CVE-2026-13603 in pretix-oppwa
Summary
by MITRE • 07/01/2026
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.
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Analysis
by VulDB Data Team • 07/01/2026
The vulnerability in the pretix-oppwa payment integration represents a critical insecure direct object reference flaw that enables unauthorized access to sensitive payment data through improper input validation. This issue specifically affects the integration with VR Payment, Hobex, and other Oppwa-based payment providers where the system processes redirect URLs containing resourcePath parameters. The technical implementation failed to properly sanitize or validate URL components before concatenating them with the base API endpoint, creating a path traversal condition that allows attackers to manipulate the target server address.
The flaw stems from a lack of proper input validation and sanitization mechanisms within the URL construction process. When the system receives a redirect from payment providers containing query parameters such as ?resourcePath=/v1/checkouts/{checkoutId}/payment, it directly appends this path to the base API URL without ensuring proper delimiter handling or parameter validation. This omission becomes particularly dangerous when the base URL lacks a trailing forward slash, creating an injection vector that allows attackers to override the intended API endpoint and redirect requests to arbitrary servers.
The security implications of this vulnerability are severe as it directly exposes the access token mechanism used for authentication with Oppwa payment providers. The compromised access tokens grant full access to sensitive transaction data, customer information, and financial records stored within the payment provider's systems. This represents a classic case of credential leakage through improper URL handling that aligns with CWE-20 (Improper Input Validation) and CWE-611 (Improper Restriction of XML External Entity Reference). The vulnerability also maps to ATT&CK technique T1566.001 (Phishing via Social Media) and T1071.004 (Application Layer Protocol: DNS) when attackers leverage manipulated redirect URLs to exfiltrate credentials.
The exploitation process involves an attacker manipulating the resourcePath parameter in the redirect URL to construct a malicious API request that bypasses the intended server and targets an attacker-controlled endpoint. This allows for credential theft, data exfiltration, and potential financial fraud. The vulnerability demonstrates poor secure coding practices related to URL construction and input validation, with the fix requiring strict validation of API endpoints before URL concatenation operations.
The recommended remediation approach involves implementing proper URL validation mechanisms that ensure all resource paths are properly sanitized and that base URLs maintain consistent formatting including trailing delimiters. Organizations should implement a comprehensive security update process that includes immediate access token rotation following deployment of patches, as well as ongoing monitoring for unauthorized access attempts. The solution must enforce strict parameter validation and implement proper input sanitization to prevent similar injection attacks in future implementations. Additionally, regular security assessments should validate URL handling mechanisms across all payment integration components to ensure consistent application of secure coding practices throughout the system architecture.