CVE-2021-21333Network attack vector

Forged email content via disabled autoescaping

Published Mar 26, 2021 · Updated Aug 3, 2024

HTML injection in Matrix.org Synapse before 1.27.0 allows remote attackers to forge content in missed-message notification emails. The template loader creates Jinja2 environments with autoescaping disabled, so message-controlled variables enter notif.html, room.html, and notif_mail.html without HTML escaping. Exploitation requires a recipient configured for missed-message email to receive and view the forged notification; account-expiry injection is not attacker-controlled.

CVSS severity6.1
Medium
EPSS probability1.39%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

HTML injection in Matrix.org Synapse before 1.27.0 allows remote attackers to forge content in missed-message notification emails. The template loader creates Jinja2 environments with autoescaping disabled, so message-controlled variables enter notif.html, room.html, and notif_mail.html without HTML escaping. Exploitation requires a recipient configured for missed-message email to receive and view the forged notification; account-expiry injection is not attacker-controlled.

The record

CVE
CVE-2021-21333
Published
Mar 26, 2021
Updated
Aug 3, 2024
Vendor
Matrix.org
Product
Synapse
Classifications
CWE-79, CWE-74, T1566
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

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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

6.1GitHub, Inc.CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:N/I:H/A:N
2.6NIST NVDAV:N/AC:H/Au:N/C:N/I:P/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')
CWE-74Improper Neutralization of Special Elements in Output Used by a Downstream Component ('Injection')
T1566Phishing

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

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Your stack

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