Denial of Service (DoS) Affecting matrix-synapse package, versions [1.62.0,1.68.0rc1)

matrix-synapse is an ecosystem for open federated Instant Messaging and VoIP.

Affected versions of this package are vulnerable to Denial of Service (DoS) by being forced to reject all events from a vulnerable homeserver. A malicious homeserver can trick Synapse into accepting previously rejected events into its view of the current state of that room. The malicious homeserver can then cause all further messages and state changes sent in that room from the vulnerable homeserver to be rejected. Synapse homeservers in rooms available over public federation are affected.

NOTE: Synapse homeservers are not vulnerable if federation is disabled (e.g. by setting federation_domain_whitelist to an empty list - []) or only trusted servers are federated.

Denial of Service (DoS) describes a family of attacks, all aimed at making a system inaccessible to its intended and legitimate users.

Unlike other vulnerabilities, DoS attacks usually do not aim at breaching security. Rather, they are focused on making websites and services unavailable to genuine users resulting in downtime.

One popular Denial of Service vulnerability is DDoS (a Distributed Denial of Service), an attack that attempts to clog network pipes to the system by generating a large volume of traffic from many machines.

When it comes to open source libraries, DoS vulnerabilities allow attackers to trigger such a crash or crippling of the service by using a flaw either in the application code or from the use of open source libraries.

Two common types of DoS vulnerabilities:

• High CPU/Memory Consumption- An attacker sending crafted requests that could cause the system to take a disproportionate amount of time to process. For example, commons-fileupload:commons-fileupload.

• Crash - An attacker sending crafted requests that could cause the system to crash. For Example, npm ws package

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