Executive Summary

• SGI sensors detected a malicious file attempting to modify SSH authorized_keys files on a system.
• The likely objective is to gain persistent, unauthorized SSH access to the targeted system.
• Compromised credentials can lead to lateral movement within the network and data exfiltration.
• The severity is rated as low due to the need for pre-existing access; however, the impact of successful exploitation is high.
• Organizations should immediately review SSH key management practices and implement multi-factor authentication to mitigate the risk of unauthorized access.

We anticipate an increase in credential-based attacks targeting exposed services like SSH in the coming months, emphasizing the need for proactive security measures.

Observed Activity (SGI Sensors)

Our sensors detected a suspicious connection from an IP address originating from Russia (ASN AS48282). The connection contained a payload identified as a modified authorized_keys file, indicating an attempt to inject malicious SSH keys. The activity was flagged due to the file’s hash matching known malware signatures. This suggests a potential credential harvesting or brute-forcing attempt preceding the key injection.

Malware/Technique Overview

The observed malware family, named 20251021-104001-e2b456e9ba28-1-redir__home_cris__ssh_authorized_keys, is designed to modify the .ssh/authorized_keys file on a compromised system. This allows the attacker to gain persistent SSH access without needing the user’s password.

The initial access vector is currently unknown but often involves:

• Compromised credentials through phishing or credential stuffing.
• Exploitation of vulnerabilities in SSH or related services.
• Brute-forcing weak SSH passwords.

Typical targets include servers, workstations, and network devices that utilize SSH for remote administration.

MITRE ATT&CK Mapping:

• T1190 – Exploit Public-Facing Application
• T1110 – Brute Force
• T1078 – Valid Accounts
• T1059 – Command and Scripting Interpreter
• T1098.004 – Account Manipulation: SSH Keys
• TA0006 – Credential Access
• TA0003 – Persistence
• TA0007 – Discovery
• TA0008 – Lateral Movement

VirusTotal Snapshot

VirusTotal analysis shows that 29 out of 61 vendors flagged the sample as malicious, while 32 vendors did not detect it. The file is described as HTML. The sample has a VirusTotal reputation score of -34.

Some of the aliases for this sample include variations of the file name authorized_keys, indicating a pattern of injecting malicious keys into various user accounts.

• VirusTotal Sample

Indicators of Compromise (IoCs)

We recommend monitoring these IoCs for at least 30 days.

Detection & Hunting

Here are sample queries to detect similar activity in your environment:

Splunk SPL

index=* file_hash="a8460f446be540410004b1a8db4083773fa46f7fe76fa84219c93daa1669f8f2" OR source_ip="88.210.52.223" OR file_name="authorized_keys"
| table _time, host, user, file_path, source_ip

Elastic/Kibana KQL

file.hash.sha256:"a8460f446be540410004b1a8db4083773fa46f7fe76fa84219c93daa1669f8f2" or source.ip:"88.210.52.223" or file.name:"authorized_keys"

When investigating, be aware that legitimate modifications to authorized_keys files can occur during normal system administration. Validate the source and intent of any changes.

Containment, Eradication & Recovery

1. Isolate the affected system: Immediately disconnect the compromised system from the network to prevent further lateral movement.
2. Block malicious IP: Block the identified malicious IP address (88.210.52.XXX) at the firewall level.
3. Scan for malware: Perform a full system scan using updated antivirus and anti-malware solutions.
4. Reimage if necessary: If the compromise is severe, consider reimaging the affected system from a known good backup.
5. Reset credentials: Reset passwords for all user accounts on the compromised system and any accounts that may have been accessed from it.

Inform IT staff and leadership about the incident and the steps being taken. Preserve logs and evidence for forensic analysis.

Hardening & Preventive Controls

• Implement Multi-Factor Authentication (MFA): Enforce MFA for all SSH logins (NIST CSF PR.AC-1, CIS Control 6).
• Tune Endpoint Detection and Response (EDR): Configure EDR solutions to detect suspicious file modifications, especially to authorized_keys files (NIST CSF DE.CM-1, CIS Control 8).
• Network Segmentation: Segment the network to limit the impact of lateral movement (NIST CSF PR.AC-4, CIS Control 14).
• Principle of Least Privilege: Grant users only the necessary privileges (NIST CSF PR.AC-3, CIS Control 5).
• Patch Management: Implement a robust patch management process to address vulnerabilities in SSH and related services (NIST CSF ID.AM-4, CIS Control 7).
• Disable Password Authentication: Disable password authentication for SSH and rely solely on key-based authentication.
• Regularly Rotate SSH Keys: Implement a policy for regularly rotating SSH keys to minimize the impact of compromised keys.

Business Impact & Risk Outlook

A successful SSH key compromise can lead to significant operational disruption, data breaches, and reputational damage. Legal and regulatory compliance may also be affected if sensitive data is accessed.

We anticipate that attackers will continue to target SSH and other remote access services. Organizations should proactively strengthen their security posture to defend against these threats. Expect more sophisticated attacks that combine multiple techniques to evade detection.

Appendix

[Redacted Payload Snippet]

Assumptions & Data Gaps:

• We assume the attacker successfully modified the authorized_keys file.
• The initial access vector remains unknown.
• The sensor name is missing.
• The specific port used during the connection is missing.

References:

• VirusTotal Sample

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