[bitcoin-dev] JSONRPC vulnerability in Electrum 2.6 to 3.0.4

[Thomas Voegtlin via bitcoin-dev]

A vulnerability has been found in Electrum, and patched in version
3.0.5. Please update your software if you are running an earlier version.

The following is a copy of the summary and guidelines we posted on our
website: https://github.com/spesmilo/electrum-docs/blob/master/cve.rst

A CVE number for the issue has been requested 2 days ago, and has not
been attributed yet.



JSONRPC vulnerability in Electrum 2.6 to 3.0.4
==============================================

On January 6th, a vulnerability was disclosed in the Electrum wallet
software, that allows malicious websites to execute wallet commands
through JSONRPC executed in a web browser. The bug affects versions
2.6 to 3.0.4 of Electrum, on all platforms. It also affects clones of
Electrum such as Electron Cash.


Can funds be stolen?
--------------------

Wallets that are not password protected are at risk of theft, if they
are opened with a version of Electrum older than 3.0.5 while a web
browser is active.

In addition, the vulnerability allows an attacker to modify user
settings, the list of contacts in a wallet, and the "payto" and
"amount" fields of the user interface while Electrum is running.

Although there is no known occurrence of Bitcoin theft occurring
because of this vulnerability, the risk increases substantially now
that the vulnerability has been made public.


Can wallet data be leaked?
--------------------------

Yes, an attacker can obtain private data, such as: Bitcoin addresses,
transaction labels, address labels, wallet contacts and master public
keys.


Can a password-protected wallet be bruteforced?
-----------------------------------------------

Not realistically. The vulnerability does not allow an attacker to
access encrypted seed or private keys, which would be needed in order
to perform an efficient brute force attack. Without the encrypted
seed, an attacker must try passwords using the JSONRPC interface,
while the user is visiting a malicious page. This is several orders of
magnitude slower than an attack with the encrypted seed, and
restricted in time. Even a weak password will protect against that.


What should users do?
---------------------

All users should upgrade their Electrum software, and stop using old
versions.

Users who did not protect their wallet with a password should create a
new wallet, and move their funds to that wallet. Even if it never
received any funds, a wallet without password should not be used
anymore, because its seed might have been compromised.

In addition, users should review their settings, and delete all
contacts from their contacts list, because the Bitcoin addresses of
their contacts might have been modified.


How to upgrade Electrum
-----------------------

Stop running any version of Electrum older than 3.0.5, and install
Electrum the most recent version. On desktop, make sure you download
Electrum from https://electrum.org and no other website. On Android,
the most recent version is available in Google Play.

If Electrum 3.0.5 (or any later version) cannot be installed or does
not work on your computer, stop using Electrum on that computer, and
access your funds from a device that can run Electrum 3.0.5. If you
really need to use an older version of Electrum, for example in order
to access wallet seed, make sure that your computer is offline, and
that no web browser is running on the computer at the same time.


Should all users move their funds to a new address?
---------------------------------------------------

We do not recommend moving funds from password protected wallets. For
wallets that were not password protected, moving funds is an extreme
precaution, that might not be necessary; indeed, if a wallet was
compromised, it is very likely that the attacker would have stolen the
funds immediately.


When was the issue reported and fixed?
--------------------------------------

The absence of password protection in the JSONRPC interface was
reported on November 25th, 2017 by user jsmad:
https://github.com/spesmilo/electrum/issues/3374

jsmad's report was about the Electrum daemon, a piece of software that
runs on web servers and is used by merchants in order to receive
Bitcoin payments. In that context, connections to the daemon from the
outside world must be explicitly authorized, by setting 'rpchost' and
'rpcport' in the Electrum configuration.

On January 6th, 2018, Tavis Ormandy demonstrated that the JSONRPC
interface could be exploited against the Electrum GUI, and that the
attack could be carried out by a web browser running locally, visiting
a webpage with specially crafted JavaScript.

We released a new version (3.0.4) in the hours following Tavis' post,
with a patch written by mithrandi (Debian packager), that addressed
the attack demonstrated by Tavis. In addition, the Github issue
remained open, because mithrandi's patch was not adding password
protection to the JSONRPC interface.

Shortly after the 3.0.4 release we started to work on adding proper
password protection to the JSONRPC interface of the daemon, and that
part was ready on Sunday, January 7th. We also learned on Sunday
afternoon that the first patch was not effective against another,
similar attack, using POST. This is why we did not delay the 3.0.5
release, which includes password protection, and completely disables
JSONRPC in the GUI.