# CVE-2020-26257: Denial of service attack via incorrect parameters in Matrix Synapse

Source: https://junglewise.ai/threats/cve-2020-26257-denial-of-service-attack-via-incorrect-parameters-in-matrix  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2020-12-09  
License: CC BY 4.0 with a link back

CVE-2020-26257 · Severity: low · CVSS 3.1 · Published 2020-12-09

Matrix is an ecosystem for open federated Instant Messaging and VoIP. Synapse is a reference "homeserver" implementation of Matrix. A malicious or poorly-implemented homeserver can inject malformed events into a room by specifying a different room id in the path of a `/send_join`, `/send_leave`, `/invite` or `/exchange_third_party_invite` request. This can lead to a denial of service in which future events will not be correctly sent to other servers over federation. This affects any server which accepts federation requests from untrusted servers. The Matrix Synapse reference implementation before version 1.23.1 the implementation is vulnerable to this injection attack. Issue is fixed in version 1.23.1. As a workaround homeserver administrators could limit access to the federation API to trusted servers (for example via `federation_domain_whitelist`).

Technologies: [matrix-synapse (PyPI)](https://junglewise.ai/threats/technologies/matrix-synapse). Vendors: [PyPI](https://junglewise.ai/threats/vendors/pypi).

## Quick answers

**Is CVE-2020-26257 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2020-26257 being exploited in the wild.

**How severe is CVE-2020-26257?** Low severity, with a CVSS score of 3.1, published 9 December 2020.

**Which products and versions are affected?** matrix-synapse (PyPI) ECOSYSTEM:introduced=0,fixed=1.23.1, GIT:introduced=0,fixed=3ce2f303f15f6ac3dc352298972dc6e04d9b7a8b|ECOSYSTEM:introduced=0,fixed=1.23.1.

**Is there a fix for CVE-2020-26257?** Yes. matrix-synapse (PyPI): upgrade to 1.23.1 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**Am I affected by CVE-2020-26257?** Check whether you run matrix-synapse (PyPI) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## How to fix

- matrix-synapse (PyPI): upgrade to 1.23.1 or later

  ```sh
  pip install "matrix-synapse>=1.23.1"
  ```


## Affected products

- matrix-synapse (PyPI) ECOSYSTEM:introduced=0,fixed=1.23.1, GIT:introduced=0,fixed=3ce2f303f15f6ac3dc352298972dc6e04d9b7a8b|ECOSYSTEM:introduced=0,fixed=1.23.1

## Identifiers

- CVE-2020-26257
- GHSA-hxmp-pqch-c8mm
- PYSEC-2020-236

## Related threats

- [PYSEC-2026-2620 - matrix-sydent and matrix-synapse Use Cryptographically Weak PRNG](https://junglewise.ai/threats/cve-2019-11842-matrix-sydent-and-matrix-synapse-use-cryptographically-weak-prng)
- [PYSEC-2026-1614 - Synapse vulnerable to federation denial of service via malformed events](https://junglewise.ai/threats/cve-2025-30355-synapse-vulnerable-to-federation-denial-of-service-via-malformed)
- [PYSEC-2026-1610 - Synapse Matrix has a partial room state leak via Sliding Sync](https://junglewise.ai/threats/cve-2024-53867-synapse-matrix-has-a-partial-room-state-leak-via-sliding-sync)
- [Synapse can be forced to thumbnail unexpected file formats, invoking external, potentially untrustworthy decoders](https://junglewise.ai/threats/cve-2024-53863-synapse-can-be-forced-to-thumbnail-unexpected-file-formats)
- [PYSEC-2026-1611 - Synapse allows a a malformed invite to break the invitee's `/sync`](https://junglewise.ai/threats/cve-2024-52815-synapse-allows-a-a-malformed-invite-to-break-the-invitee-s-sync)