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GHSA-4mhg-xv73-xq2x
No affected components available
Impact
Synapse versions before 1.106 are vulnerable to a disk fill attack, where an unauthenticated adversary can induce Synapse to download and cache large amounts of remote media. The default rate limit strategy is insufficient to mitigate this. This can lead to a denial of service, ranging from further media uploads/downloads failing to completely unavailability of the Synapse process, depending on how Synapse was deployed.
Patches
Synapse 1.106 introduces a new "leaky bucket" rate limit on remote media downloads to reduce the amount of data a user can request at a time. This does not fully address the issue, but does limit an unauthenticated user's ability to request large amounts of data to be cached.
Workarounds
Synapse deployments can currently decrease the maximum file size allowed, as well as increase request rate limits. However, this does not as effectively address the issue as a dedicated rate limit on remote media downloads.
Server operators may also wish to consider putting media on a dedicated disk or volume, reducing the impact of a disk fill condition.
References
- https://en.wikipedia.org/wiki/Leaky_bucket#As_a_meter
For more information
If you have any questions or comments about this advisory, please email us at security at element.io.
The vulnerability can be exploited over the network without needing physical access. It is easy for an attacker to exploit this vulnerability. An attacker does not need any special privileges or access rights. No user interaction is needed for the attacker to exploit this vulnerability. The impact is confined to the system where the vulnerability exists. There is a high impact on the availability of the system.
Exploitation attempts have been detected. Elevated vigilance and prompt remediation are advised.
The exploit probability is very low. The vulnerability is unlikely to be exploited in the next 30 days.
We did not find any exploit available. Neither in GitHub repositories nor in the Exploit-Database.
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