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Medium

Synapse has URL deny list bypass via oEmbed and image URLs when generating previews

  • Disclosed: 06.06.2023
  • Updated: 11.10.2026 11:20

Impact

A discovered oEmbed or image URL can bypass the url_preview_url_blacklist setting potentially allowing server side request forgery or bypassing network policies. Impact is limited to IP addresses allowed by the url_preview_ip_range_blacklist setting (by default this only allows public IPs) and by the limited information returned to the client:

  • For discovered oEmbed URLs, any non-JSON response or a JSON response which includes non-oEmbed information is discarded.
  • For discovered image URLs, any non-image response is discarded.

Systems which have URL preview disabled (via the url_preview_enabled setting) or have not configured a url_preview_url_blacklist are not affected.

Because of the uncommon configuration required, the limited information a malicious user, and the amount of guesses/time the attack would need; the severity is rated as low.

Patches

The issue is fixed by #15601.

Workarounds

The default configuration of the url_preview_ip_range_blacklist should protect against requests being made to internal infrastructure, URL previews of public URLs is expected.

Alternately URL previews could be disabled using the url_preview_enabled setting.

Affected software

Click a technology to see all vulnerabilities affecting it.

PlatformPackageVulnerable versionsFixed in
Python (PyPI) matrix-synapse < 1.85.0 1.85.0

References

CVSS score 3.5/10
CVSS vector
CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:U/C:N/I:L/A:N
Weakness (CWE)
CWE-863, CWE-918
Platform
Python (PyPI)
Package
matrix-synapse
Disclosed
06.06.2023
Updated
11.10.2026 11:20

Sources and licenses

OSV / GitHub Advisory Database - PYSEC-2023-85 - PyPA Advisory Database; CC BY 4.0

OSV / GitHub Advisory Database - GHSA-98px-6486-j7qc - 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 has URL deny list bypass via oEmbed and image URLs when generating previews this entry
    Medium · 3.5 CVE-2023-32683 < 1.85.0 1.85.0
  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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