The attack was performed on Sunday, April 29 against #matrix:matrix.org and #matrix-dev:matrix.org that made the rooms temporarily unusable.
Vulnerable component: Synapse
CVSSv3 score: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:N/A:H/E:H/RL:O/RC:C
CWE-ID: CWE-20 - Improper Input Validation
The vulnerability allows a remote attacker to perform a denial of service (DoS) attack.
The vulnerability exists due to an input validation error where malicious events injected with depth = 2^63 - 1 render rooms unusable, related to federation/federation_base.py and handlers/message.py. A remote attacker can send malicious messages and perform a denial of service attack.
Note: this vulnerability has been exploited in the wild in April 2018.