Rethinking Neural Decompilation: Candidate Selection as the Hidden Bottleneck

Abstract

Reverse engineering obfuscated binaries is a core task in malware analysis and software security auditing. While neural decompilers based on Large Language Models (LLMs) have improved the readability and functional fidelity of recovered code over heuristic tools such as Ghidra and IDA Pro, a substantial gap remains between best-sample performance and practical single-output accuracy, particularly under code obfuscation. We study this gap through the lens of candidate generation and candidate selection. Using the LLM4Decompile architecture, we show that ground-truth tokens lie within the model's top-five next-token predictions in 95.6% of positions on average and that multi-sample decoding reaches a pass@100 of 93% on HumanEval-Decompile, indicating that correct reconstructions are typically already present in the model's sampling space. We then evaluate a generate-then-select pipeline that ranks 100 candidates per input with text-, syntax-, embedding-, consensus-, and likelihood-based selectors. Negative perplexity emerges as the strongest static selector on HumanEval-Decompile (73.3% pass@1), while execution-guided selection nearly closes the oracle gap on ExeBench (86.3% pass@1). Combined with one epoch of fine-tuning on our newly released ExeBench-Obfuscated dataset containing 362,647 obfuscated OLLVM samples, the same pipeline increases the pass@1 rate for obfuscated binaries to 88.8%, directly contradicting prior claims that released LLM4Decompile checkpoints are unsuitable for obfuscated inputs.