DistilBERT Phishing URL Classifier: 4-bit NF4 (Distilled + Quantized)

The last stage of a distil-then-quantise pipeline. This is the 4-layer DistilBERT student with its linear layers quantised to 4-bit NF4 (bitsandbytes, double quantisation). It classifies a URL as phishing or benign.

BERT-base teacher  ──KD──▶  DistilBERT student  ──NF4──▶  4-bit student (this)
 438 MB · acc 89.7%          211 MB · acc 96.0%           110 MB · acc 95.8%

Results

All three rows are on the full test split (33,000 URLs, balanced):

• 4× smaller than the teacher and ~48% smaller than the fp32 student. Only the nn.Linear weights are quantised. The 30,522 × 768 word-embedding table stays in fp32 and now makes up most of the file.
• Quantisation costs only 0.18 points of accuracy (0.9601 → 0.9583) compared with the fp32 student. It also shifts the balance slightly toward recall (0.948 → 0.959) at the cost of precision (0.971 → 0.958), so fewer phishing URLs are missed.

_{Teacher test numbers come from the student card. The student and 4-bit numbers were re-measured on 27 Sep 2026 with max_length=128. The 4-bit row uses the stored NF4 weights, dequantised to bf16. This is the arithmetic bitsandbytes runs on GPU with bnb_4bit_compute_dtype=bfloat16.}

Quantisation config

Distillation settings for the student (T = 3.0, α = 0.6 KL + 0.4 CE, 4 epochs) are documented on the student card.

Usage

Needs a CUDA GPU with bitsandbytes installed. The quantisation config is stored in config.json, so the model loads already quantised.

import torch
from transformers import AutoTokenizer, AutoModelForSequenceClassification

repo = "Akash-Sakala/bert-phishing-classifier_student_4bit"
tokenizer = AutoTokenizer.from_pretrained(repo)   # standard distilbert-base-uncased vocab
model = AutoModelForSequenceClassification.from_pretrained(repo, device_map="cuda").eval()

urls = ["paypal.com.secure-login.verify-account.ru/webscr", "wikipedia.org/wiki/Phishing"]
inputs = tokenizer(urls, return_tensors="pt", truncation=True, padding=True, max_length=128).to(model.device)
with torch.no_grad():
    probs = model(input_ids=inputs["input_ids"], attention_mask=inputs["attention_mask"]).logits.softmax(-1)[:, 1]
for u, p in zip(urls, probs):
    print(f"{p:.2f}  {'PHISHING' if p > 0.5 else 'benign'}  {u}")

For CPU-only inference, use the fp32 student. bitsandbytes' 4-bit CPU kernels don't yet support this model's 2-class output layer.

Limitations

• The model sees only the URL string. Use it as one signal alongside reputation feeds and page analysis.
• Most training URLs have no http(s):// scheme. Strip the scheme before inference.
• Accuracy on new phishing campaigns will be lower than on this static test set.

Related

• Teacher: bert-phishing-classifier_teacher
• fp32 student: bert-phishing-classifier_student
• Dataset: phishing-site-classification