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EUVD-2026-22225

fkie_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.

GHSA-w9fg-xffh-p362 — Denial of service (via resource exhaustion) due to improper input validation on third-party identifier endpoints

Affected: PyPI: matrix-synapse

### Impact 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.

### Patches The issue is fixed by #9321.

### Workarounds Depending on the needs and configuration of the homeserver a few options are available:

1. Using email as third-party identifiers be disabled by not configuring the `email` setting. 2. Using phone numbers as third-party identifiers can be disabled by ensuring that `account_threepid_delegates.msisdn` is not configured. 3. Additionally, the affected endpoint patterns can be blocked at a reverse proxy:

* `^/_matrix/client/(r0|unstable)/register/email` * `^/_matrix/client/(r0|unstable)/register/msisdn` * `^/_matrix/client/(r0|unstable)/account/password` * `^/_matrix/client/(r0|unstable)/account/3pid`

gsd-2021-21394

PYSEC-2021-27

Affected: PyPI: matrix-synapse

UBUNTU-CVE-2021-21394

Affected: Ubuntu:Pro:18.04:LTS: matrix-synapse, Ubuntu:Pro:20.04:LTS: matrix-synapse, Ubuntu:Pro:22.04:LTS: matrix-synapse