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High

Synapse Denial of service due to incorrect application of event authorization rules during state resolution

  • Disclosed: 24.05.2023
  • Updated: 11.10.2026 11:20

Impact

If Synapse and a malicious homeserver are both joined to the same room, the malicious homeserver can trick Synapse into accepting previously rejected events into its view of the current state of that room. This can be exploited in a way that causes all further messages and state changes sent in that room from the vulnerable homeserver to be rejected.

Synapse homeservers are affected by this issue if and only if they are joined to rooms which members of untrusted homeservers are joined or invited to.

  • Synapse homeservers in rooms available over public federation are affected.
  • Synapse homeservers with federation disabled are not affected.
  • Synapse homeservers in a closed federation containing only trusted servers are not affected.
  • Synapse homeservers which are only joined to rooms with federation disabled[^1] are not affected.

Patches

Administrators of homeservers with federation enabled are advised to upgrade to 1.68.0 or higher.

Workarounds

  • Federation can be disabled by setting federation_domain_whitelist to an empty list ([]). from the vulnerable homeserver to be rejected. This issue has been patched in version 1.68.0

References

[^1]: See m.federate in the m.room.create definition.

For more information

If you have any questions or comments about this advisory, e-mail us at security@matrix.org.

Affected software

Click a technology to see all vulnerabilities affecting it.

PlatformPackageVulnerable versionsFixed in
Python (PyPI) matrix-synapse >= 1.62.0, < 1.68.0rc1 1.68.0rc1

References

CVSS score 6.5/10
CVSS vector
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
Weakness (CWE)
CWE-400
Platform
Python (PyPI)
Package
matrix-synapse
Disclosed
24.05.2023
Updated
11.10.2026 11:20

Sources and licenses

OSV / GitHub Advisory Database - PYSEC-2023-66 - PyPA Advisory Database; CC BY 4.0

OSV / GitHub Advisory Database - GHSA-p9qp-c452-f9r7 - CC BY 4.0

Vulnerability history: matrix-synapse

All published vulnerabilities of this package (Python (PyPI)), newest first. Shows whether this is a one-off slip or a package that needs watching.

All (45) in the archive
Total entries
45
First → latest
03.2019 05.2026
Without a fix
0
Exploited in attacks
0

Severity breakdown

  • High 21
  • Medium 21
  • Low 3

Disclosures per year

Severity:
  1. Synapse CPU starvation (Denial of Service)
    High · 5.5 CVE-2026-45078 < 1.152.1 1.152.1
  2. Synapse vulnerable to federation denial of service via malformed events
    High · 7.1 CVE-2025-30355 < 1.127.1 1.127.1
  3. Synapse can be forced to thumbnail unexpected file formats, invoking external, potentially untrustworthy decoders
    High CVE-2024-53863 < 1.120.1 1.120.1
  4. Synapse allows a a malformed invite to break the invitee's `/sync`
    High CVE-2024-52815 < 1.120.1 1.120.1
  5. Synapse allows unsupported content types to lead to memory exhaustion
    High CVE-2024-52805 < 1.120.1 1.120.1
  6. Synapse denial of service through media disk space consumption
    High · 7.5 CVE-2024-37302 < 1.106 1.106
  7. Synapse Denial of service due to incorrect application of event authorization rules during state resolution this entry
    High · 6.5 CVE-2022-39374 >= 1.62.0, < 1.68.0rc1 1.68.0rc1
  8. Synapse does not apply enough checks to servers requesting auth events of events in a room
    High · 5.0 CVE-2022-39335 < 1.69.0 1.69.0
  9. Denial of service due to incorrect application of event authorization rules
    High · 7.5 CVE-2022-31152 < 1.62.0rc1 1.62.0rc1
  10. URL previews of unusual or maliciously-crafted pages can crash Synapse media repositories or Synapse monoliths
    High · 6.5 CVE-2022-31052 < 1.61.1 1.61.1
  11. Matrix Synapse DoS
    High · 7.5 CVE-2018-10657 < 0.28.1 0.28.1
  12. Matrix Synapse Improper Signature Validation
    High · 8.8 CVE-2018-16515 >= 0.33.3, < 0.33.3.1, < 0.33.2.1 0.33.3.1, 0.33.2.1
  13. Matrix Synapse Authorization Error
    High · 7.5 CVE-2018-12423 < 0.31.2 0.31.2
  14. Matrix Synapse Security Filtering Flaw
    High · 7.5 CVE-2018-12291 < 0.31.1 0.31.1
  15. Path traversal in Matrix Synapse
    High · 7.5 CVE-2021-41281 < 1.47.1 1.47.1
  16. Open redirect via transitional IPv6 addresses on dual-stack networks
    High · 6.3 CVE-2021-21392 < 1.28.0rc1 1.28.0rc1
  17. Denial of service attack via incorrect parameters in Matrix Synapse
    High · 6.5 CVE-2020-26257 < 1.23.1 1.23.1
  18. Denial of service attack due to invalid JSON
    High · 7.5 CVE-2020-26890 < 1.20.0 1.20.0
  19. Improper Verification of Cryptographic Signature in matrix-synapse
    High · 8.6 CVE-2019-18835 < 1.5.0 1.5.0
  20. matrix-sydent and matrix-synapse Use Cryptographically Weak PRNG
    High · 7.5 CVE-2019-11842 < 0.99.3.1 0.99.3.1
  21. Matrix Synapse before 0.34.0.1, when the macaroon_secret_key authentication parameter is not set, uses a predictable value to derive a secret key and other secrets which could allow remote attackers t
    High · 7.5 CVE-2019-5885 < 0.34.0.1 0.34.0.1

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