Hackers usually discover and exploit vulnerabilities existing in the entry point before invading a corporate environment. The web server exploration and spams are two popular means used by hackers to gain access to the enterprise computer systems. In this paper, we focus on protecting a web server in dealing with such cybersecurity intrusion threat. During the discovery stage, a web vulnerability investigation scanner (e.g., SQLMap, NMap, and Kali) is used by hackers to learn the web server versions and other related vulnerabilities. Then, in the exploitation stage, hackers develop a customized intrusion method which exploits those previously learned vulnerabilities to launch a subsequent attack. Currently, the most popular defense approaches (e.g., IDS, WAF) detect web server intrusion events through domain expert rules and anomaly pattern matches. For example, ModSecurity is an open source WAF which only detects known malware signature by domain expert rules. Thus, those approaches are good to defend the first discovery stage intrusion. However, they are not effective to deal with the customized intrusion in the second exploitation stage since no rules or signatures are available for such kind of intrusion detection. In this paper, in order to resolve the above problem, we propose an unsupervised data-driven anomaly detection known as WebHound. It not only identifies hackers reconnaissance but also detects the customized intrusion means deployed by hackers by analyzing large-scale web access logs. Moreover, WebHoundalso provides intrusion evidence using storyline for recovering intrusion procedure. Among numerous experiments and case studies, we applied WebHoundto a special government case for the intrusion evidence investigation and at the same time, we compared our results with the work done by computer forensic experts. The results showed that WebHoundcould discover more intrusion evidence than human experts. We also compared WebHoundwith ModSecurity which is updated with the newest domain expert rules running in a virtualized corporate environment. The experimental results show that WebHoundhas a better accuracy rate than ModSecurity. In summary, WebHoundalleviates the heavy demand on expert knowledge and human efforts to detect cyber-attack on a web server, and it also enhances detection accuracy and recall rate. Moreover, WebHoundcould provide more evidence for forensic experts to trace the original entry points.