The payment integration pretix-oppwa provides support for the payment providers VR Payment, Hobex, and potentially others based on Oppwa's

The payment integration pretix-oppwa provides support
for the payment providers VR Payment, Hobex, and potentially others
based on Oppwa's technology. The integration of Oppwa, following their
official documentation, includes a step where the user is redirected
from the payment provider back to our system with a query parameter like
?resourcePath=/v1/checkouts/{checkoutId}/payment in the URL. Our system is then supposed to fetch the status of the transaction from the URL given by baseUrl + resourcePath.

Our plugin pretix-oppwa did so insecurely by
concatenating the parameter form the URL to the base domain of the API
without further validation and, critically, without a / at the end of the baseUrl. Therefore, an attacker could inject a resourcePath argument in a way that causes pretix to call a different
server instead. Since the request includes the access token (API key)
of the Oppwa account, this would leak the access token, giving access to
data contained in the payment provider's system. This is fixed with the
release today by strictly validating the given API URL.

After installing the update, we recommend asking your payment provider for a new access token and updating it in pretix.