CVE-2020-15867: Gogs Git Hook RCE Vulnerability

CVE-2020-15867 Overview

CVE-2020-15867 is a remote code execution vulnerability affecting Gogs, a self-hosted Git service. The git hook feature in Gogs versions 0.5.5 through 0.12.2 allows authenticated users to execute arbitrary code on the server. This vulnerability presents a significant privilege escalation risk when access to the hook feature is granted to users who do not have administrative privileges. Notably, while this functionality is documented, it is not clearly warned about in the user interface, which the CVE notes could be considered a "Product UI does not Warn User of Unsafe Actions" issue.

Critical Impact

Authenticated attackers can achieve remote code execution on Gogs servers, potentially leading to complete system compromise, data theft, and lateral movement within the network.

Affected Products

• Gogs versions 0.5.5 through 0.12.2
• Self-hosted Gogs Git service installations
• Environments where non-administrative users have access to git hook features

Discovery Timeline

• 2020-10-16 - CVE CVE-2020-15867 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2020-15867

Vulnerability Analysis

This vulnerability exists in the git hook feature of Gogs, which allows users to configure server-side scripts that execute during git operations such as pre-receive, post-receive, and update hooks. The core issue is that this powerful functionality can be assigned to non-administrative users without adequate warning or access controls.

Git hooks are shell scripts that run on the server during various git events. When a user with access to this feature configures a malicious hook, they can execute arbitrary commands with the privileges of the Gogs server process. This effectively transforms a standard user account into an attack vector for complete server compromise.

The vulnerability is particularly dangerous because the user interface does not adequately warn administrators about the security implications of granting hook access to non-admin users. While the documentation mentions this capability, the lack of UI warnings creates an operational security gap.

Root Cause

The root cause of this vulnerability is insufficient access control and warning mechanisms around the git hook feature. Gogs allows administrators to grant hook access to regular users, but fails to:

1. Adequately restrict which commands can be executed within hooks
2. Warn administrators in the UI about the security implications of granting hook access
3. Implement sandboxing or command filtering for hook execution

This design flaw means that any authenticated user with hook access can leverage this feature to execute arbitrary system commands.

Attack Vector

The attack vector for CVE-2020-15867 involves an authenticated user with access to the git hook feature. The attacker can exploit this vulnerability through the following steps:

1. Authenticate to the Gogs instance with a user account that has git hook permissions
2. Navigate to a repository where they have hook configuration access
3. Create or modify a git hook (such as post-receive) to include malicious shell commands
4. Trigger the hook execution by performing the corresponding git operation (e.g., pushing code to the repository)
5. The malicious commands execute with the privileges of the Gogs server process

The vulnerability requires authentication but does not require administrative privileges if hook access has been inappropriately delegated. For detailed technical analysis, see the FZI Security Vulnerability Analysis and the Packet Storm RCE Exploit documentation.

Detection Methods for CVE-2020-15867

Indicators of Compromise

• Unexpected modifications to git hook files in repository directories (typically located in hooks/ subdirectories)
• Unusual process spawning from the Gogs server process
• Suspicious network connections originating from the Gogs server
• Unauthorized file system access or modifications in server directories
• Anomalous git operations followed by unexpected system activity

Detection Strategies

• Monitor git hook file modifications across all repositories for unauthorized changes
• Implement file integrity monitoring on the Gogs installation directory and repository hooks
• Review Gogs access logs for users accessing hook configuration endpoints
• Deploy endpoint detection to identify suspicious process chains originating from the Gogs process
• Audit user permissions to identify non-admin accounts with hook access

Monitoring Recommendations

• Enable verbose logging in Gogs to capture hook-related activities
• Set up alerts for any modifications to git hook scripts in production repositories
• Monitor for unusual command execution patterns from the Gogs service account
• Implement network monitoring to detect unexpected outbound connections from the Gogs server
• Regularly review user permission assignments, especially hook access delegation

How to Mitigate CVE-2020-15867

Immediate Actions Required

• Audit all Gogs user accounts and revoke git hook access from non-administrative users
• Review existing git hooks across all repositories for malicious or unauthorized content
• Update Gogs to the latest available version beyond 0.12.2
• Implement network segmentation to limit the impact of potential server compromise
• Consider temporarily disabling the git hook feature if not operationally required

Patch Information

Administrators should upgrade Gogs to a version newer than 0.12.2 that addresses this vulnerability. Check the official Gogs releases for security patches. Given the severity of this vulnerability, immediate patching is recommended. Organizations should also consider migrating to alternative solutions if timely patches are not available.

Workarounds

• Restrict git hook permissions to only trusted administrative users
• Implement additional access controls at the operating system level to limit what commands hooks can execute
• Run Gogs with minimal system privileges using a dedicated service account
• Use containerization or sandboxing to isolate the Gogs instance from critical infrastructure
• Deploy application-level firewalls to restrict outbound connections from the Gogs server

# Configuration example - Restrict hook access at the OS level
# Create a dedicated low-privilege user for Gogs
useradd -r -s /sbin/nologin gogs-service

# Set restrictive permissions on the hooks directory
chmod 750 /path/to/gogs/repositories/*/hooks
chown root:gogs-service /path/to/gogs/repositories/*/hooks

# Consider using SELinux or AppArmor to further restrict Gogs process capabilities