Talos Vulnerability Report

Summary

An exploitable improper authorization vulnerability exists in miner_stop API of cpp-ethereum’s JSON-RPC (commit 4e1015743b95821849d001618a7ce82c7c073768). A JSON request can cause an access to the restricted functionality resulting in authorization bypass. An attacker can send JSON to trigger this vulnerability.

Tested Versions

Ethereum commit 4e1015743b95821849d001618a7ce82c7c073768

Product URLs

http://cpp-ethereum.org

CVSSv3 Score

4.0 - CVSS:3.0/AV:N/AC:H/PR:N/UI:N/S:C/C:N/I:N/A:L

CWE

CWE-285: Improper Authorization

Details

CPP-Ethereum is a C++ ethereum client, one of the 3 most popular clients for the ethereum platform.  One of the components that is part of cpp-ethereum is a JSON-RPC server which exposes various APIs to manage client/node functionality.  Improper authorization checks in the implementation of the `miner_stop` API allows a remote attacker without any credentials to triggers functionality reserved only for a user with administrator privileges. We can observe a similar approach in two other clients (lack of any kind of authorization) but in this case the situation is exacerbated by the fact that:

-	By default interface is bound to 0.0.0.0, which means it’s exposed to the world
-	The Content-Type set to ‘application/json’ during requests is not enforced, which means that even if eth JSON-RPC daemon is ran on machine behind a NAT 
	the JSON-RPC APIs can still be easily triggered by CSRF or SSRF attacks.
-	older version of the same API had implemented an authorization check
-	there is no visible option to change the default JSON-RPC interface to localhost

For comparison let us take geth (the go ethereum client) which also implements a JSON-RPC interface but using much better security practices: - by default the interface is bound to localhost - The “Content-Type” request header value must be set to ‘application/json’ - CORS settings are set to block by default all “cross-domain” requests

Let us take a look at miner_stop and describe in details improper/consistency check of authorization.

Line 247	bool AdminEth::miner_stop()
Line 248	{
Line 249		m_eth.stopSealing();
Line 250		return true;
Line 251	}

As we can see there is no check for calling user privileges which is done in couple other APIs via RPC_ADMIN macro. Same functionality is exposed over admin_eth_setMinig API where at the beginning of API body, privileges check is made:

Line 22	bool AdminEth::admin_eth_setMining(bool _on, string const& _session)
Line 23	{
Line 24		RPC_ADMIN;
Line 25		if (_on)
Line 26			m_eth.startSealing();
Line 27		else
Line 28			m_eth.stopSealing();
Line 29		return true;
Line 30	}
Line 31

We are aware that this client is not recommended for mining and that the mentioned functionality related with the administrator interface is turned off by default. However when enabled the default behavior is insecure and can allow a remote attacker to perform unauthenticated RPC requests.

Crash Information

icewall@ubuntu:~/bugs/cpp-ethereum/build/eth$ ./eth -j --ipc --private 123 --no-discovery --datadir `pwd`/data --config config.json --admin-via-http 
cpp-ethereum, a C++ Ethereum client
cpp-ethereum 1.3.0
  By cpp-ethereum contributors, (c) 2013-2016.
  See the README for contributors and credits.
Networking disabled. To start, use netstart or pass --bootstrap or a remote host.
JSONRPC Admin Session Key: Zt9zxSANHZs=
  ℹ  03:09:10 AM.197|miner0  Loading full DAG of seedhash: #00000000…
  ℹ  03:09:10 AM.978|miner0  Full DAG loaded
  
icewall@ubuntu:~/bugs/cpp-ethereum$ curl -X POST --data '{"jsonrpc":"2.0","method":"miner_stop","params":[1],"id":1}' localhost:8545

Timeline

2017-12-06 - Vendor Disclosure
2018-01-09 - Public Release