Web Server Reconnaissance via Unusual User Agents

This rule detects unusual spikes in web server requests with uncommon or suspicious user-agent strings, which may indicate reconnaissance attempts by attackers trying to identify vulnerabilities in web applications or servers. These user-agents are often associated with automated tools used for scanning, vulnerability assessment, or brute-force attacks. The rule is designed to detect activity from tools like Nikto, Nessus, SQLMap, WPScan, Feroxbuster, Masscan, Ffuf, Dirsearch, Dirb, Dirbuster, Gobuster, Nmap, Hydra, w3af, Arachni, Skipfish, OpenVAS, Acunetix, ZAP, and Burp Suite. It analyzes logs from web servers such as Nginx, Apache, Apache Tomcat, IIS, and Traefik, looking for a high volume of requests with diverse URLs originating from a single source IP address.

Attack Chain

1. Attacker uses a scanning tool (e.g., Nikto, WPScan, Dirsearch) to send HTTP requests to the target web server.
2. The scanning tool employs a specific User-Agent header that identifies it (e.g., "nikto", "wpscan").
3. The web server logs the HTTP requests, including the source IP address, User-Agent, and requested URL.
4. The attacker's tool iterates through numerous URLs, probing for common vulnerabilities, hidden paths, or administrative interfaces.
5. The web server responds to each request, generating distinct log entries for each URL accessed.
6. The rule detects an unusual spike in requests with suspicious user agents from a single source IP within a defined time interval.
7. If successful, the attacker gains information about the web server's configuration, software versions, and potential vulnerabilities.
8. The attacker may then exploit these vulnerabilities to gain unauthorized access or control.

Impact

Successful reconnaissance can lead to the discovery of vulnerabilities in web applications, potentially allowing attackers to gain unauthorized access, escalate privileges, or exfiltrate sensitive data. While this rule flags potential reconnaissance activity, it does not directly indicate a successful breach. The impact can range from minor information leakage to complete system compromise, depending on the vulnerabilities discovered and exploited. Organizations can experience data breaches, service disruptions, and reputational damage if attackers successfully leverage the information gathered during reconnaissance.

Recommendation

• Deploy the Sigma rule Web Server Suspicious User Agent - High Volume to detect high-volume scanning activity based on user-agent strings.
• Enable logging for web servers, including Nginx, Apache, Apache Tomcat, IIS, and Traefik, to ensure the process_creation log source is available.
• Investigate any alerts generated by the Sigma rules to determine if the detected activity is legitimate vulnerability scanning or malicious reconnaissance attempts.
• Implement rate limiting and blocking mechanisms for suspicious IP addresses detected by the Sigma rule at the WAF/CDN or edge firewall.
• Monitor web server access logs for requests matching the user-agent strings listed in the rule's query section.
• Implement WAF signatures for known scanner user agents to prevent malicious reconnaissance attempts.