{"id":"CVE-2021-21394","aliases":["GHSA-w9fg-xffh-p362","PYSEC-2021-27"],"title":"Denial of service (via resource exhaustion) due to improper input validation on third-party identifier endpoints","summary":"Denial of service (via resource exhaustion) due to improper input validation on third-party identifier endpoints","severity":"medium","cvss":5.3,"cvssVector":"CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H","vendor":"matrix-synapse","product":"matrix-synapse","ecosystem":"pip","affected":["matrix-synapse < 1.28.0"],"patched":["matrix-synapse 1.28.0"],"published":"2021-04-13","updated":"2026-07-08","source":"OSV","sourceUrl":"https://osv.dev/vulnerability/GHSA-w9fg-xffh-p362","references":[{"url":"https://github.com/matrix-org/synapse/security/advisories/GHSA-w9fg-xffh-p362"},{"url":"https://nvd.nist.gov/vuln/detail/CVE-2021-21394"},{"url":"https://github.com/matrix-org/synapse/pull/9321"},{"url":"https://github.com/matrix-org/synapse/pull/9393"},{"url":"https://github.com/matrix-org/synapse"},{"url":"https://github.com/pypa/advisory-database/tree/main/vulns/matrix-synapse/PYSEC-2021-27.yaml"},{"url":"https://lists.fedoraproject.org/archives/list/package-announce@lists.fedoraproject.org/message/TNNAJOZNMVMXM6AS7RFFKB4QLUJ4IFEY"},{"url":"https://pypi.org/project/matrix-synapse"}],"tags":["osv","pip"],"epss":0.01538,"epssPercentile":0.73794,"ingestedAt":"2026-07-08T18:25:53.589Z","slug":"CVE-2021-21394","body":"## Overview\n\n### Impact\nMissing input validation of some parameters on the endpoints used to confirm third-party identifiers could cause excessive use of disk space and memory leading to resource exhaustion.\n\n### Patches\nThe issue is fixed by #9321.\n\n### Workarounds\nDepending on the needs and configuration of the homeserver a few options are available:\n\n1. Using email as third-party identifiers be disabled by not configuring the `email` setting.\n2. Using phone numbers as third-party identifiers can be disabled by ensuring that `account_threepid_delegates.msisdn` is not configured.\n3. Additionally, the affected endpoint patterns can be blocked at a reverse proxy:\n\n    * `^/_matrix/client/(r0|unstable)/register/email`\n    * `^/_matrix/client/(r0|unstable)/register/msisdn`\n    * `^/_matrix/client/(r0|unstable)/account/password`\n    * `^/_matrix/client/(r0|unstable)/account/3pid`\n\n## Affected packages\n\n- `matrix-synapse < 1.28.0`\n\n## Remediation\n\nUpgrade to a patched release:\n\n- `matrix-synapse 1.28.0`","depth":"sunlit","depthScore":29,"depthScoreParts":{"impact":29.2,"likelihood":0.3,"exploitation":0,"ransomware":0},"changes":[]}