CyberCane: a privacy-bounded neuro-symbolic retrieval framework for phishing detection under distribution shift

arXiv:2604.23563v2 Announce Type: replace-cross Abstract: Organizations usually calibrate phishing detectors on public corpora, because privacy rules make their own labeled legitimate mail hard to collect. This practice breaks the false-positive budget of trained detectors: calibrated for a 1% false-positive rate (FPR) on older public mail and deployed on DataPhish 2025, they realize 20.2-22.9% FPR, 9-94 times the budget across three calibration sources. The cause is calibration transfer, not ranking: the classifiers separate the deployment mail well, but their thresholds do not transfer. We introduce CyberCane, a privacy-bounded neuro-symbolic detector that scores email only by similarity to known phishing in a redacted, training-only index, plus deterministic rules, and calibrates verdicts to an explicit FPR budget. Because legitimate mail never enters its score, only its threshold depends on the calibration source. Under the same shift CyberCane realizes 5.4% FPR at 86.4% recall, reducing the overrun 2.7-6.1 times across sources, seeds, a second shift family, and a local embedder. A segment-level variant, scoring an email's most phishing-like part, lowers the realized FPR a further 1.2-2.7 times at equal ranking quality and withstands benign padding. CyberCane screens an email locally in 0.20 s, sending no text out; a 70B zero-shot LLM ranks better at strict FPR but is 14-24 times slower and must receive each email. We also show that LLM rewriting does not evade embedding-based detectors and that a random split of DataPhish 2025 leaks 96% of its test set. All protocols were fixed in advance, and the released splits and code let a deployer measure realized FPR before trusting a budget.