{"cve":"CVE-2021-29471","nvd":{"published":"2021-05-11T15:15:08.507","modified":"2026-06-17T03:47:42.517","status":"Modified","description":"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 1.33.2 \"Push rules\" can specify conditions under which they will match, including `event_match`, which matches event content against a pattern including wildcards. Certain patterns can cause very poor performance in the matching engine, leading to a denial-of-service when processing moderate length events. The issue is patched in version 1.33.2. A potential workaround might be to prevent users from making custom push rules, by blocking such requests at a reverse-proxy.","cvss_v4":null,"cvss_v3":{"version":"3.1","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L","score":5.3},"cwes":["CWE-331","CWE-400"],"references":["https://github.com/matrix-org/synapse/commit/03318a766cac9f8b053db2214d9c332a977d226c","https://github.com/matrix-org/synapse/releases/tag/v1.33.2","https://github.com/matrix-org/synapse/security/advisories/GHSA-x345-32rc-8h85","https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/TNNAJOZNMVMXM6AS7RFFKB4QLUJ4IFEY/","https://github.com/matrix-org/synapse/commit/03318a766cac9f8b053db2214d9c332a977d226c"],"products":["fedoraproject:fedora","matrix:synapse"],"source":"nvd","synced_at":"2026-10-19T21:18:09.669877188+00:00"},"epss":{"cve":"CVE-2021-29471","score":0.01647,"percentile":0.7647,"date":"2026-11-01"},"kev":null,"osv":[{"id":"GHSA-x345-32rc-8h85","url":"https://osv.dev/vulnerability/GHSA-x345-32rc-8h85","summary":"Denial of service attack via push rule patterns in matrix-synapse","modified":"2026-07-08T06:27:40.962217171Z","packages":[{"ecosystem":"PyPI","name":"matrix-synapse","purl":"pkg:pypi/matrix-synapse","ranges":["<1.33.2"],"fixed":["1.33.2"]}]},{"id":"PYSEC-2021-135","url":"https://osv.dev/vulnerability/PYSEC-2021-135","summary":"","modified":"2023-11-08T04:05:34.987105Z","packages":[{"ecosystem":"PyPI","name":"matrix-synapse","purl":"pkg:pypi/matrix-synapse","ranges":["<1.33.2"],"fixed":["1.33.2"]}]}],"attribution":["This product uses the NVD API but is not endorsed or certified by the NVD.","EPSS scores: FIRST Exploit Prediction Scoring System, https://www.first.org/epss","Known exploited vulnerabilities: CISA Known Exploited Vulnerabilities Catalog, https://www.cisa.gov/known-exploited-vulnerabilities-catalog","Package advisories: OSV (https://osv.dev), from the GitHub Advisory Database, PyPA, Go, RustSec and other ecosystem databases, under CC-BY 4.0 or CC0","CVE records: the CVE Program's CVE List (https://www.cve.org), under its terms of use (https://www.cve.org/Legal/TermsOfUse). CVE is a registered trademark of The MITRE Corporation; this product is not affiliated with the CVE Program."]}