TXTing 101: Finding Security Issues in the Long Tail of DNS TXT Records

The DNS TXT resource record is the one that without doubt provide users with the most flexibility of content, as it is a largely unstructured. Although it might be the ideal basis for storing any form of text-based information, it also poses a security threat, as TXT records can also be used for malicious and unintended practices. Yet, we reckon that TXT records are often overlooked in security research. In this paper, we present the first structured study of the uses of TXT records, with a specific focus on security implications. We are able to classify over 99.54% of all TXT records in our dataset, finding security issues including accidentally published private keys and exploit delivery attempts. We also report our lessons learned while dealing with a large-scale, systematic analysis of TXT records.