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No exploitation reportedFix available

CVE-2021-29430

No exploitation reported by the sources we read.

  • Published2021-04-15
  • Fixed in2.3.0
  • WeaknessCWE-20 · CWE-400 · CWE-770

Am I affected?

With a domain, we read the version its public headers and pages expose. A distribution build can carry the fix under an older number.

Description

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.

Timeline

dates in UTC
  1. 2021-04-1521:15CVE publishedNVD

Observed activity

isMalicious corpus · updated 2026-10-23 21:37 UTC
Indicators tagged with this CVE, per day0 indicators over 7 days
Block these indicators
  • Firewall list (EDL, text): https://api.ismalicious.com/edl/cve/CVE-2021-29430?format=txt
  • CSV export: https://api.ismalicious.com/edl/cve/CVE-2021-29430?format=csv
  • TAXII collection of exploited CVEs
  • The EDL and CSV take an API key on the Pro plan.

Versions per branch

NVD · OSV
Matrix sydent
BranchAffectedFixed in
2.3< 2.3.02.3.0

as publishedRanges as NVD and OSV publish them; a distribution’s build can be fixed under an older number.

Fix

  1. Update Matrix sydent to 2.3.0

Advisories and patches

Detection

Nuclei template
none so far
ProjectDiscovery
Exploit-DB
no entry
Exploit-DB
Proof-of-concept repositories
none listed
PoC-in-GitHub

Sources

  • CNACVSS 3.17.5read 2026-10-23
  • NVDCVSS 3.17.5read 2026-10-23
  • NVDCVSS 2.05read 2026-10-23
  • CNAweakness (CWE)CWE-20, CWE-400read 2026-10-23
  • NVDweakness (CWE)CWE-770read 2026-10-23
  • FIRST EPSSEPSS score1.8 %2026-10-22
  • OSVaffected packages—

Technical details

CVSS v3 Vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Published
2021-04-15
Last Modified
2026-06-17
NVD status
Modified

Frequently asked questions

What is CVE-2021-29430?

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.

Is CVE-2021-29430 actively exploited?

Active exploitation of CVE-2021-29430 has not been confirmed. Its EPSS score was 1.8% on 2026-10-22, the estimated probability of exploitation in the next 30 days.

What is the CVSS score for CVE-2021-29430?

CVE-2021-29430 has a CVSS v3 base score of 7.5 (HIGH severity), with vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H.

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