Subject: Import token permissions checking not enforced

CVE: CVE-2021-3127

Background:

NATS.io is a high performance open source pub-sub distributed communication
technology, built for the cloud, on-premise, IoT, and edge computing.
The server is written in Go and there are client libraries in many languages
and frameworks.

The nats-io JWT library provides the handling for JWTs common to the NATS.io
projects.


Problem Description:

The NATS server provides for Subjects which are namespaced by Account;
all Subjects are supposed to be private to an account, with an
Export/Import system used to grant cross-account access to some
Subjects.  Some Exports are public, such that anyone can import the
relevant subjects, and some Exports are private, such that the Import
requires a token JWT to prove permission.

The JWT library's validation of the bindings in the Import Token
incorrectly warned on mismatches, instead of outright rejecting the
token.

As a result, any account can take an Import token used by any other
account and re-use it for themselves because the binding to the
importing account is not rejected, and use it to import *any* Subject
from the Exporting account, not just the Subject referenced in the
Import Token.

The NATS account-server system treats account JWTs as semi-public
information, such that an attacker can easily enumerate all account JWTs
and retrieve all Import Tokens from those account JWTs.

The CVE identifier should cover the JWT library repair and the
nats-server containing the fixed JWT library, and any other application
depending upon the fixed JWT library.


Affected versions:

JWT library:
 * all versions prior to 2.0.1
 * fixed after nats-io/jwt PR 149 landed (2021-03-14)

NATS Server:
 * Version 2 prior to 2.2.0
   + 2.0.0 through and including 2.1.9 are vulnerable
 * fixed with nats-io/nats-server commit 423b79440c (2021-03-14)


Impact:

In deployments with untrusted accounts able to update the Account Server
with imports, a malicious account can access any Subject from an account
which provides Exported Subjects.

Abuse of this facility requires the malicious actor to upload their
tampered Account JWT to the Account Server, providing the service
operator with a data-store which can be scanned for signs of abuse.


Workaround:

Deny access to clients to update their account JWT in the account
server.


Solution:

Upgrade the JWT dependency in any application using it.

Upgrade the NATS server if using NATS Accounts (with private Exports;
Account owners can create those at any time though).

Audit all accounts JWTs to scan for exploit attempts.

---------------------------8< jwt-audit.py >8---------------------------
#!/usr/bin/env python3

"""jwt-audit: audit a packfile of JWTs for security issues.

See CVE-2021-3127 for more details.
"""

import argparse
import base64
import json
import os
import pathlib
import sys

from dataclasses import dataclass

import requests

SCRIPT_DIR = pathlib.Path(__file__).absolute().parent


class Error(Exception):
    """Base class for exceptions from audit."""
    pass


class Exit(Exception):
    """Base class for exceptions which exit without a stack trace."""
    pass


@dataclass
class ReadAccount:
    lineno1: int
    account_id: str
    b64_jwt: str


class AuditAccount:
    def __init__(self, read_account):
        self.read_account = read_account
        self.acct = self.read_account.account_id
        self.problem_count = 0

    def report(self, message):
        print('{}: {}'.format(self.acct, message), file=sys.stderr)
        self.problem_count += 1

    def b64_json_decode(self, raw):
        decode = raw + '=' * (4 - len(raw) % 4)
        return json.loads(base64.urlsafe_b64decode(decode))

    def payload_from_jwt(self, raw):
        return self.b64_json_decode(raw.split('.', 2)[1])

    def populate(self):
        self.jwt = self.payload_from_jwt(self.read_account.b64_jwt)

    def audit(self):
        self.populate()
        if self.jwt["sub"] != self.acct:
            self.report(f'account does not match JWT subject {self.jwt["sub"]}')
        if 'nats' not in self.jwt:
            self.report(f'JWT missing "nats" top-level key')
            return
        if 'imports' not in self.jwt['nats']:
            return
        for impIndex0, Import in enumerate(self.jwt['nats']['imports']):
            if 'token' not in Import:
                continue
            index = impIndex0 + 1
            token = self.payload_from_jwt(Import['token'])
            if token["type"] != 'activation':
                self.report(f'import {index}: non-activation token {token["type"]!r}')
                continue
            if token["nats"]["type"] == 'stream':
                # we don't audit these
                continue
            if token["nats"]["type"] != 'service':
                self.report(f'import {index}: import not of type service but is {token["nats"]["type"]!r}')
            if token["iss"] != Import["account"]:
                self.report(f'import {index}: issuer {token["iss"]!r} != import-from account {Import["account"]!r}')
            if token["nats"]["subject"] != Import["to"]:
                self.report(f'import {index}: ABUSE: token grants {token["nats"]["subject"]!r} but used to access {Import["to"]!r}')


class AccountCollection:
    def __init__(self, *, options):
        self.options = options
        if options.file:
            self.data_reader = self.from_file(options.file)
        elif options.url:
            self.data_reader = self.from_url(options.url)
        else:
            raise Error('code bug: unhandled origin for AccountCollection')

    def from_file(self, filename):
        yield from self.from_opener(repr(filename), open(filename))

    def from_url(self, url):
        response = requests.get(url, stream=True)
        iterator = [line.decode(response.encoding) for line in response.iter_lines()]
        yield from self.from_opener('<'+url+'>', iterator)

    def from_opener(self, label, opened):
        for lineno0, line in enumerate(opened):
            try:
                line = line.strip()
                if not line:
                    continue
                acct, jwt = line.split('|', 1)
                yield ReadAccount(lineno0+1, acct, jwt)
            except ValueError as e:
                l = lineno0 + 1
                print(f'warning: {label} line {l}: {e}', file=sys.stderr)

    def audit(self):
        for index, ra in enumerate(self.data_reader):
            AuditAccount(ra).audit()


def _main(args, argv0):
    parser = argparse.ArgumentParser(
        description=__doc__,
        formatter_class=argparse.RawDescriptionHelpFormatter)
    origin = parser.add_mutually_exclusive_group(required=True)
    origin.add_argument('-f', '--file',
                        help='Audit a pack-file of JWTs')
    origin.add_argument('--url',
                        help='Pull packfile of accounts from given URL')
    parser.add_argument('-v', '--verbose',
                        action='count', default=0,
                        help='Be more verbose')
    options = parser.parse_args(args=args)

    accounts = AccountCollection(options=options)

    accounts.audit()


if __name__ == '__main__':
    argv0 = pathlib.Path(sys.argv[0]).name
    if argv0.endswith('.py'):
        argv0 = argv0[:-3]
    try:
        rv = _main(sys.argv[1:], argv0=argv0)
    except Exit as e:
        for arg in e.args:
            print('{}: {}'.format(argv0, arg), file=sys.stderr)
        sys.exit(1)
    sys.exit(rv)
---------------------------8< jwt-audit.py >8---------------------------