Phishing is an attempt to steal a user's identity. This is typically accomplished by sending an email message to a user, with a link directing the user to a web site used to collect personal information. Phishing detection systems typically rely on content filtering techniques, such as Latent Dirichlet Allocation (LDA), to identify phishing messages. In the case of spear phishing, however, this may be ineffective because messages from a trusted source may contain little content. In order to handle such emerging spear phishing behavior, we propose as a first step the use of Spectral Clustering to analyze messages based on traffic behavior. In particular, Spectral Clustering analyzes the links between URL substrings for web sites found in the message contents. Cluster membership is then used to construct a Random Forest classifier for phishing. Data from the Phishing Email Corpus and the Spam Assassin Email Corpus are used to evaluate this approach. Performance evaluation metrics include the Area Under the receiver operating characteristic Curve (AUC), as well as accuracy, precision, recall, and the (harmonic mean) F measure. Performance of the integrated Spectral Clustering and Random Forest approach is found to provide significant improvements in all the metrics listed, compared to a content filtering technique such as LDA coupled with text message deletion done randomly or in an adaptive fashion using adversarial learning. The Spectral Clustering approach is robust against the absence of content. In particular, we show that Spectral Clustering yields (99.8%, 97.8%) for (AUC, F measure) compared to LDA that yields (94.6%, 89.4%) and (79.6%, 57.9%) when the content of the messages is reduced to 10% of their original size using random and adversarial deletion, respectively. The difference is most striking at low False Positive (FP) rates.