CVE-2021-20727
Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.
NVD · uneditedCross-site scripting vulnerability in Zettlr from 0.20.0 to 1.8.8 allows an attacker to execute an arbitrary script by loading a file or code snippet containing an invalid iframe into Zettlr.
Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.
dbcve analysis · high confidenceZettlr markdown editor fails to properly sanitize or neutralize invalid iframe tags when loading files or code snippets. An attacker can embed malicious JavaScript in an intentionally malformed iframe within a loaded document, which then executes when the victim opens that file in the vulnerable Zettlr version.
Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.
Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.
NVD · CPE data>= 0.20.0, <= 1.8.8CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.
From the vector- Attack vector
- Network
- Complexity
- Low
- Privileges
- None
- User interaction
- Required
- Scope
- Changed
- Confidentiality
- Low
- Integrity
- Low
- Availability
- None
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.
dbcve checksWork through these to decide whether this CVE applies to you.
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Check installed Zettlr versionOpen Zettlr, then go to Help > About, or check the application version through your system package manager. Compare the version number to the affected range: 0.20.0 through 1.8.8.Affected if The installed version falls within 0.20.0 to 1.8.8 inclusive.
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Identify if you open external or shared markdown filesDetermine whether you regularly open markdown files from untrusted sources, shared documents, or files downloaded from the internet in Zettlr.Affected if You open markdown files from sources outside your trusted circle, as the vulnerability triggers when loading a file containing the malformed iframe.
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Inspect your markdown files for malformed iframe tagsSearch your opened or recently opened markdown files for any iframe HTML tags, particularly malformed ones with incomplete attributes or suspicious URLs.Affected if Any loaded file contains an iframe tag, especially one that appears malformed or contains unexpected URLs.
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Check if file loading with code snippets is enabledIn Zettlr settings, verify if markdown file rendering or code snippet support is active, as the vulnerability exploits how Zettlr processes iframe content when loading files.Affected if File rendering and HTML content processing is enabled when opening markdown files.
You are affected if Zettlr version 0.20.0 through 1.8.8 is installed AND you open markdown files that may contain malicious malformed iframe tags.
Generated from the published advisory. Verify against your own configuration.
Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.
From vendor dataUpgrade Zettlr to version 1.8.9 or later, which contains the fix for this XSS vulnerability.
- Consultation1.0 h
- Implementation2.0 h
- Testing2.0 h
- Review / QA1.0 h
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Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2021-20727 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.
References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.
Primary sourcesPractitioner notes
ContributedPeer-ranked notes from engineers who’ve handled CVE-2021-20727 in production — separate from our analysis above.
The advisory tells you what broke. It rarely tells you what actually worked. If you’ve dealt with this one, that detail is what the next engineer is searching for.
- The version that genuinely resolved it — not the one the vendor claimed
- A config change or rule that shut the vector down
- A gotcha in the upgrade path that cost you an afternoon
No notes yet
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- Verified mitigations, workarounds, and config changes
- Version or environment caveats, and links to real fixes
- No weaponised exploit code, or anything meant to cause harm
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