In Synapse before 0.31.2, unauthorised users can hijack rooms when there is no m.room.power_levels event in force.
https://matrix.org/blog/2018/06/14/security-update-synapse-0-31-2/
https://github.com/matrix-org/matrix-doc/issues/1304
https://bugs.debian.org/901549
Source: Mitre, NVD
Published: 2018-06-14
Updated: 2019-10-03
Base Score: 5
Vector: CVSS2#AV:N/AC:L/Au:N/C:N/I:P/A:N
Severity: Medium
Base Score: 7.5
Vector: CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N
Severity: High