CVE-2023-35179: SolarWinds Serv-U Auth Bypass Vulnerability

CVE-2023-35179 Overview

A vulnerability has been identified within SolarWinds Serv-U 15.4 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. This authentication bypass vulnerability (CWE-284: Improper Access Control) poses a significant risk to organizations relying on MFA as a critical security layer for their file transfer infrastructure.

Critical Impact

Attackers with administrator-level access can bypass multi-factor authentication controls, potentially gaining unauthorized access to sensitive file transfer operations and data while circumventing security policies designed to prevent account compromise.

Affected Products

• SolarWinds Serv-U 15.4.0
• SolarWinds Serv-U installations prior to Hotfix 1

Discovery Timeline

• August 11, 2023 - CVE-2023-35179 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2023-35179

Vulnerability Analysis

This vulnerability represents an authentication bypass flaw in SolarWinds Serv-U 15.4, specifically targeting the multi-factor authentication (MFA) mechanism. The improper access control weakness (CWE-284) allows privileged users with administrative access to circumvent the MFA verification process that would normally be required for authentication.

The vulnerability is network-accessible, requiring no user interaction for exploitation. While the prerequisite of administrator-level access limits the attack surface, successful exploitation can lead to complete compromise of confidentiality, integrity, and availability of the affected system.

Root Cause

The root cause lies in improper access control implementation within the Serv-U authentication workflow. The MFA verification layer can be bypassed when requests originate from accounts with administrative privileges, indicating a flaw in how the application enforces authentication controls across different privilege levels. This suggests that the MFA enforcement logic does not properly validate that all authentication factors have been completed before granting access, particularly for administrative sessions.

Attack Vector

The attack requires network access to the Serv-U application and existing administrator credentials. An attacker who has compromised or obtained administrator-level credentials can leverage this vulnerability to bypass MFA protections that would normally provide a second layer of defense. This attack vector is particularly concerning in scenarios where:

• Administrator credentials have been phished or stolen
• An insider threat with legitimate admin access wants to operate without MFA audit trails
• Attackers seek to maintain persistent access while avoiding MFA-based detection

The vulnerability enables attackers to access administrative functions without completing MFA challenges, effectively reducing the security posture from multi-factor to single-factor authentication for privileged accounts.

Detection Methods for CVE-2023-35179

Indicators of Compromise

• Administrative login events that do not show corresponding MFA verification completion
• Unusual administrative sessions originating from unexpected IP addresses or geographic locations
• Multiple administrative access attempts in rapid succession without MFA prompts
• Log entries showing successful authentication for admin accounts without two-factor authentication events

Detection Strategies

• Monitor Serv-U authentication logs for administrative logins that bypass MFA verification steps
• Implement SIEM rules to correlate admin login events with MFA challenge/response events and alert on discrepancies
• Deploy behavioral analytics to detect anomalous administrative session patterns
• Review audit trails for administrative actions performed without corresponding MFA confirmations

Monitoring Recommendations

• Enable verbose logging on Serv-U authentication events including MFA verification status
• Configure alerting for any administrative authentication that does not include MFA completion
• Regularly audit administrative account activity and correlate with MFA logs
• Implement network monitoring to detect unauthorized access to Serv-U management interfaces

How to Mitigate CVE-2023-35179

Immediate Actions Required

• Apply SolarWinds Serv-U 15.4 Hotfix 1 immediately to remediate the vulnerability
• Audit all administrative accounts for unauthorized access or suspicious activity
• Review authentication logs for evidence of MFA bypass attempts
• Implement network segmentation to restrict access to Serv-U administrative interfaces

Patch Information

SolarWinds has released Hotfix 1 for Serv-U 15.4 to address this vulnerability. Organizations should download and apply the patch from the SolarWinds Serv-U Hotfix 1 page. Additional details are available in the SolarWinds Security Advisory for CVE-2023-35179.

Workarounds

• Restrict network access to Serv-U administrative interfaces to trusted IP ranges only
• Implement additional network-layer authentication controls for administrative access
• Enable enhanced audit logging for all administrative operations until the patch can be applied
• Consider temporarily disabling remote administrative access if not operationally required

# Example: Restrict administrative access by IP using firewall rules
# Limit Serv-U admin interface access to specific trusted networks
iptables -A INPUT -p tcp --dport 443 -s 10.0.0.0/8 -j ACCEPT
iptables -A INPUT -p tcp --dport 443 -j DROP

# Enable enhanced logging in Serv-U configuration
# Review Serv-U-log.txt for authentication events
# Location: <Serv-U Install Directory>\Serv-U-log.txt