CVE-2021-20727

Arbitrary script execution via unsanitized iframe attributes

Published May 27, 2021 · Updated Aug 3, 2024

Cross-site scripting in Zettlr 0.20.0 through 1.8.8 allows attackers to execute arbitrary scripts when a user opens crafted Markdown. The Markdown editor's render-iframes plugin inserted the complete matched iframe element, including attacker-controlled srcdoc content, into the editor DOM. Exploitation requires a user to load attacker-supplied Markdown or an iframe snippet while iframe rendering is enabled, allowing script execution in the desktop application's context.

CVSS severity5.0
Medium
EPSS probability1.04%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Cross-site scripting in Zettlr 0.20.0 through 1.8.8 allows attackers to execute arbitrary scripts when a user opens crafted Markdown. The Markdown editor's render-iframes plugin inserted the complete matched iframe element, including attacker-controlled srcdoc content, into the editor DOM. Exploitation requires a user to load attacker-supplied Markdown or an iframe snippet while iframe rendering is enabled, allowing script execution in the desktop application's context.

The record

CVE
CVE-2021-20727
Published
May 27, 2021
Updated
Aug 3, 2024
Vendor
Zettlr
Product
Zettlr
Classifications
CWE-79, T1204.002
Attack vector
local
Privileges
unauthenticated

Timeline

How it unfolded

  1. May 27, 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=from 0.20.0 to 1.8.8

What conditions does exploitation require?

Attack vectorlocal
Required privilegesunauthenticated

What is affected?

Zettlr · Zettlrversion=from 0.20.0 to 1.8.8

Published CVSS scores

4.3JPCERT/CCAV:N/AC:M/Au:N/C:N/I:P/A:N
5.0JPCERT/CCCVSS:3.0/AV:L/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
6.1NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/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 9, 2026Sep 15, 2026
Latest reporting daySep 15, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectorlocal
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-79Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
T1204.002Malicious File

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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