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Medium

Synapse V2 state resolution weakness allows Denial of Service (DoS)

  • Disclosed: 23.04.2024
  • Updated: 11.10.2026 15:17

Impact

A remote Matrix user with malicious intent, sharing a room with Synapse instances before 1.105.1, can dispatch specially crafted events to exploit a weakness in how the auth chain cover index is calculated. This can induce high CPU consumption and accumulate excessive data in the database of such instances, resulting in a denial of service.

Servers in private federations, or those that do not federate, are not affected.

Patches

Server administrators should upgrade to 1.105.1 or later.

Workarounds

One can:

  • ban the malicious users or ACL block servers from the rooms; and/or
  • leave the room and purge the room using the admin API

For more information

If you have any questions or comments about this advisory, please email us at security AT element.io.

Affected software

Click a technology to see all vulnerabilities affecting it.

PlatformPackageVulnerable versionsFixed in
Python (PyPI) matrix-synapse < 1.105.1 1.105.1

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-770
Platform
Python (PyPI)
Package
matrix-synapse
Disclosed
23.04.2024
Updated
11.10.2026 15:17

Sources and licenses

OSV / GitHub Advisory Database - PYSEC-2024-50 - PyPA Advisory Database; CC BY 4.0

OSV / GitHub Advisory Database - GHSA-3h7q-rfh9-xm4v - 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 V2 state resolution weakness allows Denial of Service (DoS) this entry
    Medium · 6.5 CVE-2024-31208 < 1.105.1 1.105.1
  8. Synapse Denial of service due to incorrect application of event authorization rules during state resolution
    High · 6.5 CVE-2022-39374 >= 1.62.0, < 1.68.0rc1 1.68.0rc1
  9. 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
  10. Denial of service due to incorrect application of event authorization rules
    High · 7.5 CVE-2022-31152 < 1.62.0rc1 1.62.0rc1
  11. 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
  12. Matrix Synapse DoS
    High · 7.5 CVE-2018-10657 < 0.28.1 0.28.1
  13. 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
  14. Matrix Synapse Authorization Error
    High · 7.5 CVE-2018-12423 < 0.31.2 0.31.2
  15. Matrix Synapse Security Filtering Flaw
    High · 7.5 CVE-2018-12291 < 0.31.1 0.31.1
  16. Path traversal in Matrix Synapse
    High · 7.5 CVE-2021-41281 < 1.47.1 1.47.1
  17. Open redirect via transitional IPv6 addresses on dual-stack networks
    High · 6.3 CVE-2021-21392 < 1.28.0rc1 1.28.0rc1
  18. Denial of service attack via incorrect parameters in Matrix Synapse
    High · 6.5 CVE-2020-26257 < 1.23.1 1.23.1
  19. Denial of service attack due to invalid JSON
    High · 7.5 CVE-2020-26890 < 1.20.0 1.20.0
  20. Improper Verification of Cryptographic Signature in matrix-synapse
    High · 8.6 CVE-2019-18835 < 1.5.0 1.5.0
  21. matrix-sydent and matrix-synapse Use Cryptographically Weak PRNG
    High · 7.5 CVE-2019-11842 < 0.99.3.1 0.99.3.1
  22. 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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