CVE-2023-5356: GitLab Auth Bypass Vulnerability

CVE-2023-5356 Overview

CVE-2023-5356 is an authorization bypass vulnerability affecting GitLab Community Edition (CE) and Enterprise Edition (EE). The flaw stems from incorrect authorization checks in the Slack and Mattermost integration features, allowing authenticated users to execute slash commands as another user. This vulnerability enables attackers to impersonate other users and perform unauthorized actions through these chat integrations.

Critical Impact

Authenticated attackers can abuse Slack/Mattermost integrations to execute slash commands as other users, potentially leading to unauthorized data access, privilege escalation, and impersonation attacks within GitLab environments.

Affected Products

• GitLab Community Edition (CE) versions 8.13 through 16.5.5
• GitLab Enterprise Edition (EE) versions 8.13 through 16.5.5
• GitLab CE/EE versions 16.6.0 through 16.6.3
• GitLab CE/EE versions 16.7.0 and 16.7.1

Discovery Timeline

• 2024-01-12 - CVE-2023-5356 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2023-5356

Vulnerability Analysis

This vulnerability is classified as CWE-863 (Incorrect Authorization), indicating a fundamental flaw in how GitLab validates user permissions when processing commands through third-party chat integrations. The issue resides in the authorization logic that handles Slack and Mattermost integration endpoints.

When a user initiates a slash command through these integrations, GitLab fails to properly verify that the requesting user is authorized to execute the command in the context of the target user. This broken access control allows an attacker to craft requests that execute slash commands with the privileges of another GitLab user, effectively bypassing the intended authorization model.

The vulnerability is exploitable over the network with low attack complexity, requiring only basic authentication. No user interaction is needed for exploitation, making this a particularly dangerous authorization bypass that could be automated at scale.

Root Cause

The root cause of CVE-2023-5356 lies in insufficient authorization validation within GitLab's Slack and Mattermost integration modules. The application fails to properly verify that the authenticated user initiating a slash command request is the same user whose context will be used for command execution. This authorization check gap allows command execution context manipulation, enabling authenticated attackers to impersonate other users within the GitLab platform.

Attack Vector

The attack vector for this vulnerability involves the network-accessible Slack and Mattermost integration endpoints in GitLab. An authenticated attacker with low privileges can exploit the authorization bypass by manipulating integration requests to execute slash commands in the context of another user.

The exploitation flow involves:

1. Attacker authenticates to GitLab with a low-privilege account
2. Attacker identifies a target user with higher privileges or access to sensitive projects
3. Attacker crafts malicious requests to the Slack/Mattermost integration endpoints
4. GitLab processes the request without proper authorization validation
5. Slash commands execute in the context of the victim user

This enables horizontal privilege escalation (accessing another user's resources) and potentially vertical privilege escalation if the target user has administrative privileges. The impact includes unauthorized access to confidential project data, repository manipulation, and potential CI/CD pipeline abuse.

Detection Methods for CVE-2023-5356

Indicators of Compromise

• Unusual slash command activity originating from Slack or Mattermost integrations
• Discrepancies between authenticated user sessions and slash command execution contexts
• Multiple slash commands executed by different user contexts from a single integration session
• Unexpected project or repository modifications attributed to users who did not initiate them

Detection Strategies

• Monitor GitLab audit logs for slash command executions through Slack/Mattermost integrations
• Implement correlation rules to detect user context mismatches in integration requests
• Review integration access patterns for anomalous behavior indicative of impersonation attacks
• Configure alerting for high-privilege actions executed through chat integrations

Monitoring Recommendations

• Enable verbose logging for Slack and Mattermost integration endpoints
• Implement user behavior analytics to detect abnormal command execution patterns
• Establish baseline metrics for integration usage and alert on significant deviations
• Regularly audit integration configurations and connected user accounts

How to Mitigate CVE-2023-5356

Immediate Actions Required

• Upgrade GitLab CE/EE to version 16.5.6, 16.6.4, or 16.7.2 or later immediately
• Review audit logs for any suspicious slash command activity prior to patching
• Temporarily disable Slack/Mattermost integrations if immediate patching is not possible
• Assess potential impact by identifying users with active integrations configured

Patch Information

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

• Version 16.5.6 for users on the 16.5.x branch
• Version 16.6.4 for users on the 16.6.x branch
• Version 16.7.2 for users on the 16.7.x branch

For detailed patch information and upgrade instructions, refer to the GitLab Issue Report and the HackerOne Report #2188868.

Workarounds

• Disable Slack and Mattermost integrations at the instance level until patching is complete
• Restrict integration configuration permissions to trusted administrators only
• Implement network-level controls to limit access to integration endpoints
• Use GitLab's IP allowlisting features to restrict integration access to known Slack/Mattermost IP ranges