Hallucinating Certificates: Differential Testing of TLS Certificate Validation Using Generative Language Models

Abstract

Certificate validation is a crucial step in Transport Layer Security (TLS), the de facto standard network security protocol. Prior research has shown that differentially testing TLS implementations with synthetic certificates can reveal critical security issues, such as accidentally accepting untrusted certificates. Leveraging known techniques, like random input mutations and program coverage guidance, prior work created corpora of synthetic certificates. By testing the certificates with multiple TLS libraries and comparing the validation outcomes, they discovered new bugs. However, they cannot generate the corresponding inputs efficiently, or they require to model the programs and their inputs in ways that scale poorly.

In this paper, we introduce a new approach, MLCerts, to generate synthetic certificates for differential testing that leverages generative language models to more extensively test software implementations. Recently, these models have become (in)famous for their applications in generating content, writing code, and conversing with users, as well as for “hallucinating” syntactically correct yet semantically nonsensical output. In this paper, we provide and leverage two novel insights: (a) TLS certificates can be expressed in natural-like language, namely in the X.509 standard that aids human readability, and (b) differential testing can benefit from hallucinated malformed test cases.

Using our approach MLCerts, we find significantly more distinct discrepancies between the five TLS implementations OpenSSL, LibreSSL, GnuTLS, MbedTLS, and MatrixSSL than the state-of-the-art benchmark Transcert (+30%; 20 vs 26, out of a maximum possible of 30) and an order of magnitude more than the seminal work Frankencerts (+1,200%; 2 vs 26). Finally, we show that the diversity of MLCerts-generated certificates reveals a range of previously unobserved and interesting behavior with security implications.