CVE-2021-21393mediumCWE-20

CVE-2021-21393: medium-severity vulnerability in matrix-org synapse

Denial of service (via resource exhaustion) due to improper input validation on groups/communities endpoints

Published · Updated

Follow this CVE+ Monitor synapse+ Monitor matrix-synapse

E-mail alert when it lands in KEV, gets a public PoC or its score goes up.

13Vexday Risk Score

No sign of exploitation. No public exploitation artifact known so far.

ssvc Trackcvss 5.3epss 1.6%
In short

Synapse (a Matrix chat server) doesn't properly validate user input on certain endpoints, allowing attackers to send specially crafted requests that consume excessive disk space and memory, potentially crashing the server.

Technical detail

CVE-2021-21393 is a denial-of-service vulnerability in Synapse < 1.28.0 affecting groups/communities endpoints due to improper input validation (CWE-20). The attack vector is network-based via crafted requests to third-party identifier confirmation endpoints; no authentication is required. The impact is resource exhaustion leading to service disruption.

Summary generated and translated by AI from the official description.
Synapse is a Matrix reference homeserver written in python (pypi package matrix-synapse). Matrix is an ecosystem for open federated Instant Messaging and VoIP. In Synapse before version 1.28.0 Synapse is missing input validation of some parameters on the endpoints used to confirm third-party identifiers could cause excessive use of disk space and memory leading to resource exhaustion. Note that the groups feature is not part of the Matrix specification and the chosen maximum lengths are arbitrary. Not all clients might abide by them. Refer to referenced GitHub security advisory for additional details including workarounds.
CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H
Affected products
matrix-org · synapse
Related CVEs — matrix-org synapse

In the same product, most dangerous first.

CVE-2020-26257MEDIUMDenial of service attack via incorrect parameters to federation APIsEPSS 2.4%CVE-2021-21274MEDIUMDenial of service attack via .well-known lookupsEPSS 2.2%CVE-2021-21273LOWOpen redirects on some federation and push requestsEPSS 1.8%CVE-2022-31052MEDIUMURL previews can crash Synapse media repositories or Synapse monolithsEPSS 1.7%CVE-2021-29471LOWDenial of service in Matrix SynapseEPSS 1.6%CVE-2021-41281HIGHPath traversal in Matrix SynapseEPSS 1.6%