LangChain serialization injection vulnerability enables secret extraction

## Context A serialization injection vulnerability exists in LangChain JS's `toJSON()` method (and subsequently when string-ifying objects using `JSON.stringify()`. The method did not escape objects with `'lc'` keys when serializing free-form data in kwargs. The `'lc'` key is used internally by LangChain to mark serialized objects. When user-controlled data contains this key structure, it is treated as a legitimate LangChain object during deserialization rather than plain user data. ### Attack surface The core vulnerability was in `Serializable.toJSON()`: this method failed to escape user-controlled objects containing `'lc'` keys within kwargs (e.g., `additional_kwargs`, `metadata`, `response_metadata`). When this unescaped data was later deserialized via `load()`, the injected structures were treated as legitimate LangChain objects rather than plain user data. This escaping bug enabled several attack vectors: 1. **Injection via user data**: Malicious LangChain object structures could be injected through user-controlled fields like `metadata`, `additional_kwargs`, or `response_metadata` 2. **Secret extraction**: Injected secret structures could extract environment variables when `secretsFromEnv` was enabled (which had no explicit default, effectively defaulting to `true` behavior) 3. **Class instantiation via import maps**: Injected constructor structures could instantiate any class available in the provided import maps with attacker-controlled parameters **Note on import maps:** Classes must be explicitly included in import maps to be instantiatable. The core import map includes standard types (messages, prompts, documents), and users can extend this via `importMap` and `optionalImportsMap` options. This architecture naturally limits the attack surface, an `allowedObjects` parameter is not necessary because users control which classes are available through the import maps they provide. **Security hardening:** This patch fixes the escaping bug in `toJSON()` and introduces new restrictive defaults in `load()`: `secretsFromEnv` now explicitly defaults to `false`, and a `maxDepth` parameter protects against DoS via deeply nested structures. JSDoc security warnings have been added to all import map options. ## Who is affected? Applications are vulnerable if they: 1. **Serialize untrusted data via `JSON.stringify()` on Serializable objects, then deserialize with `load()`** , Trusting your own serialization output makes you vulnerable if user-controlled data (e.g., from LLM responses, metadata fields, or user inputs) contains `'lc'` key structures. 2. **Deserialize untrusted data with `load()`** , Directly deserializing untrusted data that may contain injected `'lc'` structures. 3. **Use LangGraph checkpoints** , Checkpoint serialization/deserialization paths may be affected. The most common attack vector is through **LLM response fields** like `additional_kwargs` or `response_metadata`, which can be controlled via prompt injection and then serialized/deserialized in streaming operations. ## Impact Attackers who control serialized data can extract environment variable secrets by injecting `{"lc": 1, "type": "secret", "id": ["ENV_VAR"]}` to load environment variables during deserialization (when `secretsFromEnv: true`). They can also instantiate classes with controlled parameters by injecting constructor structures to instantiate any class within the provided import maps with attacker-controlled parameters, potentially triggering side effects such as network calls or file operations. Key severity factors: - Affects the serialization path, applications trusting their own serialization output are vulnerable - Enables secret extraction when combined with `secretsFromEnv: true` - LLM responses in `additional_kwargs` can be controlled via prompt injection ## Exploit example ```typescript import { load } from "@langchain/core/load"; // Attacker injects secret structure into user-controlled data const attackerPayload =

CVSS 3.1

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:N/A:N

Severity from

GitHub (reviewed advisory)

Weakness

CWE-502

Also known as

CVE-2025-68665

• github.com/langchain-ai/langchainjs/security/advisories/GHSA-r399-636x-v7f6
• nvd.nist.gov/vuln/detail/CVE-2025-68665
• github.com/langchain-ai/langchainjs/commit/e5063f9c6e9989ea067dfdff39262b9e7b6aba62
• github.com/langchain-ai/langchainjs
• github.com/langchain-ai/langchainjs/releases/tag/%40langchain%2Fcore%401.1.8
• github.com/langchain-ai/langchainjs/releases/tag/langchain%401.2.3