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Sydent is a reference Matrix identity server. Sydent does not limit the size of requests it receives from HTTP clients. A malicious user could send an HTTP request with a very large body, leading to memory exhaustion and denial of service. Sydent also does not limit response size for requests it makes to remote Matrix homeservers. A malicious homeserver could return a very large response, again leading to memory exhaustion and denial of service. This affects any server which accepts registration requests from untrusted clients. This issue has been patched by releases 89071a1, 0523511, f56eee3. As a workaround request sizes can be limited in an HTTP reverse-proxy. There are no known workarounds for the problem with overlarge responses.
Cite this page
CVE-2021-29430. CVEDatabase.com. Retrieved 3 May 2026. https://cvedatabase.com/cve/CVE-2021-29430
Use CWE-20, Matrix vendor hub and Sydent product page to widen CVE-2021-29430 into its surrounding weakness, vendor, and product context.
Compare it with CVE-2023-38686, CVE-2021-29431 and CVE-2019-11842 for nearby disclosures in the same product family.