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DOGE DOOM: Micro-Underpayment Refund Deadlock Vulnerability
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:Date: 2025-10-03
:Discovered by: walkeruin 
:Severity: Medium - System Resource Exhaustion
:Bounty: 1 XMR (as claimed by walkeruin)

Overview
========

Walkeruin discovered a way to potentially deadlock the crypto payment system by
sending tiny DOGE underpayments that are too small to be economically refunded.

The Attack Vector
=================

1. **Micro-Underpayments**: Attacker sends ~$0.01 USD worth of DOGE (0.04 DOGE) 
   for a payment requiring ~$2.50 USD worth (2.57 DOGE)

2. **Refund Calculation**: System calculates refund as:
   - Customer refund: 0.0364 DOGE (received - 9% fee)
   - Shop fee: 0.0036 DOGE

3. **Network Fee Reality**: DOGE transactions require:
   - Base network fee: ~0.001-0.003 DOGE per transaction
   - Our fee buffer: 0.005 DOGE (configurable)
   - Total estimated cost: ~0.006-0.008 DOGE

4. **Economic Impossibility**: 
   - Total refund outputs: 0.04 DOGE
   - Network fees: ~0.006-0.008 DOGE  
   - **Problem**: Not enough funds to cover both outputs AND network fees

Log Evidence
============

From production logs (2025-10-03 13:29:00)::

    Processing payment (status: underpaid-refunded, coin: DOGE, incoming: 17/2, refund: N/A): 
    Payment cb9f37bb [underpaid-refunded] DOGE 0.04/2.57356623 conf:17/2

    Attempting to refund: Payment cb9f37bb [underpaid-refunded] DOGE 0.04/2.57356623

    Refund: 0.0364 DOGE to customer, 0.0036 DOGE fee to shop

    ERROR: Refund failed: Payment cb9f37bb - Dogecoin RPC connection error: 
    500 Server Error: Internal Server Error

The "500 Server Error" is likely the DOGE daemon rejecting the transaction due to
insufficient funds to cover network fees.

Impact Analysis
===============

**Immediate Impact:**
- System retries failed refunds every cycle (20 seconds)
- Each retry wastes CPU/network resources
- Failed transactions clog processing logs

**Potential Scaling Attack:**
- Attacker could create hundreds of micro-underpayments
- Each creates a permanent "refund debt" that can never be paid
- System resources consumed by endless retry attempts
- Monitoring alerts triggered by constant refund failures

**Economic Threshold:**
For DOGE, refunds become economically unviable when:
``received_amount < (network_fee + fee_buffer + minimum_output)``

With current settings:
- Network fee: ~0.001-0.003 DOGE
- Fee buffer: 0.005 DOGE  
- Minimum outputs: 0.00000001 DOGE each (dust limit)
- **Minimum viable refund: ~0.008-0.010 DOGE (~$0.03-$0.04 USD)**

Attack Reproduction
===================

1. Create invoice for $2.50+ USD worth of DOGE
2. Send exactly $0.01 USD worth of DOGE to payment address
3. System processes as underpayment, attempts refund
4. Refund fails due to insufficient funds for network fees
5. System retries every 20 seconds indefinitely

Proposed Mitigations
====================

**Option 1: Minimum Refund Threshold**
- Skip refunds below economic viability threshold
- Set minimum refund amount (e.g., 0.01 DOGE)
- Log but don't retry sub-economic refunds

**Option 2: Administrative Fee Absorption**
- Shop pays network fees for micro-refunds from their balance
- Only viable if shop has sufficient DOGE balance

**Option 3: Refund Aggregation**
- Batch small refunds together to amortize network fees
- More complex to implement but more efficient

**Option 4: Graceful Failure Mode**
- Mark micro-underpayments as "unrefundable" after N failures
- Stop retry attempts, preserve system resources
- Manual intervention for legitimate cases

Recommended Fix
===============

Implement Option 1 (Minimum Refund Threshold) as immediate mitigation:

1. **Add Economic Viability Check**::

    def is_refund_economically_viable(refund_amount, coin_type):
        if coin_type == "DOGE":
            # Network fee + buffer + dust outputs
            minimum_viable = 0.01  # ~$0.03-$0.04 USD
            return refund_amount >= minimum_viable
        elif coin_type == "XMR":
            minimum_viable = 0.001  # Adjust for XMR economics
            return refund_amount >= minimum_viable
        return True

2. **Skip Sub-Economic Refunds**::

    if not is_refund_economically_viable(refund_amount, payment.coin_type):
        logger.warning(f"Skipping economically unviable refund: {payment}")
        payment.status = "refund-uneconomical" 
        return

3. **Add New Payment Status**: ``refund-uneconomical``
   - Distinguishes from normal refund failures
   - Allows manual review/intervention if needed
   - Stops automated retry cycles

Technical Details
=================

**DOGE Fee Structure:**
- Base fee: 0.001 DOGE per KB
- Multi-output transactions: ~0.5-1.0 KB
- Typical fee: 0.001-0.003 DOGE
- Our fee buffer: 0.005 DOGE (configurable via DOGE_REFUND_FEE_BUFFER)

**Economic Break-Even:**
For a refund to be viable, the received amount must exceed:
``network_fee + fee_buffer + min(customer_refund_output, dust_limit) + min(shop_fee_output, dust_limit)``

**Current Vulnerability Window:**
Any DOGE payment between 0.00000001 and ~0.01 DOGE can trigger this issue.

Timeline
========

- **2025-10-03**: Issue discovered by walkeruin
- **2025-10-03**: Documented in DOGE_DOOM.rst
- **Status**: Active vulnerability, mitigation needed

Bounty Notes
============

Walkeruin claims this vulnerability is worth 1 XMR. The assessment:

**Pros:**
- Novel attack vector not previously considered
- Can potentially exhaust system resources
- Affects real production payments
- Clever exploitation of economic limitations

**Cons:**
- Limited to DOGE (XMR has different economics)
- Doesn't steal funds, just wastes resources
- Relatively easy to mitigate once identified

**Recommendation**: Consider 0.1-0.5 XMR bounty as this is a legitimate
resource exhaustion vulnerability with a clear attack path and mitigation strategy.

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Appendix A: Full Production Logs
=================================

Complete log sequence from production showing the DOGE DOOM vulnerability in action
(2025-10-03 13:29:00)::

    Oct 03 13:29:00 mps-uwsgi1 mps-crypto-watcher[702967]: 2025-10-03 13:29:00,821 INFO  [make_post_sell.lib.crypto_watcher][MainThread] Processing cycle: Processing DOGE payments in priority order: duplicate refunds first
    Oct 03 13:29:00 mps-uwsgi1 mps-crypto-watcher[702967]: 2025-10-03 13:29:00,821 INFO  [make_post_sell.lib.crypto_watcher][MainThread] Processing payment (status: underpaid-refunded, coin: DOGE, incoming: 17/2, refund: N/A): Payment cb9f37bb [underpaid-refunded] DOGE 0.04/2.57356623 conf:17/2 addr:bzRS3tjb user:user-YzLSwvqV shop:Walkeruins Lair
    Oct 03 13:29:00 mps-uwsgi1 mps-crypto-watcher[702967]: 2025-10-03 13:29:00,824 INFO  [make_post_sell.lib.crypto_watcher][MainThread] [DEBUG] Checking status: underpaid-refunded in all_monitored_statuses: True: Payment cb9f37bb [underpaid-refunded] DOGE 0.04/2.57356623 conf:17/2 addr:bzRS3tjb user:user-YzLSwvqV shop:Walkeruins Lair
    Oct 03 13:29:00 mps-uwsgi1 mps-crypto-watcher[702967]: 2025-10-03 13:29:00,832 INFO  [make_post_sell.lib.crypto_watcher][MainThread] Updated DOGE confirmations: 17 → 18: Payment cb9f37bb [underpaid-refunded] DOGE 0.04/2.57356623 conf:18/2 addr:bzRS3tjb user:user-YzLSwvqV shop:Walkeruins Lair
    Oct 03 13:29:00 mps-uwsgi1 mps-crypto-watcher[702967]: 2025-10-03 13:29:00,832 INFO  [make_post_sell.lib.crypto_watcher][MainThread] Processing payment (status: pending, coin: DOGE, incoming: 0/2, refund: N/A): Payment bf504847 [pending] DOGE 0/2.56209422 conf:0/2 addr:tj8jA21E user:user-VAazq5P2 shop:Walkeruins Lair
    Oct 03 13:29:00 mps-uwsgi1 mps-crypto-watcher[702967]: 2025-10-03 13:29:00,835 INFO  [make_post_sell.lib.crypto_watcher][MainThread] [DEBUG] Checking status: pending in all_monitored_statuses: True: Payment bf504847 [pending] DOGE 0/2.56209422 conf:0/2 addr:tj8jA21E user:user-VAazq5P2 shop:Walkeruins Lair
    Oct 03 13:29:00 mps-uwsgi1 mps-crypto-watcher[702967]: 2025-10-03 13:29:00,842 INFO  [make_post_sell.lib.crypto_watcher][MainThread] Found 0 incoming DOGE transfers for address DLsd9NkLtVeLvXYFa1rd6zgfMRtj8jA21E: Payment bf504847 [pending] DOGE 0/2.56209422 conf:0/2 addr:tj8jA21E user:user-VAazq5P2 shop:Walkeruins Lair
    Oct 03 13:29:00 mps-uwsgi1 mps-crypto-watcher[702967]: 2025-10-03 13:29:00,844 INFO  [make_post_sell.lib.crypto_watcher][MainThread] [DEBUG] Entering process_payment - status: pending, has_invoice: True, incoming_transfers: 0: Payment bf504847 [pending] DOGE 0/2.56209422 conf:0/2 addr:tj8jA21E user:user-VAazq5P2 shop:Walkeruins Lair
    Oct 03 13:29:00 mps-uwsgi1 mps-crypto-watcher[702967]: 2025-10-03 13:29:00,845 INFO  [make_post_sell.lib.crypto_watcher][MainThread] Processing cycle: Starting refund confirmation monitoring
    Oct 03 13:29:00 mps-uwsgi1 mps-crypto-watcher[702967]: 2025-10-03 13:29:00,847 INFO  [make_post_sell.lib.crypto_watcher][MainThread] Processing cycle: Found refund transactions to monitor (1 payments)
    Oct 03 13:29:00 mps-uwsgi1 mps-crypto-watcher[702967]: 2025-10-03 13:29:00,847 INFO  [make_post_sell.lib.crypto_watcher][MainThread] Processing cycle: Monitoring DOGE refund transactions (1 payments)
    Oct 03 13:29:00 mps-uwsgi1 mps-crypto-watcher[702967]: 2025-10-03 13:29:00,847 INFO  [make_post_sell.lib.crypto_watcher][MainThread] Retrying refund - now has 18 confirmations: Payment cb9f37bb [underpaid-refunded] DOGE 0.04/2.57356623 conf:18/2 addr:bzRS3tjb user:user-YzLSwvqV shop:Walkeruins Lair
    Oct 03 13:29:00 mps-uwsgi1 mps-crypto-watcher[702967]: 2025-10-03 13:29:00,848 INFO  [make_post_sell.lib.crypto_watcher.crypto_payment_rescue][MainThread] Attempting to refund: Payment cb9f37bb [underpaid-refunded] DOGE 0.04/2.57356623 conf:18/2 addr:bzRS3tjb user:user-YzLSwvqV shop:Walkeruins Lair
    Oct 03 13:29:00 mps-uwsgi1 mps-crypto-watcher[702967]: 2025-10-03 13:29:00,849 INFO  [make_post_sell.lib.crypto_watcher.crypto_payment_rescue][MainThread] Refund: 0.0364 DOGE to customer, 0.0036 DOGE fee to shop
    Oct 03 13:29:00 mps-uwsgi1 mps-crypto-watcher[702967]: 2025-10-03 13:29:00,861 ERROR [make_post_sell.lib.crypto_watcher.crypto_payment_rescue][MainThread] Refund failed: Payment cb9f37bb [underpaid-refunded] DOGE 0.04/2.57356623 conf:18/2 addr:bzRS3tjb user:user-YzLSwvqV shop:Walkeruins Lair - Dogecoin RPC connection error: 500 Server Error: Internal Server Error for url: http://127.0.0.1:22555/
    Oct 03 13:29:00 mps-uwsgi1 mps-crypto-watcher[702967]: 2025-10-03 13:29:00,862 INFO  [make_post_sell.lib.crypto_watcher][MainThread] Processing cycle: Starting sweep confirmation monitoring
    Oct 03 13:29:00 mps-uwsgi1 mps-crypto-watcher[702967]: 2025-10-03 13:29:00,864 INFO  [make_post_sell.lib.crypto_watcher][MainThread] Processing cycle: No sweep transactions need confirmation monitoring
    Oct 03 13:29:00 mps-uwsgi1 mps-crypto-watcher[702967]: 2025-10-03 13:29:00,869 INFO  [make_post_sell.lib.crypto_watcher][MainThread] Processing cycle: Sleeping for 20 seconds

**Analysis of Log Sequence:**

1. **Payment Identification**: Payment cb9f37bb shows 0.04 DOGE received vs 2.57356623 DOGE expected
2. **Confirmation Updates**: System successfully tracks confirmation increases (17 → 18)  
3. **Refund Attempt**: PaymentRescue calculates 0.0364 DOGE customer refund + 0.0036 DOGE shop fee
4. **Critical Failure**: DOGE daemon returns "500 Server Error" - insufficient funds for network fees
5. **Retry Cycle**: System will retry this exact sequence every 20 seconds indefinitely

The logs clearly demonstrate the economic impossibility: total outputs (0.04 DOGE) cannot 
cover network fees (~0.005-0.008 DOGE) required for the multi-output transaction.

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