sc-business-logic - SKILL.md Agent Skill

name: sc-business-logic description: Business logic flaw detection — price manipulation, workflow bypass, race conditions, and abuse vectors license: MIT metadata: author: ersinkoc category: security version: "1.0.0"

SC: Business Logic Flaws

Purpose

Detects business logic vulnerabilities that arise from flawed application workflows rather than technical implementation errors. Covers price/quantity manipulation, coupon abuse, workflow step bypassing, negative value attacks, account enumeration, and reward system abuse. These vulnerabilities cannot be detected by pattern matching alone — they require understanding of the application's intended behavior.

Activation

Called by sc-orchestrator during Phase 2. Runs against web applications with business workflows.

Phase 1: Discovery

Keyword Patterns to Search

"price", "amount", "quantity", "discount", "coupon",
"total", "balance", "credit", "transfer", "payment",
"checkout", "cart", "order", "refund", "reward",
"referral", "bonus", "limit", "quota", "threshold"

Vulnerability Categories

1. Price/Quantity Manipulation:

// VULNERABLE: Trusting client-side price
app.post('/checkout', (req, res) => {
  const total = req.body.items.reduce((sum, item) =>
    sum + item.price * item.quantity, 0  // Client sends price!
  );
  charge(req.user, total);
});

// SAFE: Server-side price lookup
app.post('/checkout', (req, res) => {
  const total = await Promise.all(req.body.items.map(async item => {
    const product = await Product.findById(item.productId);
    return product.price * item.quantity;
  })).then(prices => prices.reduce((a, b) => a + b, 0));
});

2. Negative Quantity/Amount:

# VULNERABLE: No negative check
def transfer(request):
    amount = int(request.POST['amount'])  # amount = -1000
    sender.balance -= amount   # Adds 1000!
    receiver.balance += amount  # Subtracts 1000!

# SAFE
amount = int(request.POST['amount'])
if amount <= 0:
    raise ValidationError("Amount must be positive")

3. Workflow Step Bypass:

// VULNERABLE: Can skip directly to confirmation without payment
app.post('/order/confirm', auth, (req, res) => {
  Order.update(req.body.orderId, { status: 'confirmed' });
});

// SAFE: Verify payment was completed
app.post('/order/confirm', auth, async (req, res) => {
  const order = await Order.findById(req.body.orderId);
  if (order.status !== 'paid') {
    return res.status(400).json({ error: 'Payment required' });
  }
  order.status = 'confirmed';
  await order.save();
});

4. Coupon/Discount Abuse: Check for: single-use coupons applied multiple times, percentage discounts exceeding 100%, stacking multiple exclusive coupons, applying expired coupons.

5. Account Enumeration: Different responses for "user not found" vs "wrong password" reveals account existence.

Severity Classification

Critical: Price manipulation allowing free purchases, negative amount enabling fund theft
High: Workflow bypass skipping payment, unlimited coupon use
Medium: Account enumeration, minor discount abuse
Low: Referral gaming, cosmetic logic flaws

Output Format

Finding: BIZ-{NNN}

Title: {Business logic flaw type}
Severity: Critical | High | Medium | Low
Confidence: 0-100
File: file/path:line
Vulnerability Type: CWE-840 (Business Logic Errors)
Description: {What was found and how it violates intended business rules}
Impact: Financial loss, service abuse, competitive advantage.
Remediation: {Server-side validation, workflow state machine, invariant checks}
References: https://cwe.mitre.org/data/definitions/840.html

Common False Positives

Admin override capabilities — admin users legitimately bypassing restrictions
Internal tools — backend tools with intentionally relaxed validation
Test/staging environments — relaxed limits for testing purposes