CVE-2026-40190: langchain-ai langsmith-sdk Prototype Pollution

Vulnerability summary

CVE-2026-40190 is a critical (CVSS 9.8) Prototype Pollution vulnerability in langchain-ai langsmith-sdk. EPSS estimates a 0.39% chance of exploitation in the next 30 days, higher than 31.1% of scored vulnerabilities. 1 public exploit is recorded.

Timeline

1. 10-04-2026
   PublishedVulnerability first disclosed.
2. 13-04-2026
   Last modifiedVulnerability information updated.

Description

LangSmith Client SDKs provide SDK's for interacting with the LangSmith platform. Prior to 0.5.18, the LangSmith JavaScript/TypeScript SDK (langsmith) contains an incomplete prototype pollution fix in its internally vendored lodash set() utility. The baseAssignValue() function only guards against the __proto__ key, but fails to prevent traversal via constructor.prototype. This allows an attacker who controls keys in data processed by the createAnonymizer() API to pollute Object.prototype, affecting all objects in the Node.js process. This vulnerability is fixed in 0.5.18.

• CWE-1321Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution')The product receives input from an upstream component that specifies attributes that are to be initialized or updated in an object, but it does not properly control modifications of attributes of the object prototype.