CVE-2021-21392: Open redirect via transitional IPv6 addresses on dual-stack networks
First published: Mon Apr 12 2021(Updated: )
### Impact 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. ### Patches This issue is fixed by #9240. ### Workarounds Outbound requests to the following address ranges can be blocked by a firewall, if unused for internal communication between systems: * `::ffff/80` * `::0000/80` (note that this IP range is considered deprecated by the IETF) * `2002::/16` (note that this IP range is considered deprecated by the IETF) ### References * [RFC3056](https://tools.ietf.org/html/rfc3056) * [RFC4291](https://tools.ietf.org/html/rfc4291)
Credit: security-advisories@github.com security-advisories@github.com
| Affected Software | Affected Version | How to fix |
|---|---|---|
| Matrix Synapse | <1.28.0 | |
| Fedoraproject Fedora | =34 | |
| pip/matrix-synapse | <1.28.0rc1 | 1.28.0rc1 |
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Reference Links
- https://github.com/matrix-org/synapse/pull/9240
- https://github.com/matrix-org/synapse/security/advisories/GHSA-5wrh-4jwv-5w78
- https://lists.fedoraproject.org/archives/list/package-announce@lists.fedoraproject.org/message/TNNAJOZNMVMXM6AS7RFFKB4QLUJ4IFEY/
- https://pypi.org/project/matrix-synapse/
- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/TNNAJOZNMVMXM6AS7RFFKB4QLUJ4IFEY/
- https://nvd.nist.gov/vuln/detail/CVE-2021-21392
- https://github.com/matrix-org/synapse/commit/4ca054a4eaa714d0befb4fc30b19a1131e52c9cc
- https://github.com/pypa/advisory-database/tree/main/vulns/matrix-synapse/PYSEC-2021-25.yaml
- https://lists.fedoraproject.org/archives/list/package-announce@lists.fedoraproject.org/message/TNNAJOZNMVMXM6AS7RFFKB4QLUJ4IFEY
- https://pypi.org/project/matrix-synapse
- https://github.com/advisories/GHSA-5wrh-4jwv-5w78 (Advisory)
Frequently Asked Questions
What is CVE-2021-21392?
CVE-2021-21392 is a vulnerability in Synapse, a Matrix reference homeserver, that allows requests to user provided domains without restrictions on IP addresses.
How does CVE-2021-21392 affect Matrix Synapse?
CVE-2021-21392 affects Matrix Synapse versions up to and excluding 1.28.0.
What is the severity of CVE-2021-21392?
CVE-2021-21392 has a severity rating of 6.3 (medium).
How do I fix CVE-2021-21392 in Matrix Synapse?
To fix CVE-2021-21392, update to version 1.28.0 or newer of Matrix Synapse.
Are there any references for CVE-2021-21392?
Yes, here are some references for CVE-2021-21392: [GitHub Pull Request](https://github.com/matrix-org/synapse/pull/9240), [GitHub Security Advisory](https://github.com/matrix-org/synapse/security/advisories/GHSA-5wrh-4jwv-5w78), [Fedora Project Announcement](https://lists.fedoraproject.org/archives/list/package-announce@lists.fedoraproject.org/message/TNNAJOZNMVMXM6AS7RFFKB4QLUJ4IFEY/).
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