No Peer, no Cry: Network Application Fuzzing via Fault Injection

TL;DR

This paper introduces Fuzztruction-Net, a novel fault injection-based fuzzing approach for network applications that effectively uncovers bugs by creating abnormal peer states without message modification.

Contribution

It presents the first fault injection method for network application fuzzing that can test both server and client sides, overcoming limitations of message replay and modification.

Findings

Uncovered 23 new bugs in popular network software

Fuzztruction-Net outperforms existing fuzzers in coverage and bug detection

Successfully tested both server and client network applications

Abstract

Network-facing applications are commonly exposed to all kinds of attacks, especially when connected to the internet. As a result, web servers like Nginx or client applications such as curl make every effort to secure and harden their code to rule out memory safety violations. One would expect this to include regular fuzz testing, as fuzzing has proven to be one of the most successful approaches to uncovering bugs in software. Yet, surprisingly little research has focused on fuzzing network applications. When studying the underlying reasons, we find that the interactive nature of communication, its statefulness, and the protection of exchanged messages render typical fuzzers ineffective. Attempts to replay recorded messages or modify them on the fly only work for specific targets and often lead to early termination of communication. In this paper, we discuss these challenges in detail, highlighting how the focus of existing work on protocol state space promises little relief. We propose a fundamentally different approach that relies on fault injection rather than modifying messages. Effectively, we force one of the communication peers into a weird state where its output no longer matches the expectations of the target peer, potentially uncovering bugs. Importantly, this weird peer can still properly encrypt/sign the protocol message, overcoming a fundamental challenge of current fuzzers. In effect, we leave the communication system intact but introduce small corruptions. Since we can turn either the server or the client into the weird peer, our approach is the first that can effectively test client-side network applications. Evaluating 16 targets, we show that Fuzztruction-Net outperforms other fuzzers in terms of coverage and bugs found. Overall, Fuzztruction-Net uncovered 23 new bugs in well-tested software, such as the web servers Nginx and Apache HTTPd and the OpenSSH client.