CVE-2018–20586 is a log injection vulnerability which allows any software with access to the RPC port to create fake or confusing entries in the debug log. Valid authentication (username/password/cookie) for the RPC service is NOT required to exploit this vulnerability, only the ability to connect to the RPC port (which is by default only exposed to the local machine).

The vulnerability was introduced in 40b556d3742a1f65d67e2d4c760d0b13fe8be5b7 (“libevent-based http server”) and first released in Bitcoin Core v0.12.0rc1 in 2016 Jan 13. A fix was hidden in 79358817e53ac0a7afa64c747115d492a74e3155 (“rpc: Make HTTP RPC debug logging more informative”) released in v0.17.1, 2018 Dec 22.

Note that as of today, this fix has NOT been backported to older versions. When/if v0.16.4 is released, it may also include a fix, but due to the minor severity of this vulnerability, it does not merit a dedicated release on its own. (The 0.16 git branch is also NOT fixed at this time.)

To be vulnerable, the malicious software must either be running on the same machine as the node, have the ability to proxy connections to the node via the local machine, or the node must be configured to accept RPC connections from a network via which the attacker can connect. Additionally, a human user must read the debug log and act on or otherwise believe the injected data, in a way that is somehow harmful.

Because the node would log the arbitrary POST request from any connection, an attacker can add nearly any content to the request to inject it into the log. To ensure their entire request is injected, standard spaces would need to be replaced with alternative whitespace characters, and newlines would need to become other control characters (such as “\r\v”). Because the injected data must use such non-standard characters, it is most likely to not fool other software parsing the debug log, and only a human visually reading it.

To fix this vulnerability, POST requests are now sanitised before being logged, removing all characters that shouldn’t be in an ordinary POST request.

Credit goes to practicalswift (https://twitter.com/practicalswift) for discovering and fixing the vulnerability.

Timeline:

- 2015–01–18: Vulnerability introduced in PR #5677.

- 2015–09–04: Vulnerability merged to master git repository.

- 2016–01–13: Vulnerability published in v0.12.0rc1.

- 2016–02–18: Vulnerability released in v0.12.0.

…

- 2018–10–25: practicalswift discloses vulnerability to security team.

- 2018–10–31: practicalswift opens PR #14618 to quietly fix vulnerability.

- 2018–11–05: Fix merged to master git repository.

- 2018–11–30: Fix merged to 0.17 git repository.

- 2018–12–07: Fix published in v0.17.1rc1.

- 2018–12–22: Fix released in v0.17.1.

…

- 2019–06–22: Vulnerability existence disclosed to bitcoin-dev ML.

- 2019–11–22: Vulnerability details disclosure to bitcoin-dev ML.