Affected: PyPI: langsmith

# Summary

An attacker who can send an HTTP request to a server running the LangSmith SDK's `TracingMiddleware` can cause that server to read an arbitrary file from its local filesystem and upload the contents to LangSmith as a trace attachment. Depending on how the distributed trace system is deployed, triggering a read may not require authentication. Retrieving the contents requires read access to the LangSmith workspace the traces are sent to. The net effect is a trust-boundary crossing: a party with workspace trace-read access (for example a low-privilege workspace member, a contractor, or a compromised teammate account) gains the ability to read files from any server running `TracingMiddleware`, a capability outside that workspace's intended trust boundary.

# Impact

Confidentiality (High): arbitrary read of files accessible to the server process, exposed to anyone with workspace trace-read access.

# Details

Two defects combine. A field supplied through a tracing-propagation header was merged into the run without validation, allowing injection of run attributes including attachments (CWE-346). A type check intended to gate filesystem access did not match the type of the decoded input, so the guard never engaged (CWE-843). As a result, an attacker-named file is opened by the server and uploaded as a trace attachment by the background tracing thread (CWE-22).

## Who can exploit this

- Anyone reachable by HTTP can trigger the file read. Depending on how the distribute…