CVE-2021-21332Network attack vector

Browser script execution via unescaped password-reset template

Published Mar 26, 2021 · Updated Aug 3, 2024

Cross-site scripting in Matrix.org Synapse before 1.27.0 allows remote attackers to run script in a user's browser via password reset links. The password-reset confirmation endpoint renders attacker-controlled variables through Jinja2 HTML templates with autoescaping disabled. A victim must open a crafted link, and exposure depends on same-domain or parent-domain hosting; successful script can read cookies and browser data or issue same-origin requests.

CVSS severity6.9
Medium
EPSS probability1.22%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
Coverage for this vulnerability

See if you're affected

Explore vulnerability checks for your environment with Hinoki.

Book a demo

Summary

What happened

Cross-site scripting in Matrix.org Synapse before 1.27.0 allows remote attackers to run script in a user's browser via password reset links. The password-reset confirmation endpoint renders attacker-controlled variables through Jinja2 HTML templates with autoescaping disabled. A victim must open a crafted link, and exposure depends on same-domain or parent-domain hosting; successful script can read cookies and browser data or issue same-origin requests.

The record

CVE
CVE-2021-21332
Published
Mar 26, 2021
Updated
Aug 3, 2024
Vendor
Matrix.org
Product
Synapse
Classifications
CWE-79, T1189
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Mar 26, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 3, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

Compare your product and version with the public record. A matching version still requires validation against your environment.

Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=< 1.27.0

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Matrix.org · Synapseversion=< 1.27.0

Published CVSS scores

4.3NIST NVDAV:N/AC:M/Au:N/C:N/I:P/A:N
6.9GitHub, Inc.CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:H/I:L/A:N
8.2NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:L/A:N

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-79Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
T1189Drive-by Compromise

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

See the directory against your own environment.

Your stack

Check the software in your environment

Book a demo to see how Hinoki identifies affected software and validates exploitability in your environment.

Book a demo