CVE-2026-73501 in kin-openapi
Summary
by MITRE • 08/13/2026
kin-openapi is a Go project for handling OpenAPI files. Prior to 0.144.0, ValidationHandler.Load() in openapi3filter/validation_handler.go silently replaces a nil AuthenticationFunc with NoopAuthenticationFunc, which returns nil without checking credentials. This substitution causes every OpenAPI security requirement to be satisfied for unauthenticated requests when an application relies on ValidationHandler as its enforcement middleware. The no-op callback prevents the fail-closed ErrAuthenticationServiceMissing path from being reached and forwards the request to protected handlers that may require an API key, OAuth token, or another security scheme. This issue is fixed in version 0.144.0.
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Analysis
by VulDB Data Team • 08/13/2026
The vulnerability in kin-openapi affects the OpenAPI validation handler component that processes security requirements for API endpoints. This flaw exists in versions prior to 0.144.0 where the ValidationHandler.Load() method performs an implicit substitution of nil AuthenticationFunc instances with a NoopAuthenticationFunc implementation. The technical mechanism behind this vulnerability involves a silent defaulting behavior that bypasses proper authentication enforcement mechanisms within the OpenAPI validation pipeline. When developers do not explicitly define authentication functions for their security schemes, the system automatically substitutes these nil references with a no-operation function that unconditionally returns nil without performing any credential verification.
The operational impact of this vulnerability creates a significant security risk by enabling unauthorized access to protected API endpoints. The NoopAuthenticationFunc implementation effectively neutralizes all authentication requirements defined in OpenAPI specifications, allowing malicious actors to bypass security controls entirely. This behavior transforms legitimate security enforcement mechanisms into ineffective pass-through filters that forward requests to downstream handlers regardless of authentication status. The vulnerability specifically impacts applications that rely on ValidationHandler as their primary enforcement middleware, creating a dangerous scenario where API endpoints configured with security requirements become accessible without proper authentication. This issue directly violates the fail-closed principle that should govern security systems, as it prevents the proper ErrAuthenticationServiceMissing error path from being triggered and instead allows unauthorized requests to proceed.
The flaw manifests as a design weakness in the validation handler's initialization process and demonstrates poor adherence to secure coding practices. According to CWE-254, this vulnerability represents a weakness in authentication mechanisms where security controls are bypassed due to improper default handling. The issue also aligns with ATT&CK technique T1078 which involves valid accounts being used for unauthorized access, as the system effectively allows unauthorized access by treating all requests as authenticated through the no-op substitution. The vulnerability creates an implicit trust relationship that should not exist between unauthenticated requests and protected API resources, fundamentally undermining the security posture of applications using affected versions of kin-openapi.
Organizations using affected versions of kin-openapi should immediately upgrade to version 0.144.0 or later to remediate this vulnerability. The fix ensures proper authentication enforcement by eliminating the automatic substitution of nil AuthenticationFunc instances with no-operation implementations. Additionally, developers should review their OpenAPI specifications to ensure explicit authentication function definitions are provided for all security requirements, rather than relying on default behaviors. Security monitoring should be enhanced to detect unauthorized access patterns that might indicate exploitation attempts, and regular vulnerability assessments should verify that all components in the API stack maintain proper authentication enforcement mechanisms. The mitigation strategy must also include code reviews to prevent similar implicit behavior patterns that could introduce additional security vulnerabilities in the system architecture.
This vulnerability highlights the critical importance of explicit security control implementation in API gateways and validation middleware. The silent nature of the substitution makes this particularly dangerous as it can remain undetected during security testing and operational monitoring. The fix implemented in version 0.144.0 represents a proper security-by-design approach that enforces explicit authentication requirements rather than relying on implicit default behaviors that could compromise system integrity. Organizations should conduct comprehensive assessments of their API security configurations to identify other potential instances where similar implicit trust relationships might exist, ensuring that all security controls function as intended without unintended bypass mechanisms.