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High

Denial of service attack due to invalid JSON

  • Disclosed: 24.11.2020
  • Updated: 11.10.2026 13:19

Impact

A denial of service attack against Matrix clients can be exploited by sending an event including invalid JSON data to Synapse. Synapse would relay the data to clients which could crash or hang. Impact is long-lasting if the event is made part of the room state.

Patches

At a minimum #8106 and #8291 must be applied. #7372 and #8124 include additional checks.

Workarounds

There are no known workarounds.

Upgrading notes

If an invalid event is accepted by an earlier Synapse it can become part of the room state and will not be fixed by upgrading Synapse. Redacting the invalid event should avoid clients receiving the invalid event.

Affected software

Click a technology to see all vulnerabilities affecting it.

PlatformPackageVulnerable versionsFixed in
Python (PyPI) matrix-synapse < 1.20.0 1.20.0

References

CVSS score 7.5/10
CVSS vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Weakness (CWE)
CWE-20
Platform
Python (PyPI)
Package
matrix-synapse
Disclosed
24.11.2020
Updated
11.10.2026 13:19

Sources and licenses

OSV / GitHub Advisory Database - PYSEC-2020-237 - PyPA Advisory Database; CC BY 4.0

OSV / GitHub Advisory Database - GHSA-4mp3-385r-v63f - 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
    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 this entry
    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
Show more (1) Showing 20 of 21

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