Prototype Pollution in Langchain Langsmith
CVE-2026-40190
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.
Vulnerability class: Prototype Pollution
Published · last modified .
CVSS v3 metric
CVSS v3 base score 5.6 (Medium). Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:L.
EPSS exploit prediction
EPSS: 0.004 (30.0th percentile), scored .
Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 30th percentile — 30.0% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.
EPSS over last 30 days · oldest: 0.003 · newest: 0.004 · change: +0.001
Affected products
- Langchain Langsmith
- Langchain-Ai Langsmith-Sdk — versions < 0.5.18
Weakness classification (CWE)
References
- github.com/langchain-ai/langsmith-sdk/security/advisories/GHSA-fw9q-39r9-c252 (Exploit, Mitigation, Vendor Advisory)
Frequently asked questions
- What is CVE-2026-40190?
- CVE-2026-40190 is a medium-severity vulnerability in Langchain Langsmith, classified under Improperly Controlled Modification of Object Prototype Attributes (Prototype Pollution). CVSS score: 5.6/10. Published 2026-04-10.
- How severe is CVE-2026-40190?
- Medium severity. CVSS v3 base score is 5.6 out of 10.