| CVE-2021-21274 | Date: (C)2021-02-27 (M)2023-12-22 |
Synapse is a Matrix reference homeserver written in python (pypi package matrix-synapse). Matrix is an ecosystem for open federated Instant Messaging and VoIP. In Synapse before version 1.25.0, 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. Issue is resolved in version 1.25.0. As a workaround the `federation_domain_whitelist` setting can be used to restrict the homeservers communicated with over federation.
CVSS Score and Metrics +CVSS Score and Metrics -| CVSS V3 Severity: | CVSS V2 Severity: |
| CVSS Score : 6.5 | CVSS Score : 4.3 |
| Exploit Score: 2.8 | Exploit Score: 8.6 |
| Impact Score: 3.6 | Impact Score: 2.9 |
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| CVSS V3 Metrics: | CVSS V2 Metrics: |
| Attack Vector: NETWORK | Access Vector: NETWORK |
| Attack Complexity: LOW | Access Complexity: MEDIUM |
| Privileges Required: NONE | Authentication: NONE |
| User Interaction: REQUIRED | Confidentiality: NONE |
| Scope: UNCHANGED | Integrity: NONE |
| Confidentiality: NONE | Availability: PARTIAL |
| Integrity: NONE | |
| Availability: HIGH | |
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