kin-openapi openapi3filter: nil-pointer panic in ValidateResponse on a response header declared with content

Executive Intelligence Brief

A high-severity vulnerability has been discovered in the kin-openapi library, specifically in the openapi3filter package. The vulnerability, tracked as CVE-2026-100349, can cause a nil-pointer panic when validating a response header declared with 'content' instead of 'schema'. This affects versions up to and including v0.145.0 of the library. The vulnerability can lead to a process crash, and in certain scenarios, it can be triggered remotely, making it a significant concern for applications using this library. Immediate action is recommended to mitigate this vulnerability.

Threat Overview

The kin-openapi library is a popular Go library used for working with OpenAPI specifications. It provides features for loading, validating, and processing OpenAPI documents. The library is widely used in the industry for building API gateways, linters, and other tools that interact with OpenAPI specifications. The vulnerability is caused by a nil-pointer panic in the `ValidateResponse` function when a response header is declared with 'content' instead of 'schema'. This can happen when an OpenAPI specification declares a response header with a 'content' type but does not provide a schema for it. When the `ValidateResponse` function is called with such a specification, it will panic with a nil-pointer exception.

Technical Deep Dive

Vulnerability Classification

The vulnerability can be classified as a CWE-476: NULL Pointer Dereference. This type of vulnerability occurs when a program attempts to access or manipulate data through a null pointer, resulting in a crash or unexpected behavior. The CVSS score for this vulnerability is 8.2, indicating a high severity level.

Root Cause Analysis

The root cause of the vulnerability is the lack of a nil check in the `decodeValue` function when processing a response header with a 'content' type. Specifically, the `validate_response.go:181` file passes the `headerRef.Value.Schema` directly to the `decodeValue` function without checking if it is nil. This causes a nil-pointer panic when the `req_resp_decoder.go:273` file tries to access the `schema.Value.AllOf` field.

Attack Vector & Chain

The attack vector for this vulnerability involves an attacker providing a malicious OpenAPI specification that declares a response header with a 'content' type but does not provide a schema for it. When the `ValidateResponse` function is called with such a specification, it will panic with a nil-pointer exception. The attack chain involves the following steps: 1. The attacker provides a malicious OpenAPI specification. 2. The specification is loaded and validated by the kin-openapi library. 3. The `ValidateResponse` function is called with the malicious specification. 4. The function panics with a nil-pointer exception.

Exploitation Scenario Walkthrough

**Scenario: kin-openapi Library Crash via Malicious OpenAPI Specification** 1. **Reconnaissance**: An attacker discovers that a target application uses the kin-openapi library to validate OpenAPI specifications. 2. **Weaponization**: The attacker creates a malicious OpenAPI specification that declares a response header with a 'content' type but does not provide a schema for it. 3. **Delivery & Exploitation**: The attacker provides the malicious specification to the target application, which loads and validates it using the kin-openapi library. When the `ValidateResponse` function is called with the malicious specification, it panics with a nil-pointer exception, causing the application to crash. 4. **Post-Exploitation**: The attacker can exploit the crash to cause a denial-of-service (DoS) attack on the target application. 5. **Impact Realization**: The target application crashes, causing a significant impact on its availability and potentially leading to a loss of business.

Impact Analysis

Direct Impact

The direct impact of this vulnerability is a process crash, which can lead to a denial-of-service (DoS) attack on the target application. In certain scenarios, the vulnerability can be triggered remotely, making it a significant concern for applications using this library.

Downstream & Cascading Effects

The downstream and cascading effects of this vulnerability include: * A potential loss of business due to the unavailability of the target application. * A potential impact on the reputation of the organization that developed the target application. * A potential increase in support requests and bug reports related to the vulnerability.

Affected Products & Versions

The kin-openapi library, specifically the openapi3filter package, is affected by this vulnerability. The affected versions are up to and including v0.145.0.

Detection & Threat Hunting

Indicators of Compromise

The indicators of compromise (IoCs) for this vulnerability include: * A nil-pointer panic in the `ValidateResponse` function. * A crash or unexpected behavior in the target application.

Detection Rules & Signatures

The detection rules and signatures for this vulnerability include: * Monitoring for nil-pointer panics in the `ValidateResponse` function. * Monitoring for crashes or unexpected behavior in the target application.

Remediation & Hardening

Immediate Actions (0-24 hours)

The immediate actions to remediate this vulnerability include: * Upgrading the kin-openapi library to a version that fixes the vulnerability (e.g., v0.146.0 or later). * Applying a patch or workaround to the affected versions.

Short-Term Hardening (1-7 days)

The short-term hardening actions include: * Implementing additional security controls, such as input validation and error handling. * Monitoring for potential attacks and anomalies.

Strategic Recommendations

The strategic recommendations for this vulnerability include: * Regularly updating and patching dependencies. * Implementing a robust security testing and validation process. * Using a web application firewall (WAF) to detect and prevent attacks.

Analyst Assessment

The analyst assessment for this vulnerability is that it is a high-severity vulnerability that requires immediate attention. The vulnerability can lead to a process crash and potentially be triggered remotely, making it a significant concern for applications using this library. The recommended course of action is to upgrade to a fixed version or apply a patch or workaround.