Themis: Ambiguity-Aware Network Intrusion Detection based on Symbolic Model Comparison

Zhongjie Wang, Shitong Zhu, Keyu Man, Pengxiong Zhu, Yu Hao, Zhiyun Qian, Srikanth V. Krishnamurthy, Tom La Porta, Michael J. De Lucia

Research output: Chapter in Book/Report/Conference proceedingConference contribution


Network intrusion detection systems (NIDS) can be evaded by carefully crafted packets that exploit implementation-level discrepancies between how they are processed on the NIDS and at the endhosts. These discrepancies arise due to the plethora of endhost implementations and evolutions thereof. It is prohibitive to proactively employ a large set of implementations at the NIDS and check incoming packets against all of those. Hence, NIDS typically choose simplified implementations that attempt to approximate and generalize across the different endhost implementations. Unfortunately, this solution is fundamentally flawed since such approximations are bound to have discrepancies with some endhost implementations. In this paper, we develop a lightweight system Themis, which empowers the NIDS in identifying these discrepancies and reactively forking its connection states when any packets with "ambiguities"are encountered. Specifically, Themis incorporates an offline phase in which it extracts models from various popular implementations using symbolic execution. During runtime, it maintains a nondeterministic finite automaton to keep track of the states for each possible implementation. Our extensive evaluations show that Themis is extremely effective and can detect all evasion attacks known to date, while consuming extremely low overhead. En route, we also discovered multiple previously unknown discrepancies that can be exploited to bypass current NIDS.

Original languageEnglish (US)
Title of host publicationCCS 2021 - Proceedings of the 2021 ACM SIGSAC Conference on Computer and Communications Security
PublisherAssociation for Computing Machinery
Number of pages16
ISBN (Electronic)9781450384544
StatePublished - Nov 12 2021
Event27th ACM Annual Conference on Computer and Communication Security, CCS 2021 - Virtual, Online, Korea, Republic of
Duration: Nov 15 2021Nov 19 2021

Publication series

NameProceedings of the ACM Conference on Computer and Communications Security
ISSN (Print)1543-7221


Conference27th ACM Annual Conference on Computer and Communication Security, CCS 2021
Country/TerritoryKorea, Republic of
CityVirtual, Online

All Science Journal Classification (ASJC) codes

  • Software
  • Computer Networks and Communications

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