Open WebUI vulnerable to Path Traversal in `POST /api/v1/audio/transcriptions`

Summary

An unsanitised filename field in the speech-to-text transcription endpoint allows any authenticated non-admin user to trigger a FileNotFoundError whose message — including the server's absolute DATA_DIR path — is returned verbatim in the HTTP 400 response body, confirming information disclosure on all default deployments.

Details

backend/open_webui/routers/audio.py:1197 extracts a file extension from the raw multipart filename using file.filename.split(".")[-1] with no path sanitisation. The result is concatenated into a filesystem path and passed to open():

ext       = file.filename.split(".")[-1]       # attacker-controlled, no sanitisation
filename  = f"{id}.{ext}"                      # may contain "/"
file_path = f"{file_dir}/{filename}"
with open(file_path, "wb") as f:
    f.write(contents)

If the filename is audio./etc/passwd, split(".")[-1] yields /etc/passwd and the assembled path becomes:

{CACHE_DIR}/audio/transcriptions/{uuid}./etc/passwd

open() fails with FileNotFoundError. The outer except block at line 1231 returns the exception via ERROR_MESSAGES.DEFAULT(e), leaking the full absolute path in the response body.

The MIME-type guard at line 1190 checks Content-Type (a separate multipart field) and does not constrain filename. Setting Content-Type: audio/wav satisfies the guard regardless of the filename value.

This handler is the only file upload path in the codebase that omits os.path.basename(). Both sibling handlers apply it explicitly:

# files.py:244
filename = os.path.basename(file.filename)

# pipelines.py:206
filename = os.path.basename(file.filename)

Recommended fix — match the existing pattern and suppress path leakage in errors:

# audio.py:1197 — sanitise extension
from pathlib import Path
safe_name = Path(file.filename).name
ext = Path(safe_name).suffix.lstrip(".") or "bin"

# audio.py:1231 — suppress internal path in error response
except Exception as e:
    log.exception(e)
    raise HTTPException(status_code=400, detail="Transcription failed.")

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PoC

Requirements: a running Open WebUI instance and one standard (non-admin) user account.