Security Is Relative: Training-Free Vulnerability Detection via Multi-Agent Behavioral Contract Synthesis

Computer Science > Cryptography and Security [Submitted on 21 Apr 2026] Security Is Relative: Training-Free Vulnerability Detection via Multi-Agent Behavioral Contract Synthesis Yongchao Wang, Zhiqiu Huang Deep learning for vulnerability detection has shown promising results on early benchmarks, but recent evaluations reveal catastrophic degradation: models achieving F1 > 0.68 on legacy datasets collapse to 0.031 under strict deduplication. We identify the root cause as the semantic ambiguity problem: identical code can be secure or vulnerable depending on project-specific behavioral contracts, rendering global classification fundamentally inadequate. We propose Phoenix, a training-free multi-agent framework that resolves this ambiguity through Behavioral Contract Synthesis. Phoenix decomposes detection into three stages: a Semantic Slicer extracting minimal vulnerability-relevant context, a Requirement Reverse Engineer synthesizing Gherkin behavioral specifications encoding the security contract, and a Contract Judge evaluating code against these specifications via strict compliance checking. On PrimeVul Paired, Phoenix achieves F1 = 0.825 and Pair-Correct = 64.4%, surpassing RASM-Vul (F1 = 0.668) and VulTrial (F1 = 0.563) while using open-source models up to 48x smaller (7-14B vs. 671B). Ablation across 25 configurations demonstrates Gherkin specifications as the decisive driver (+0.09 to +0.35 F1). Error analysis reveals 18% of "False Positives" identify genuine security concerns in patched code, demonstrating that security is a relative property defined against behavioral contracts, not an absolute property of code syntax. Subjects: Cryptography and Security (cs.CR); Software Engineering (cs.SE) Cite as: arXiv:2604.19012 [cs.CR]   (or arXiv:2604.19012v1 [cs.CR] for this version)   https://doi.org/10.48550/arXiv.2604.19012 Focus to learn more Submission history From: Yongchao Wang [view email] [v1] Tue, 21 Apr 2026 03:02:34 UTC (655 KB) Access Paper: HTML (experimental) view license Current browse context: cs.CR < prev   |   next > new | recent | 2026-04 Change to browse by: cs cs.SE References & Citations NASA ADS Google Scholar Semantic Scholar Export BibTeX Citation Bookmark Bibliographic Tools Bibliographic and Citation Tools Bibliographic Explorer Toggle Bibliographic Explorer (What is the Explorer?) Connected Papers Toggle Connected Papers (What is Connected Papers?) Litmaps Toggle Litmaps (What is Litmaps?) scite.ai Toggle scite Smart Citations (What are Smart Citations?) Code, Data, Media Demos Related Papers About arXivLabs Which authors of this paper are endorsers? | Disable MathJax (What is MathJax?)