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Denial of service attack via .well-known lookups

Moderate severity GitHub Reviewed Published Feb 25, 2021 in matrix-org/synapse • Updated Jan 29, 2023

Package

pip matrix-synapse (pip)

Affected versions

>= 0.99.0, < 1.25.0

Patched versions

1.25.0

Description

Impact

A malicious homeserver could redirect requests to their .well-known file to a large file. This can lead to a denial of service attack where homeservers will consume significantly more resources when requesting the .well-known file of a malicious homeserver.

This affects any server which accepts federation requests from untrusted servers.

Patches

Issue is resolved by #8950. A bug not affecting the security aspects of this was fixed in #9108.

Workarounds

The federation_domain_whitelist setting can be used to restrict the homeservers communicated with over federation.

References

@clokep clokep published to matrix-org/synapse Feb 25, 2021
Published by the National Vulnerability Database Feb 26, 2021
Reviewed Mar 1, 2021
Published to the GitHub Advisory Database Mar 1, 2021
Last updated Jan 29, 2023

Severity

Moderate

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v3 base metrics

Attack vector
Network
Attack complexity
Low
Privileges required
None
User interaction
Required
Scope
Unchanged
Confidentiality
None
Integrity
None
Availability
Low

CVSS v3 base metrics

Attack vector: More severe the more the remote (logically and physically) an attacker can be in order to exploit the vulnerability.
Attack complexity: More severe for the least complex attacks.
Privileges required: More severe if no privileges are required.
User interaction: More severe when no user interaction is required.
Scope: More severe when a scope change occurs, e.g. one vulnerable component impacts resources in components beyond its security scope.
Confidentiality: More severe when loss of data confidentiality is highest, measuring the level of data access available to an unauthorized user.
Integrity: More severe when loss of data integrity is the highest, measuring the consequence of data modification possible by an unauthorized user.
Availability: More severe when the loss of impacted component availability is highest.
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:L

EPSS score

0.174%
(55th percentile)

Weaknesses

CVE ID

CVE-2021-21274

GHSA ID

GHSA-2hwx-mjrm-v3g8

Source code

Credits

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