AI Security Vulnerability

## 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()` an

What Happened

## 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()` an

Why This Matters

Current evidence identifies a security issue involving the affected technology, but does not yet support a more specific impact claim.

Recommended Action

No confirmed vendor remediation is available in the current evidence. Confirm whether the affected technology is present in your environment and review the affected configuration.

Exposure

Exposure reason: This incident does not currently match a technology in My Interests.

Exploitation status: UNKNOWN

Primary entities:

Timeline

• Incident first seen
  Dec 24, 2025 01:38

  BugSkan first recorded this incident.

• LangChain serialization injection vulnerability enables secret extraction
  Dec 24, 2025 01:38

  GitHub Advisory Database · Research

Sources

My Interests Match