CVE-2021-26835: No filtering of cross-site scripting (XSS) payloads in the markdown-editor in Zettlr 1.8.7 allows attackers to

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MEDIUM 6.1DOWNGRADE

CVE-2021-26835 is a medium-severity flaw (CVSS 6.1) affecting zettlr. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

No filtering of cross-site scripting (XSS) payloads in the markdown-editor in Zettlr 1.8.7 allows attackers to perform remote code execution via a crafted file.

Why this priority

Public exploit reference

No fix is published yet: follow the downgrade or mitigation steps.

Risk facts

CVSS score	6.1 (Medium)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	Required
EPSS exploit chance	1.3% (top 30%)
CISA exploited list	No
Weakness	CWE-79
Published	2021-06-18
Last modified	2026-06-17

Publisher (CNA): MITRE · Vendor: zettlr

Last updated 2026-06-17 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2021-06-18 12:15 UTCPublished CVE record published
2026-06-17 03:43 UTCUpdated Record last modified

zettlr zettlr DOWNGRADE

Affected: 1.8.7

Fixed in: no fixed release known

Safe to downgrade to: a release before 1.8.7

1. Check installed version

apt-cache policy zettlr || rpm -q zettlr || zettlr --version

2. No fixed release: DOWNGRADE to a release before 1.8.7 (Debian / Ubuntu)

apt-cache madison zettlr sudo apt install zettlr=<VERSION_BEFORE_1.8.7> sudo apt-mark hold zettlr

2. No fixed release: DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list zettlr sudo dnf downgrade zettlr-<VERSION_BEFORE_1.8.7> sudo dnf versionlock add zettlr

Downgrade risk

Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.

After downgrade

Restart the service, and keep the package held until a fixed release ships.

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2021-26835?

zettlr: downgrade or apply the listed mitigation. Exact commands for each OS are on this page.

Is CVE-2021-26835 being exploited?

CVE-2021-26835 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2021-26835 affect?

CVE-2021-26835 affects zettlr. Severity is medium (CVSS 6.1). No filtering of cross-site scripting (XSS) payloads in the markdown-editor in Zettlr 1.8.7 allows attackers to perform remote code execution via a crafted file.

How urgent is CVE-2021-26835?

Priority 44/100. Public exploit reference

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CVE-2021-20727 MEDIUM 6.1 UPGRADE Cross-site scripting vulnerability in Zettlr from 0.20.0 to 1.8.8 allows an attacker to execute an arbitrary s

References

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