CVE-2021-40868: In Cloudron 6.2, the returnTo parameter on the login page is vulnerable to Reflected XSS.

This week

MEDIUM 6.1DOWNGRADE

CVE-2021-40868 is a medium-severity flaw (CVSS 6.1) affecting cloudron. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

In Cloudron 6.2, the returnTo parameter on the login page is vulnerable to Reflected XSS.

Why this priority

EPSS top 5%: very likely to be exploited
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	9.1% (top 5%)
CISA exploited list	No
Weakness	CWE-79
Published	2021-09-21
Last modified	2026-06-17

Publisher (CNA): MITRE · Vendor: cloudron

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

Am I affected?

Timeline

2021-09-21 17:15 UTCPublished CVE record published
2026-06-17 04:07 UTCUpdated Record last modified

cloudron cloudron DOWNGRADE

Affected: 6.2

Fixed in: no fixed release known

Safe to downgrade to: a release before 6.2

1. Check installed version

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

2. No fixed release: DOWNGRADE to a release before 6.2 (Debian / Ubuntu)

apt-cache madison cloudron sudo apt install cloudron=<VERSION_BEFORE_6.2> sudo apt-mark hold cloudron

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

dnf --showduplicates list cloudron sudo dnf downgrade cloudron-<VERSION_BEFORE_6.2> sudo dnf versionlock add cloudron

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.

Public exploits and detection

Public code exists for this CVE. Treat unpatched systems as at risk, and only run PoCs in a lab.

Exploit-DB: Cloudron 6.2 - 'returnTo ' Cross Site Scripting (Reflected)
Nuclei scan template: Cloudron 6.2 Cross-Site Scripting

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-2021-40868?

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

Is CVE-2021-40868 being exploited?

CVE-2021-40868 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-2021-40868 affect?

CVE-2021-40868 affects cloudron. Severity is medium (CVSS 6.1). In Cloudron 6.2, the returnTo parameter on the login page is vulnerable to Reflected XSS.

How urgent is CVE-2021-40868?

Priority 50/100. EPSS top 5%: very likely to be exploited; Public exploit reference

References

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