CVE-2026-3100: Asustor Data Master Info Disclosure Flaw

CVE-2026-3100 Overview

CVE-2026-3100 is a certificate validation bypass vulnerability in Asustor Data Master (ADM) that affects the FTP Backup functionality. The vulnerability stems from improper TLS certificate verification when connecting to FTP servers using FTPES/FTPS protocols. This security flaw enables remote attackers to intercept network traffic and perform Man-in-the-Middle (MitM) attacks, potentially compromising authentication credentials and sensitive backup data.

Critical Impact

Remote attackers can intercept, modify, or obtain sensitive information including authentication credentials and backup data through Man-in-the-Middle attacks due to improper TLS/SSL certificate validation in the FTP Backup feature.

Affected Products

• Asustor Data Master (ADM) versions 4.1.0 through 4.3.3.ROF1
• Asustor Data Master (ADM) versions 5.0.0 through 5.1.2.RE51
• All ASUSTOR NAS devices running affected ADM versions

Discovery Timeline

• 2026-02-25 - CVE-2026-3100 published to NVD
• 2026-02-26 - Last updated in NVD database

Technical Details for CVE-2026-3100

Vulnerability Analysis

This vulnerability is classified as CWE-295 (Improper Certificate Validation), which occurs when the software fails to properly validate digital certificates during TLS/SSL connections. In the context of Asustor Data Master, the FTP Backup feature does not strictly enforce TLS certificate verification when establishing secure connections to FTP servers using FTPES (FTP Explicit TLS) or FTPS (FTP Implicit TLS) protocols.

The network-based attack vector allows remote attackers positioned between the NAS device and the FTP backup server to intercept encrypted communications. The attack requires some preparation (positioning in the network path) but does not require user interaction or prior authentication to the target system. A successful exploit results in high confidentiality impact, allowing attackers to access sensitive backup data and authentication credentials, along with limited integrity impact through potential data modification.

Root Cause

The root cause of this vulnerability lies in the improper implementation of TLS/SSL certificate validation within the ADM FTP Backup module. When the backup process initiates a secure connection to an FTP server, the software fails to properly verify:

• Certificate chain validity and trust anchors
• Certificate expiration dates
• Certificate hostname matching (Subject Alternative Names)
• Certificate revocation status

This implementation flaw allows connections to proceed even when presented with invalid, self-signed, or mismatched certificates, effectively nullifying the security guarantees that TLS/SSL is designed to provide.

Attack Vector

The attack exploits the certificate validation bypass through network-based interception. An attacker must first position themselves in the network path between the ASUSTOR NAS device and the target FTP backup server. This can be achieved through various means such as ARP spoofing, DNS hijacking, or compromising network infrastructure.

Once positioned, the attacker presents a fraudulent TLS certificate to the NAS device during the FTP connection handshake. Because ADM does not properly validate the certificate, the connection proceeds with the attacker's certificate, establishing an encrypted tunnel to the attacker instead of the legitimate FTP server. The attacker can then decrypt, inspect, and potentially modify the traffic before forwarding it to the actual destination server.

The vulnerability enables attackers to capture FTP authentication credentials transmitted during the login phase, intercept backup data as it flows through the compromised connection, modify backup data in transit without detection, and potentially inject malicious content into backup streams.

Detection Methods for CVE-2026-3100

Indicators of Compromise

• Unexpected certificate warnings or errors in FTP backup logs that were subsequently ignored
• Backup connections establishing with servers presenting untrusted or self-signed certificates
• Network traffic anomalies indicating potential ARP spoofing or DNS hijacking attempts
• Discrepancies between expected and actual FTP server certificate fingerprints

Detection Strategies

• Monitor network traffic for TLS connections that complete successfully despite certificate validation failures
• Implement network-based certificate pinning detection to identify connections to unexpected certificate authorities
• Review ADM backup logs for any certificate-related warnings or connection anomalies
• Deploy network intrusion detection systems (IDS) to identify potential MitM positioning attacks

Monitoring Recommendations

• Enable verbose logging for FTP backup operations to capture certificate validation events
• Implement network monitoring to detect ARP spoofing or suspicious DNS responses
• Monitor for unusual backup traffic patterns or connections to unexpected IP addresses
• Establish baseline certificate fingerprints for legitimate FTP backup servers and alert on deviations

How to Mitigate CVE-2026-3100

Immediate Actions Required

• Update Asustor Data Master to the latest patched version as soon as available from ASUSTOR
• Review and audit all configured FTP backup connections for security posture
• Consider temporarily disabling FTP backups over untrusted networks until patches are applied
• Implement network segmentation to limit exposure of NAS devices to potential MitM attacks

Patch Information

ASUSTOR has acknowledged this vulnerability and released security advisory information. Administrators should consult the ASUSTOR Security Advisory #53 for detailed patch information and update instructions. Apply the latest ADM firmware update that addresses CVE-2026-3100 as the primary remediation step.

Workarounds

• Use VPN tunnels for FTP backup connections to add an additional layer of encryption and authentication
• Implement IPsec or other network-layer encryption between the NAS and FTP backup servers
• Restrict FTP backup operations to trusted, segmented network environments only
• Consider alternative backup methods (such as rsync over SSH) that provide proper certificate validation until a patch is applied

# Network segmentation example - restrict NAS backup traffic to specific VLAN
# Configure on your network switch/router
# Create dedicated VLAN for backup traffic
vlan 100
  name BACKUP_NETWORK
  
# Apply strict firewall rules to limit MitM attack surface
# Allow only known FTP backup server IPs
iptables -A OUTPUT -p tcp --dport 21 -d <trusted_ftp_server_ip> -j ACCEPT
iptables -A OUTPUT -p tcp --dport 990 -d <trusted_ftp_server_ip> -j ACCEPT
iptables -A OUTPUT -p tcp --dport 21 -j DROP
iptables -A OUTPUT -p tcp --dport 990 -j DROP