1. Introduction
Nessus detected one or more malicious files on the remote host, indicating a YARA File Scan (Linux) vulnerability. This means files matching patterns of known malware have been found on Linux systems. This is serious as it suggests a compromise or potential for future infection. Impact could be high to confidentiality, integrity and availability depending on the nature of the detected files.
2. Technical Explanation
The vulnerability occurs when files present on a Linux system match rules defined in YARA. These rules identify patterns associated with malicious software. An attacker does not directly exploit this; rather, it’s an indicator that malware is already present or has been previously introduced to the system. Preconditions include having files on the system which are identified by existing YARA rules.
- Root cause: The presence of files matching malicious signatures defined in YARA rules.
- Exploit mechanism: An attacker would introduce a file that matches an existing YARA rule onto the system, typically through methods like phishing, software vulnerabilities or misconfiguration.
- Scope: Linux systems are affected. Specific distributions and versions aren’t directly relevant as the vulnerability depends on the files present.
3. Detection and Assessment
Confirming a system is vulnerable involves checking for files matching malicious YARA rules. A quick check can be done by reviewing Nessus scan results. Thorough assessment requires running a full YARA scan of the file system.
- Quick checks: Review Nessus vulnerability reports for “YARA File Scan (Linux)”.
- Scanning: Use the `yara` command-line tool with appropriate rule sets to scan files and directories. Example:
yara /path/to/scan -r /path/to/ruleset. - Logs and evidence: Check system logs for unusual file creation or modification events, especially in temporary directories (/tmp) or user home directories.
yara /var/log -r /etc/yara/rules/malware_rules.yar 4. Solution / Remediation Steps
Fixing this issue requires identifying and removing the malicious files. This is a multi-step process to ensure complete eradication.
4.1 Preparation
- Dependencies: Ensure you have access to the `yara` tool and updated rule sets. Roll back plan: Restore from backup if files are incorrectly removed.
- Change window needs: A maintenance window is recommended, particularly for production systems. Approval should be sought from system owners.
4.2 Implementation
- Step 1: Identify the malicious files reported by Nessus or YARA scan results.
- Step 2: Isolate the affected system to prevent further spread of potential malware.
- Step 3: Delete the identified malicious files using the `rm` command. Example:
rm /path/to/malicious_file. - Step 4: Scan the entire file system again with YARA to confirm no remaining matches.
- Step 5: Review logs for any related activity and investigate further if needed.
4.3 Config or Code Example
Before
ls -l /tmp/suspicious_file After
rm /tmp/suspicious_file 4.4 Security Practices Relevant to This Vulnerability
Several security practices can help prevent this type of issue. Least privilege limits the impact if malware is executed. Input validation prevents malicious files from being created or uploaded in the first place. Patch cadence ensures systems are up-to-date with the latest security fixes.
- Practice 1: Implement least privilege to restrict user access and limit the potential damage caused by compromised accounts.
- Practice 2: Employ input validation on all file uploads or data transfers to block potentially malicious content.
4.5 Automation (Optional)
#!/bin/bash
# Script to remove files matching YARA rules
for file in $(yara /path/to/scan -r /path/to/ruleset --print-file); do
echo "Removing malicious file: $file"
rm "$file" # Be cautious with this command!
done
5. Verification / Validation
Confirming the fix involves re-running the YARA scan and verifying no matches are found. A service smoke test should also be performed to ensure functionality remains intact.
- Post-fix check: Run
yara /path/to/scan -r /path/to/ruleset; expected output should show no matching files. - Re-test: Re-run the Nessus scan to confirm the vulnerability is no longer reported.
- Smoke test: Verify key system functions, such as user login and basic application functionality.
- Monitoring: Monitor system logs for unusual file creation or modification events. Example query:
grep -i "malware" /var/log/syslog.
yara /var/log -r /etc/yara/rules/malware_rules.yar 6. Preventive Measures and Monitoring
- Baselines: Update system security baselines with current YARA rules and file integrity monitoring configurations.
- Pipelines: Add static analysis (SAST) tools to CI/CD pipelines to scan for malicious code in software builds.
- Asset and patch process: Implement a regular patch cycle and configuration review process to identify and address potential vulnerabilities.
7. Risks, Side Effects, and Roll Back
- Roll back: Restore the system from the backup taken prior to remediation.
8. References and Resources
- Vendor advisory or bulletin: Not applicable, as this is a detection method rather than a specific vendor issue.
- NVD or CVE entry: Not applicable, as this is a detection method rather than a specific vulnerability.
- Product or platform documentation relevant to the fix: http://virustotal.github.io/yara/