation
• Deploy web application firewall rules to filter and block common XSS payloads in user input
• Restrict user registration and profile modification capabilities until patching is complete
• Monitor and manually review user name changes through administrative controls

# Example: Configure CSP headers in GitLab's nginx configuration
# Add to /etc/gitlab/gitlab.rb before running gitlab-ctl reconfigure
nginx['custom_gitlab_server_config'] = "add_header Content-Security-Policy \"default-src 'self'; script-src 'self' 'unsafe-inline' 'unsafe-eval'; style-src 'self' 'unsafe-inline';\";"

# Apply configuration
sudo gitlab-ctl reconfigure