CVE-2026-102877 in Fider
Summary
by MITRE • 09/29/2026
Fider before 0.38.0 contains a server-side request forgery vulnerability due to a time-of-check time-of-use gap in URL validation for webhooks and custom OAuth provider endpoints. Administrators controlling DNS can perform DNS rebinding attacks to make the Fider server send requests to internal services or cloud metadata endpoints.
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Analysis
by VulDB Data Team • 09/29/2026
The identified security flaw resides within versions of the Fider application prior to release 0.38.0, specifically affecting its handling of webhook configurations and custom OAuth provider endpoint definitions. This vulnerability is classified as a Server-Side Request Forgery, which allows an attacker to induce the server-side application to make HTTP requests to arbitrary destinations under specific conditions. The root cause of this issue lies in a time-of-check-to-time-of-use race condition inherent in the URL validation logic employed by the system. When administrators configure these endpoints, the application performs an initial check on the provided Uniform Resource Locator to ensure it adheres to expected protocols and domains. However, due to the asynchronous nature of network operations or potential delays in DNS resolution processes, there exists a temporal gap between this initial validation step and the actual execution of the HTTP request.
This architectural weakness enables sophisticated attacks such as DNS rebinding. In a typical scenario involving DNS rebinding, an attacker controls a domain name that resolves to a legitimate external IP address during the initial check phase, thereby passing the application's security filters. Subsequently, before or during the time when Fider attempts to send the actual request, the attacker modifies the DNS records for their controlled domain to resolve instead to an internal private IP address or a cloud metadata endpoint. Because the validation has already passed and the connection is being established based on the now-changed DNS record, the server successfully connects to resources that are normally inaccessible from the public internet. This bypasses standard network perimeter defenses because the request appears to originate from within the trusted environment of the Fider deployment.
The operational impact of this vulnerability is significant for organizations relying on Fider for community feedback and authentication management. An attacker with control over DNS infrastructure can exploit this flaw to access internal services, such as database interfaces, administrative panels, or other microservices running on private networks behind firewalls that would otherwise block external traffic. Furthermore, the ability to reach cloud metadata endpoints poses a severe risk in containerized or virtualized environments. By accessing these endpoints, an attacker may retrieve instance identity credentials, IAM roles, and other sensitive configuration data stored by the underlying infrastructure provider. This can lead to full compromise of the host environment, lateral movement across internal networks, and unauthorized access to critical business applications that are not exposed publicly but are reachable from within the network segment where Fider is hosted.
To mitigate this risk, organizations running versions of Fider earlier than 0.38.0 must upgrade immediately to a patched version where the URL validation logic has been hardened against such race conditions and DNS rebinding techniques. In addition to upgrading software, administrators should implement strict allow-listing for outbound connections from the application server. This involves configuring network firewalls or egress filtering rules to permit HTTP traffic only to explicitly trusted domains required for webhook integrations and OAuth providers. It is also advisable to disable any functionality that allows dynamic configuration of external endpoints unless absolutely necessary, thereby reducing the attack surface. For environments where DNS resolution cannot be fully controlled, implementing local host files with static IP mappings can prevent the manipulation of domain resolutions during the critical window between validation and execution.
From a classification perspective, this vulnerability aligns with CWE-918, which describes Server-Side Request Forgery flaws resulting from insufficient URL validation. The exploitation technique leverages DNS rebinding, a method often associated with CWE-20 Improper Input Validation when applied to network protocols. In the context of adversary tactics and techniques, this scenario maps closely to ATT&CK T1598, specifically Phishing for Information within Cloud Infrastructure or Attacker-in-the-Middle variants that exploit trust relationships. The ability to pivot from a web application into internal infrastructure highlights the importance of treating outbound requests with the same level of scrutiny as inbound traffic. Security teams should audit their current configurations to ensure no other components suffer from similar time-of-check-to-time-of-use vulnerabilities in URL parsing or DNS resolution logic, ensuring that validation occurs at multiple layers including network, host, and application levels to provide defense in depth against such sophisticated bypasses.