DissecTLS: A Scalable Active Scanner for TLS Server Configurations, Capabilities, and TLS Fingerprinting

Abstract

Collecting metadata from Transport Layer Security (TLS) servers on a large scale allows to draw conclusions about their capabilities and configuration. This provides not only insights into the Internet but it enables use cases like detecting malicious Command and Control (C &C) servers. However, active scanners can only observe and interpret the behavior of TLS servers, the underlying configuration and implementation causing the behavior remains hidden. Existing approaches struggle between resource intensive scans that can reconstruct this data and light-weight fingerprinting approaches that aim to differentiate servers without making any assumptions about their inner working. With this work we propose DissecTLS, an active TLS scanner that is both light-weight enough to be used for Internet measurements and able to reconstruct the configuration and capabilities of the TLS stack. This was achieved by modeling the parameters of the TLS stack and derive an active scan that dynamically creates scanning probes based on the model and the previous responses from the server. We provide a comparison of five active TLS scanning and fingerprinting approaches in a local testbed and on toplist targets. We conducted a measurement study over nine weeks to fingerprint C &C servers and analyzed popular and deprecated TLS parameter usage. Similar to related work, the fingerprinting achieved a maximum precision of 99 % for a conservative detection threshold of 100 %; and at the same time, we improved the recall by a factor of 2.8.

EFACTLS: Effective Active TLS Fingerprinting for Large-Scale Server Deployment Characterization

“…DissecTLS provided a higher C2 detection precision compared to this work at the cost of sending more requests (24 ch per server on average [20]). Similarly, TLS server debugging tools like testssl.sh [19] or SSLyze [18] dynamically collect data about the TLS servers.…”

“…2 The average number of sent CHs observed in a study on the Tranco top 10k domains published in Ref. [20].…”

“…The only requirement is that servers behave consistently. The related work DissecTLS [20] shows that it is possible to implement such a dynamic scanning approach efficiently enough to use it for large-scale measurements.…”

“…1 This work is an extension of Ref. [10], published in the Network Traffic Measurement and Analysis Conference (TMA) in 2022. Contributions (iii), (iv), and (vi) represent novel additions compared to the previous version.…”

“…It is an extension of the study described in Ref. [10], which provides a long-term analysis showcasing the applicability and performance of detection use cases supported by active TLS fingerprinting, along with an assessment of the effectiveness of their data collection.…”