Fix double refund race condition vulnerability

[ArnavBallinCode]

Fix Double Refund Race Condition Vulnerability

Summary

Fixes a security vulnerability where concurrent refund requests could refund more money than the original payment amount, resulting in direct financial loss to event organizers.

How It Works

1. First thread acquires exclusive lock on payment row
2. Second thread waits for lock to be released
3. First thread processes refund and commits transaction (releases lock)
4. Second thread acquires lock, sees updated refunded_amount, rejects request
5. Result: Only valid refunds are processed ✓

Tested

fixes #1405

Summary by Sourcery

Prevent concurrent refund race conditions that could allow refunds exceeding the original payment.

Bug Fixes:

• Ensure refunds are processed within a database transaction with row-level locking to avoid double refunds under concurrent requests.

Enhancements:

• Clarify refund logic by explicitly computing the available refundable amount while the payment row is locked.

[sourcery-ai]

Reviewer's guide (collapsed on small PRs)

Reviewer's Guide

Adds transactional row-level locking around the order refund endpoint to prevent a double-refund race condition by ensuring refund calculations and validations occur under an exclusive database lock.

Sequence diagram for concurrent refund requests with row-level locking

sequenceDiagram
    autonumber
    actor User1
    actor User2
    participant API1 as OrderViewSet_refund_Thread1
    participant API2 as OrderViewSet_refund_Thread2
    participant DB as Database

    User1->>API1: POST /orders/{id}/refund
    User2->>API2: POST /orders/{id}/refund

    API1->>DB: BEGIN TRANSACTION
    API1->>DB: SELECT payment FOR UPDATE
    Note over DB: Row-level lock acquired by API1

    API2->>DB: BEGIN TRANSACTION
    API2->>DB: SELECT payment FOR UPDATE (blocks)

    API1->>API1: available_amount = amount - refunded_amount
    API1->>API1: validate refund <= available_amount
    API1->>DB: UPDATE payment (refunded_amount += refund)
    API1->>DB: COMMIT (releases row lock)
    API1-->>User1: 200 OK refund processed

    DB-->>API2: SELECT payment FOR UPDATE (lock acquired)
    API2->>API2: available_amount = updated_amount - updated_refunded_amount
    API2->>API2: validate refund <= available_amount (fails)
    API2-->>User2: 400 Bad Request refund exceeds available_amount
    API2->>DB: ROLLBACK

Loading

Flow diagram for secured refund endpoint with transactional locking

flowchart TD
    A[Refund endpoint called
OrderViewSet_refund] --> B[Start transaction
transaction.atomic]
    B --> C[Load payment
OrderPayment.select_for_update]
    C --> D[Parse refund amount
from request]
    D --> E[Check provider:
full_refund_possible]
    E --> F[Check provider:
partial_refund_possible]
    F --> G[Compute available_amount = payment.amount - payment.refunded_amount
under row lock]
    G --> H{Is amount <= 0?}
    H -->|Yes| Z1[Return error
invalid amount]
    H -->|No| I{Is amount > available_amount?}
    I -->|Yes| Z2[Return error
refund exceeds available]
    I -->|No| J[Process refund via provider]
    J --> K[Update payment.refunded_amount
and persist]
    K --> L[Commit transaction]
    L --> M[Return success]

Loading

File-Level Changes

Change	Details	Files
Protect refund logic with a database transaction and row-level locking to avoid concurrent over-refunds.	Wrap the refund action in a Django atomic transaction to ensure all refund operations run in a single, isolated unit of work. Replace the plain object retrieval in the refund action with a select_for_update() query on OrderPayment to acquire a row-level lock for the duration of the transaction. Compute the available refundable amount (amount - refunded_amount) while holding the lock, ensuring the validation uses up-to-date, consistent values.	app/eventyay/api/views/order.py

Assessment against linked issues

Issue	Objective	Addressed	Explanation
#1405	Prevent concurrent refund requests for the same payment from creating refunds whose total exceeds the original payment amount (fix the race condition vulnerability).	✅

Possibly linked issues

• Bug: Concurrent Refund Requests Can Exceed Payment Amount #1405: PR adds atomic transaction and select_for_update lock to stop concurrent refunds exceeding payment amount, fixing issue.

---

Tips and commands

Interacting with Sourcery

• Trigger a new review: Comment @sourcery-ai review on the pull request.
• Continue discussions: Reply directly to Sourcery's review comments.
• Generate a GitHub issue from a review comment: Ask Sourcery to create an
  issue from a review comment by replying to it. You can also reply to a
  review comment with @sourcery-ai issue to create an issue from it.
• Generate a pull request title: Write @sourcery-ai anywhere in the pull
  request title to generate a title at any time. You can also comment
  @sourcery-ai title on the pull request to (re-)generate the title at any time.
• Generate a pull request summary: Write @sourcery-ai summary anywhere in
  the pull request body to generate a PR summary at any time exactly where you
  want it. You can also comment @sourcery-ai summary on the pull request to
  (re-)generate the summary at any time.
• Generate reviewer's guide: Comment @sourcery-ai guide on the pull
  request to (re-)generate the reviewer's guide at any time.
• Resolve all Sourcery comments: Comment @sourcery-ai resolve on the
  pull request to resolve all Sourcery comments. Useful if you've already
  addressed all the comments and don't want to see them anymore.
• Dismiss all Sourcery reviews: Comment @sourcery-ai dismiss on the pull
  request to dismiss all existing Sourcery reviews. Especially useful if you
  want to start fresh with a new review - don't forget to comment
  @sourcery-ai review to trigger a new review!

Customizing Your Experience

Access your dashboard to:

• Enable or disable review features such as the Sourcery-generated pull request
  summary, the reviewer's guide, and others.
• Change the review language.
• Add, remove or edit custom review instructions.
• Adjust other review settings.

Getting Help

• Contact our support team for questions or feedback.
• Visit our documentation for detailed guides and information.
• Keep in touch with the Sourcery team by following us on X/Twitter, LinkedIn or GitHub.

[sourcery-ai]

Hey there - I've reviewed your changes - here's some feedback:

• Instead of calling self.get_object() and then re-querying with OrderPayment.objects.select_for_update(), consider using self.get_queryset().select_for_update().get(pk=...) to avoid the extra query and ensure any view-level filtering or permission constraints are preserved.
• Since @transaction.atomic is now applied at the view level, double-check whether any upstream code wraps this call in another atomic block; if so, you may want to use atomic(savepoint=False) or move the locking logic closer to the model/service layer to avoid unnecessary nesting and potential deadlocks.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- Instead of calling `self.get_object()` and then re-querying with `OrderPayment.objects.select_for_update()`, consider using `self.get_queryset().select_for_update().get(pk=...)` to avoid the extra query and ensure any view-level filtering or permission constraints are preserved.
- Since `@transaction.atomic` is now applied at the view level, double-check whether any upstream code wraps this call in another atomic block; if so, you may want to use `atomic(savepoint=False)` or move the locking logic closer to the model/service layer to avoid unnecessary nesting and potential deadlocks.

## Individual Comments

### Comment 1
<location> `app/eventyay/api/views/order.py:1252-1253` </location>
<code_context>
+    @transaction.atomic
     def refund(self, request, **kwargs):
-        payment = self.get_object()
+        # Acquire row-level lock on payment to prevent concurrent refund race conditions
+        payment = OrderPayment.objects.select_for_update().get(pk=self.get_object().pk)
+        
         amount = serializers.DecimalField(max_digits=10, decimal_places=2).to_internal_value(
</code_context>

<issue_to_address>
**suggestion:** Avoid the double query and consider integrating select_for_update with get_object to reduce overhead.

This pattern does two lookups: `self.get_object()` and then `OrderPayment.objects.select_for_update().get(...)`, adding overhead and touching the row once before it’s locked. To avoid this, consider either (a) adding `select_for_update()` in `get_queryset()` when locking is needed, or (b) introducing a `get_locked_object()` helper that reuses `get_object()`’s filtering/permission logic but issues a single `SELECT ... FOR UPDATE` query.

Suggested implementation:

```python
    @action(detail=True, methods=['POST'])
    @transaction.atomic
    def refund(self, request, **kwargs):
        # Acquire row-level lock on payment to prevent concurrent refund race conditions.
        # We reuse DRF's filtering & permission logic while issuing a single SELECT ... FOR UPDATE.
        queryset = self.filter_queryset(self.get_queryset().select_for_update())

        lookup_url_kwarg = self.lookup_url_kwarg or self.lookup_field
        lookup_value = self.kwargs[lookup_url_kwarg]
        payment = get_object_or_404(queryset, **{self.lookup_field: lookup_value})

        amount = serializers.DecimalField(max_digits=10, decimal_places=2).to_internal_value(

```

1. Ensure `get_object_or_404` is imported at the top of `app/eventyay/api/views/order.py`, for example:
   `from rest_framework.generics import get_object_or_404`
   If it is already imported elsewhere in the file, no further import changes are required.
2. This code assumes the view class defines `lookup_field` / `lookup_url_kwarg` in the standard DRF way. If the lookup is customized (e.g., using a different kwarg name or composite key), adjust the lookup construction accordingly.
</issue_to_address>

---

Sourcery is free for open source - if you like our reviews please consider sharing them ✨

• X
• Mastodon
• LinkedIn
• Facebook

_{Help me be more useful! Please click 👍 or 👎 on each comment and I'll use the feedback to improve your reviews.}

[Copilot]

Pull request overview

This PR fixes a critical race condition vulnerability in the refund endpoint that could allow multiple concurrent refund requests to process more money than the original payment amount, leading to financial loss for event organizers.

Key Changes:

• Added database transaction with row-level locking using @transaction.atomic and select_for_update() to prevent concurrent refund processing
• Implemented proper TOCTOU (Time-Of-Check-Time-Of-Use) protection by calculating available refund amount under lock
• Added clear documentation comments explaining the race condition prevention mechanism

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[Sak1012]

Please rebase and update the PR

[Copilot]

Pull request overview

Copilot reviewed 1 out of 1 changed files in this pull request and generated 3 comments.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[ArnavBallinCode]

@Sak1012 added tests in the pr #1498

[hongquan]

Putting transaction.atomic here doesn't work, because this refund function already handle an PaymentException:

try:
    r.payment_provider.execute_refund(r)
except PaymentException as e:

Read the Avoid catching exceptions inside atomic! note.