In Tor before 0.2.5.16, 0.2.6 through 0.2.8 before 0.2.8.17, 0.2.9 before 0.2.9.14, 0.3.0 before 0.3.0.13, and 0.3.1 before 0.3.1.9, relays (that have incompletely downloaded descriptors) can pick themselves in a circuit path, leading to a degradation of anonymity, aka TROVE-2017-012.
https://www.debian.org/security/2017/dsa-4054
https://bugs.torproject.org/24333
https://bugs.torproject.org/21534
https://blog.torproject.org/new-stable-tor-releases-security-fixes-0319-03013-02914-02817-02516