CVE-2020-15366 in Ajv
Summary
by MITRE
An issue was discovered in ajv.validate() in Ajv (aka Another JSON Schema Validator) 6.12.2. A carefully crafted JSON schema could be provided that allows execution of other code by prototype pollution. (While untrusted schemas are recommended against, the worst case of an untrusted schema should be a denial of service, not execution of code.)
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Analysis
by VulDB Data Team • 06/22/2024
The vulnerability identified as CVE-2020-15366 represents a critical prototype pollution flaw in the Ajv JSON schema validator library version 6.12.2. This issue arises from the insecure handling of JSON schema validation processes where maliciously crafted schemas can manipulate the prototype chain of JavaScript objects, leading to arbitrary code execution. The vulnerability specifically affects the ajv.validate() function which is widely used across Node.js applications for data validation and schema enforcement. The flaw demonstrates how seemingly benign validation operations can be exploited to compromise application security when dealing with untrusted input data.
Prototype pollution vulnerabilities occur when an application fails to properly sanitize user-provided data before using it to modify object prototypes. In this case, the Ajv library does not adequately protect against malicious schema definitions that contain properties designed to pollute the Object.prototype chain. When such schemas are processed through the validate() function, they can inject malicious properties into the prototype, which then propagate to all objects inheriting from that prototype. This behavior creates a dangerous attack surface where an attacker can manipulate core JavaScript object methods and properties, potentially leading to complete system compromise. The vulnerability is particularly concerning because it violates the fundamental security principle that untrusted input should never result in code execution, as outlined in CWE-1321.
The operational impact of this vulnerability extends far beyond simple denial of service scenarios, as the security model assumes that untrusted schemas should only cause service disruption rather than code execution. This assumption is fundamentally flawed in the presence of prototype pollution attacks, which can be leveraged to execute arbitrary JavaScript code within the application context. Attackers can exploit this vulnerability to perform server-side request forgery, execute arbitrary commands, or gain unauthorized access to sensitive data. The attack vector is particularly insidious because it requires no special privileges beyond the ability to provide a malicious JSON schema, making it accessible to attackers who can influence input validation processes. This vulnerability directly maps to ATT&CK technique T1059.007 for JavaScript and T1211 for lateral movement through compromised validation processes, as it enables attackers to establish persistent access through validation system manipulation.
Organizations using Ajv version 6.12.2 should immediately implement mitigations including upgrading to patched versions of the library, implementing strict input validation for all schema definitions, and employing sandboxing techniques for untrusted schema processing. The recommended approach involves not only updating to the patched version that addresses the prototype pollution mechanism but also applying defensive programming practices such as using Object.freeze() on prototype objects and implementing comprehensive schema validation before any processing occurs. Additionally, organizations should consider implementing runtime monitoring to detect anomalous prototype modifications and establish strict access controls for schema definition processes. The vulnerability highlights the importance of following secure coding practices as defined in OWASP Top 10 and the CWE hierarchy related to prototype pollution and input validation failures, emphasizing that validation libraries must be designed with security in mind to prevent such critical exploitation vectors.