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No exploitation reportedFix available

CVE-2021-21332

No exploitation reported by the sources we read.

  • Published2021-03-26
  • WeaknessCWE-79

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

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 1.27.0, the password reset endpoint served via Synapse was vulnerable to cross-site scripting (XSS) attacks. The impact depends on the configuration of the domain that Synapse is deployed on, but may allow access to cookies and other browser data, CSRF vulnerabilities, and access to other resources served on the same domain or parent domains. This is fixed in version 1.27.0.

Timeline

dates in UTC
  1. 2021-03-2620:15CVE publishedNVD

Observed activity

isMalicious corpus · updated 2026-10-23 18:48 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-2021-21332?format=txt
  • CSV export: https://api.ismalicious.com/edl/cve/CVE-2021-21332?format=csv
  • TAXII collection of exploited CVEs
  • The EDL and CSV take an API key on the Pro plan.

Versions per branch

NVD
Fedoraproject fedora
BranchAffectedFixed in
34.x= 34no fix published
Matrix synapse
BranchAffectedFixed in
1.27< 1.27.01.27.0

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

Fix

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 3.16.9read 2026-10-23
  • NVDCVSS 3.18.2read 2026-10-23
  • NVDCVSS 2.04.3read 2026-10-23
  • CNAweakness (CWE)CWE-79read 2026-10-23
  • NVDweakness (CWE)CWE-79read 2026-10-23
  • FIRST EPSSEPSS score1.2 %2026-10-22

Technical details

CVSS v3 Vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:L/A:N
Published
2021-03-26
Last Modified
2026-06-17
NVD status
Modified

Frequently asked questions

What is CVE-2021-21332?

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 1.27.0, the password reset endpoint served via Synapse was vulnerable to cross-site scripting (XSS) attacks. The impact depends on the configuration of the domain that Synapse is deployed on, but may allow access to cookies and other browser data, CSRF vulnerabilities, and access to other resources served on the same domain or parent domains. This is fixed in version 1.27.0.

Is CVE-2021-21332 actively exploited?

Active exploitation of CVE-2021-21332 has not been confirmed. Its EPSS score was 1.2% on 2026-10-22, the estimated probability of exploitation in the next 30 days.

What is the CVSS score for CVE-2021-21332?

CVE-2021-21332 has a CVSS v3 base score of 8.2 (HIGH severity), with vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:L/A:N.

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