CVE-2026-44843

LangChain contains older runtime code paths that deserialize run inputs, run outputs, or other application-controlled payloads using overly broad object allowlists. These paths may call load() with allowed_objects="all". This does not enable arbitrary Python object deserialization, but it does allow any trusted LangChain-serializable object to be revived, which is broader than these runtime paths require. As a result, attacker-supplied LangChain serialized constructor dictionaries may cause trusted runtime paths to instantiate classes with untrusted constructor arguments.

Applications are exposed only when all of the following are true:

1. The application accepts untrusted structured input, such as JSON, from a user or network request.
2. The application does not validate or canonicalize that input into an inert schema before invoking LangChain.
3. Attacker-controlled nested dictionaries or lists are preserved in LangChain run inputs or outputs.
4. The application uses an affected API path that later deserializes that run data.

Known affected runtime surfaces include:

• RunnableWithMessageHistory
• astream_log()
• astream_events(version="v1")

Related unsafe deserialization patterns may also affect applications that explicitly load serialized LangChain prompt or runnable objects from untrusted sources, including shared prompt stores, Hub artifacts with model configuration, or other application-controlled serialization stores.

Applications that validate incoming requests against a fixed schema, such as coercing user input to a plain string or message-content field before invoking LangChain, are unlikely to expose this deserialization primitive.

This release also fixes a related secret-marker validation bypass in the serialization and deserialization layer (_is_lc_secret). That issue creates an additional path by which attacker-controlled constructor dictionaries can avoid escaping during dumps() -> loads() round-trips and reach LangChain object revival logic.

Guidance for load() and loads()

load() and loads() should be used only with trusted LangChain manifests or serialized objects from trusted storage. Do not pass user-controlled data to load() or loads(), and do not use them as general parsers for request bodies, tool inputs, chat messages, or other attacker-controlled data.

load() and loads() are beta APIs, and their behavior may change as LangChain narrows unsafe defaults. Future LangChain versions will require callers to be explicit about which objects may be revived. Users should pass a narrow allowed_objects value appropriate for the specific trusted manifest they are loading, rather than relying on broad defaults or allowed_objects="all", which permits the full trusted LangChain serialization allowlist.

Credits

The original issue was first reported by @u-ktdi.

Similar findings were reported by @dewankpant, @shrutilohani, @Moaaz-0x, @pucagit.

A related _is_lc_secret marker bypass affecting dumps() -> loads() round-trips was reported by @yardenporat353 (and a similar report by @localhost-detect)

An attacker who can submit untrusted structured input to an affected application, and have that structure preserved in LangChain run data, may be able to inject LangChain serialized constructor payloads such as:

{
  "lc": 1,
  "type": "constructor",
  "id": ["langchain_core", "messages", "ai", "AIMessage"],
  "kwargs": {"content": "attacker-controlled content"}
}

If this payload reaches a broad load() call, LangChain may instantiate the referenced class instead of treating the payload as inert user data.

Realistic impacts include:

• Persistent chat-history poisoning when revived AIMessage, HumanMessage, or SystemMessage objects are stored by RunnableWithMessageHistory.
• Prompt injection or behavior manipulation if attacker-controlled messages are later included in model context.
• Instantiation of unexpected trusted LangChain objects with attacker-controlled constructor arguments.
• Possible credential disclosure or server-side requests if a reachable object reads environment credentials, creates clients, or contacts attacker-controlled endpoints during initialization.
• Additional prompt-template or runnable-configuration impacts in applications that separately load and execute untrusted serialized LangChain objects.

Untrusted serialized data is processed by a deserializer that can instantiate arbitrary objects or execute code as a side effect. Typical impact: arbitrary code execution or logic abuse.

CVE-2026-44843 has a CVSS score of 8.2 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (1.3.3, 0.3.85); upgrading removes the vulnerable code path.

Kodem intelligence

LangChain will deprecate the affected APIs as part of this fix:

• RunnableWithMessageHistory
• astream_log()
• astream_events(version="v1")

These are older code paths that are no longer recommended for new applications. They were not previously marked as deprecated, but recent LangChain documentation has primarily directed users toward newer streaming and memory patterns, including the stream API. Applications should migrate to the currently recommended APIs rather than continue depending on these older surfaces.

Separately, LangChain will update load() and loads() to tighten deserialization behavior so broad object revival is not applied implicitly to untrusted or application-controlled payloads. The older runtime surfaces listed above are being deprecated rather than preserved as supported paths for broad runtime deserialization.

This release also fixes a related secret-marker validation bypass in the serialization and deserialization layer (_is_lc_secret). That issue creates an additional path by which attacker-controlled constructor dictionaries can avoid escaping during dumps() -> loads() round-trips and reach LangChain object revival logic.

1. What is CVE-2026-44843? CVE-2026-44843 is a high-severity insecure deserialization vulnerability in langchain-core (pip), affecting versions >= 1.0.0, <= 1.3.2. It is fixed in 1.3.3, 0.3.85. Untrusted serialized data is processed by a deserializer that can instantiate arbitrary objects or execute code as a side effect.
2. How severe is CVE-2026-44843? CVE-2026-44843 has a CVSS score of 8.2 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of langchain-core are affected by CVE-2026-44843? langchain-core (pip) versions >= 1.0.0, <= 1.3.2 is affected.
4. Is there a fix for CVE-2026-44843? Yes. CVE-2026-44843 is fixed in 1.3.3, 0.3.85. Upgrade to this version or later.
5. Is CVE-2026-44843 exploitable, and should I be worried? Whether CVE-2026-44843 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-44843 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-44843?
   • Upgrade langchain-core to 1.3.3 or later
   • Upgrade langchain-core to 0.3.85 or later