[Security] Payment amount validation missing server-side - client can manipulate payment values to underpay for AI services

[anshul23102]

Problem

In a micropayment architecture, the payment amount must be validated server-side against the service's defined price. If the server accepts whatever amount the client claims to have paid, a client can send a 0-value payment and still access the AI service:

// Vulnerable - trusts client-supplied amount:
func handlePayment(w http.ResponseWriter, r *http.Request) {
    var req PaymentRequest
    json.NewDecoder(r.Body).Decode(&req)
    // No check that req.Amount matches the service price
    if verifyTransaction(req.TxHash) {
        serveAIResponse(w, req.Prompt)  // served regardless of amount paid
    }
}

Proposed Fix

Look up the service price from a trusted source (config or database) and validate the on-chain amount matches:

type ServiceConfig struct {
    PriceUSDC  int64  // price in micro-USDC (6 decimals)
    ServiceID  string
}

func handlePayment(w http.ResponseWriter, r *http.Request, config ServiceConfig) {
    var req PaymentRequest
    if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
        http.Error(w, "Invalid request", http.StatusBadRequest)
        return
    }

    // Verify on-chain: get actual transferred amount from the blockchain
    actualAmount, err := getOnChainTransferAmount(req.TxHash, config.ServiceID)
    if err != nil {
        http.Error(w, "Payment verification failed", http.StatusPaymentRequired)
        return
    }

    // Validate amount meets minimum price:
    if actualAmount < config.PriceUSDC {
        http.Error(w, fmt.Sprintf("Insufficient payment. Required: %d, Received: %d",
            config.PriceUSDC, actualAmount), http.StatusPaymentRequired)
        return
    }

    serveAIResponse(w, req.Prompt)
}

Acceptance Criteria

• Payment amount verified against on-chain data, not client-supplied value
• Service price defined server-side in config, not derived from request
• Underpayment returns 402 Payment Required with the expected amount
• Amount tolerance (for gas fluctuations) configurable but defaults to 0
• Test case verifies that a 0-value transaction is rejected

[anshul23102]

Hi @AnkanMisra, I'd like to work on this issue as part of GSSoC 2026. I'll add server-side validation of payment amounts against the expected service price to prevent clients from manipulating payment values to underpay. Could you please assign it to me?

/assign
/claim

[Mohitmhatre32]

/assign

[github-actions]

Assigned @Mohitmhatre32.

[anshul23102]

@AnkanMisra

I'd like to work on implementing server-side payment amount validation to prevent underpayment exploits as part of GSSoC 2026.

Implementation Details:

• Verify payment amount against on-chain data, not client-supplied values
• Define service price server-side in configuration
• Return 402 Payment Required for underpayment with expected amount
• Implement configurable amount tolerance for gas fluctuations (default: 0)
• Add test case verifying 0-value transactions are rejected
• Validate actual transferred amount from blockchain

Please assign this issue to me. For GSSoC 2026 tracking, could you also add the gssoc-approved label?

/assign
/claim