CVE-2026-5173: GitLab Auth Bypass Vulnerability

CVE-2026-5173 Overview

GitLab has remediated a significant access control vulnerability in GitLab CE/EE affecting all versions from 16.9.6 before 18.8.9, 18.9 before 18.9.5, and 18.10 before 18.10.3. This vulnerability could allow an authenticated user to invoke unintended server-side methods through websocket connections due to improper access control mechanisms.

Critical Impact

Authenticated attackers can bypass access controls to invoke unauthorized server-side methods via websocket connections, potentially leading to unauthorized data access and system manipulation.

Affected Products

• GitLab CE/EE versions from 16.9.6 before 18.8.9
• GitLab CE/EE versions 18.9 before 18.9.5
• GitLab CE/EE versions 18.10 before 18.10.3

Discovery Timeline

• 2026-04-08 - CVE CVE-2026-5173 published to NVD
• 2026-04-08 - Last updated in NVD database

Technical Details for CVE-2026-5173

Vulnerability Analysis

This vulnerability is classified as CWE-749 (Exposed Dangerous Method or Function), which occurs when an application exposes methods or functions that can be accessed by unauthorized users or in unintended ways. In the context of GitLab, the websocket implementation fails to properly validate which server-side methods an authenticated user is authorized to invoke.

The improper access control allows authenticated users to call server-side methods they should not have access to. This can lead to unauthorized information disclosure and potential system manipulation, as attackers with valid but limited credentials can escalate their capabilities within the GitLab environment.

Root Cause

The root cause of this vulnerability lies in insufficient authorization checks within GitLab's websocket connection handling. When websocket connections are established, the application fails to properly verify whether the authenticated user has the necessary permissions to invoke specific server-side methods. This gap in access control validation enables users to execute methods beyond their authorized scope.

Attack Vector

The attack is network-based and requires the attacker to have valid authentication credentials for the GitLab instance. Once authenticated, the attacker can establish a websocket connection and craft requests to invoke server-side methods that would normally be restricted. The attack does not require user interaction and can be executed with low complexity, making it a significant threat to GitLab deployments.

The websocket protocol provides a persistent connection between the client and server, which the attacker can leverage to send crafted method invocation requests. Without proper access control checks, these requests are processed by the server, allowing unauthorized actions to be performed.

Detection Methods for CVE-2026-5173

Indicators of Compromise

• Unusual websocket connection patterns from authenticated users attempting to invoke methods outside their permission scope
• Anomalous server-side method calls that do not match expected user behavior or role permissions
• Increased websocket traffic from specific user accounts that may indicate reconnaissance or exploitation attempts

Detection Strategies

• Monitor websocket connection logs for method invocation attempts that do not align with user roles and permissions
• Implement application-level logging to track all server-side method calls made through websocket connections
• Deploy anomaly detection rules to identify users invoking methods inconsistent with their typical usage patterns
• Review GitLab audit logs for unauthorized access attempts or suspicious activity patterns

Monitoring Recommendations

• Enable comprehensive audit logging for all websocket-based interactions within GitLab
• Configure alerts for method invocations from users who should not have access to specific functionality
• Regularly review access control configurations to ensure proper permission boundaries are enforced

How to Mitigate CVE-2026-5173

Immediate Actions Required

• Upgrade GitLab CE/EE to version 18.8.9, 18.9.5, or 18.10.3 or later immediately
• Review user permissions and access controls to ensure least privilege principles are enforced
• Audit websocket connection logs for any signs of exploitation prior to patching
• Consider temporarily restricting websocket functionality if immediate patching is not possible

Patch Information

GitLab has released security patches addressing this vulnerability. Organizations should upgrade to the following fixed versions:

• GitLab 18.8.9 or later for versions in the 18.8.x branch
• GitLab 18.9.5 or later for versions in the 18.9.x branch
• GitLab 18.10.3 or later for versions in the 18.10.x branch

For detailed patch information, refer to the GitLab Patch Release Announcement and the GitLab Work Item #588959.

Workarounds

• Implement network-level controls to restrict websocket connections to trusted sources only
• Review and tighten user role assignments to minimize the potential impact of exploitation
• Deploy web application firewall (WAF) rules to monitor and filter suspicious websocket traffic
• Consider implementing additional authentication layers for sensitive server-side method invocations