CVE-2021-21394

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.

• CWE-20
• CWE-20

• https://github.com/matrix-org/synapse/pull/9321Patch, Third Party Advisory
• https://github.com/matrix-org/synapse/pull/9393Patch, Third Party Advisory
• https://github.com/matrix-org/synapse/security/advisories/GHSA-w9fg-xffh-p362Patch, Third Party Advisory
• https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/TNNAJOZNMVMXM6AS7RFFKB4QLUJ4IFEY/
• https://pypi.org/project/matrix-synapse/Product, Third Party Advisory
• https://github.com/matrix-org/synapse/pull/9321Patch, Third Party Advisory
• https://github.com/matrix-org/synapse/pull/9393Patch, Third Party Advisory
• https://github.com/matrix-org/synapse/security/advisories/GHSA-w9fg-xffh-p362Patch, Third Party Advisory
• https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/TNNAJOZNMVMXM6AS7RFFKB4QLUJ4IFEY/
• https://pypi.org/project/matrix-synapse/Product, Third Party Advisory

Published

Apr 12, 2021

Last Modified

Jun 17, 2026

Status

Modified

• How Strix found a critical auth bypass in etcdStrix autonomously discovered a critical authentication bypass in etcd, later designated CVE-2026-33413.
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