CVE-2023-40060: SolarWinds Serv-U Auth Bypass Vulnerability

CVE-2023-40060 Overview

A vulnerability has been identified within SolarWinds Serv-U 15.4 and 15.4 Hotfix 1 that, if exploited, allows an actor to bypass multi-factor/two-factor authentication. The actor must have administrator-level access to Serv-U to perform this action. SolarWinds found that the issue was not completely fixed in 15.4 Hotfix 1, requiring an additional security update.

Critical Impact

Attackers with administrative access can bypass MFA protections, potentially compromising the security of the entire Serv-U file transfer infrastructure and any sensitive data managed through the platform.

Affected Products

• SolarWinds Serv-U 15.4.0
• SolarWinds Serv-U 15.4.0 Hotfix 1
• SolarWinds Serv-U deployments with MFA/2FA enabled

Discovery Timeline

• 2023-09-07 - CVE-2023-40060 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2023-40060

Vulnerability Analysis

This vulnerability is classified under CWE-284 (Improper Access Control), indicating a fundamental flaw in how the Serv-U application enforces authentication controls. The vulnerability specifically affects the multi-factor authentication mechanism within the administrative interface.

An authenticated administrator can exploit this flaw to bypass the secondary authentication factor that would normally be required for sensitive operations. This represents a significant security control failure, as MFA is specifically designed to provide defense-in-depth against compromised credentials.

The network-accessible nature of this vulnerability means that remote administrators with valid credentials can exploit this bypass without requiring physical access to the server. While the prerequisite of administrator-level access limits the attack surface, the impact is severe once this condition is met.

Root Cause

The root cause lies in improper access control within the Serv-U authentication subsystem. The application fails to properly enforce multi-factor authentication requirements under certain conditions, allowing administrators to perform actions that should require additional verification. SolarWinds acknowledged that the initial fix in Hotfix 1 did not fully address the vulnerability, indicating complexity in the authentication flow that permitted bypass conditions.

Attack Vector

The attack requires network access and valid administrator credentials to the Serv-U management interface. Once authenticated with the first factor (username/password), an attacker can exploit the vulnerability to bypass the second authentication factor. This could be leveraged in scenarios where administrator credentials have been compromised through phishing, credential stuffing, or other means—precisely the situation MFA is designed to protect against.

The exploitation does not require user interaction once the attacker has administrative credentials. The bypass allows full administrative access without completing the MFA challenge, enabling the attacker to modify configurations, access transferred files, create backdoor accounts, or exfiltrate sensitive data.

Detection Methods for CVE-2023-40060

Indicators of Compromise

• Successful administrative logins without corresponding MFA challenge completion in audit logs
• Administrative actions performed from unusual IP addresses or geographic locations
• Anomalous administrator session patterns, such as multiple simultaneous sessions
• Configuration changes to user accounts, permissions, or security settings without proper authorization workflow

Detection Strategies

• Monitor Serv-U audit logs for administrative login events and correlate with MFA verification records
• Implement alerting for administrative logins from IP addresses not on the approved list
• Deploy network monitoring to detect unusual traffic patterns to the Serv-U management interface
• Review administrative action logs for unexpected privilege changes or new account creation

Monitoring Recommendations

• Enable comprehensive logging for all administrative authentication events in Serv-U
• Configure SIEM rules to alert on administrative logins without MFA verification
• Implement user behavior analytics to detect anomalous administrator activities
• Regularly audit administrator accounts and their recent activities for signs of compromise

How to Mitigate CVE-2023-40060

Immediate Actions Required

• Upgrade SolarWinds Serv-U to version 15.4.0 Hotfix 2 or later immediately
• Audit all administrative accounts for unauthorized access or configuration changes
• Review recent administrative session logs for evidence of MFA bypass exploitation
• Rotate administrative credentials as a precautionary measure

Patch Information

SolarWinds has released Serv-U 15.4.0 Hotfix 2 to address this vulnerability. The patch can be obtained from the SolarWinds Support Center. Organizations should prioritize this update given the authentication bypass nature of the vulnerability. Additional details are available in the SolarWinds Security Advisory.

Workarounds

• Restrict network access to the Serv-U administrative interface using firewall rules or VPN requirements
• Implement network segmentation to limit access to the management interface from trusted administrative networks only
• Enable additional logging and monitoring on administrative sessions until patching is complete
• Consider temporarily disabling remote administrative access if business operations permit