CVE-2021-3154: SolarWinds Serv-U Information Disclosure

CVE-2021-3154 Overview

CVE-2021-3154 is an injection vulnerability discovered in SolarWinds Serv-U, a widely-used managed file transfer (MFT) server solution. The vulnerability allows unauthenticated attackers to retrieve cleartext passwords through macro injection attacks. This flaw affects versions of Serv-U prior to 15.2.2 and represents a significant security risk due to the ability to extract sensitive credential information without requiring any prior authentication to the system.

Critical Impact

Unauthenticated remote attackers can exploit macro injection to extract cleartext passwords, potentially leading to complete account compromise and unauthorized access to sensitive file transfer systems.

Affected Products

• SolarWinds Serv-U (versions prior to 15.2.2)

Discovery Timeline

• 2021-05-04 - CVE CVE-2021-3154 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-3154

Vulnerability Analysis

This vulnerability falls under the category of injection flaws (CWE-74: Improper Neutralization of Special Elements in Output Used by a Downstream Component). The core issue stems from inadequate input validation and sanitization within the Serv-U application, allowing attackers to inject malicious macro content that, when processed by the application, results in the disclosure of cleartext password information.

The vulnerability is particularly concerning because it can be exploited by unauthenticated attackers over the network without requiring any user interaction. The attack targets confidentiality, enabling extraction of stored credentials that could be used for further lateral movement or privilege escalation within the affected environment.

It should be noted that this vulnerability had a distinct fix relative to CVE-2020-35481, indicating that while related, these are separate security issues requiring individual remediation.

Root Cause

The root cause of CVE-2021-3154 lies in improper neutralization of special macro elements within user-controlled input. The application fails to adequately sanitize or validate input that contains macro sequences, allowing these macros to be interpreted and executed in a context where they can access and expose sensitive password data stored in cleartext. This represents a fundamental input validation failure in the application's processing logic.

Attack Vector

The attack vector is network-based and requires no authentication or user interaction. An attacker can remotely target a vulnerable Serv-U server by crafting requests that contain malicious macro injection payloads. When processed by the vulnerable application, these macros are interpreted in a way that exposes cleartext password information.

The exploitation flow typically involves:

1. Identifying a vulnerable Serv-U installation accessible over the network
2. Crafting a request containing macro injection payloads designed to extract credential information
3. Submitting the malicious request to the target server
4. Receiving the cleartext password data in the application's response

Technical details regarding specific macro syntax and injection vectors can be found in the SolarWinds Serv-U 15.2.2 Release Notes.

Detection Methods for CVE-2021-3154

Indicators of Compromise

• Unusual requests containing macro syntax or special characters targeting Serv-U services
• Unexpected credential disclosure in application responses or logs
• Network traffic patterns showing reconnaissance or exploitation attempts against Serv-U ports (typically TCP 22, 21, 443)
• Authentication attempts using credentials that may have been extracted via this vulnerability

Detection Strategies

• Monitor Serv-U application logs for suspicious request patterns containing macro injection indicators
• Implement network intrusion detection signatures for known Serv-U exploitation patterns
• Deploy SentinelOne Singularity to detect and prevent exploitation attempts targeting Serv-U installations
• Conduct regular vulnerability scanning to identify unpatched Serv-U instances in your environment

Monitoring Recommendations

• Enable verbose logging on Serv-U servers and forward logs to a SIEM for centralized analysis
• Monitor for failed and successful authentication patterns that may indicate credential misuse
• Implement alerting on access attempts from unexpected geographic locations or IP ranges
• Regularly audit user accounts and credentials for signs of unauthorized access

How to Mitigate CVE-2021-3154

Immediate Actions Required

• Upgrade SolarWinds Serv-U to version 15.2.2 or later immediately
• Audit all stored credentials and rotate passwords for any accounts managed by vulnerable Serv-U instances
• Review access logs for signs of exploitation prior to patching
• Implement network segmentation to limit exposure of Serv-U servers

Patch Information

SolarWinds has addressed this vulnerability in Serv-U version 15.2.2. Organizations running affected versions should upgrade to the patched release as soon as possible. The fix includes proper input validation and sanitization to prevent macro injection attacks. Detailed release information is available in the SolarWinds Serv-U 15.2.2 Release Notes.

Workarounds

• Restrict network access to Serv-U servers using firewall rules to limit exposure to trusted networks only
• Implement web application firewall (WAF) rules to detect and block macro injection patterns
• Enable multi-factor authentication for Serv-U access to reduce the impact of credential compromise
• Consider temporarily disabling public-facing Serv-U services until patching is completed

# Example firewall rule to restrict Serv-U access (Linux iptables)
# Allow access only from trusted management network
iptables -A INPUT -p tcp --dport 22 -s 10.0.0.0/8 -j ACCEPT
iptables -A INPUT -p tcp --dport 21 -s 10.0.0.0/8 -j ACCEPT
iptables -A INPUT -p tcp --dport 443 -s 10.0.0.0/8 -j ACCEPT
iptables -A INPUT -p tcp --dport 22 -j DROP
iptables -A INPUT -p tcp --dport 21 -j DROP
iptables -A INPUT -p tcp --dport 443 -j DROP