CVE-2021-41380: RealVNC Viewer 6.21.406 allows remote VNC servers to cause a denial of service (application crash) via crafted

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MEDIUM 6.5DOWNGRADE

CVE-2021-41380 is a medium-severity flaw (CVSS 6.5) affecting vnc viewer. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

RealVNC Viewer 6.21.406 allows remote VNC servers to cause a denial of service (application crash) via crafted RFB protocol data. NOTE: It is asserted that this issue requires social engineering a user into connecting to a fake VNC Server. The VNC Viewer application they are using will then hang, until terminated, but no memory leak occurs - the resources are freed once the hung process is terminated and the resource usage is constant during the

Why this priority

Public exploit reference

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

Risk facts

CVSS score	6.5 (Medium)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	Required
EPSS exploit chance	1.0% (top 39%)
CISA exploited list	No
Weakness	CWE-20
Published	2021-09-17
Last modified	2026-06-17

Publisher (CNA): MITRE · Vendor: realvnc

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

Am I affected?

Timeline

2021-09-17 20:15 UTCPublished CVE record published
2026-06-17 04:08 UTCUpdated Record last modified

realvnc vnc viewer DOWNGRADE

Affected: 6.21.406

Fixed in: no fixed release known

Safe to downgrade to: a release before 6.21.406

1. Check installed version

apt-cache policy vnc-viewer || rpm -q vnc-viewer || vnc-viewer --version

2. No fixed release: DOWNGRADE to a release before 6.21.406 (Debian / Ubuntu)

apt-cache madison vnc-viewer sudo apt install vnc-viewer=<VERSION_BEFORE_6.21.406> sudo apt-mark hold vnc-viewer

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

dnf --showduplicates list vnc-viewer sudo dnf downgrade vnc-viewer-<VERSION_BEFORE_6.21.406> sudo dnf versionlock add vnc-viewer

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-2021-41380?

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

Is CVE-2021-41380 being exploited?

CVE-2021-41380 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-2021-41380 affect?

CVE-2021-41380 affects vnc viewer. Severity is medium (CVSS 6.5). RealVNC Viewer 6.21.406 allows remote VNC servers to cause a denial of service (application crash) via crafted RFB protocol data. NOTE: It is asserted that this issue requires social engineering a user into connecting to a fake VNC Server.

How urgent is CVE-2021-41380?

Priority 43/100. Public exploit reference

Related CVEs

CVE-2022-41975 HIGH 7.8 UPGRADE UPGRADE RealVNC VNC Server before 6.11.0 and VNC Viewer before 6.22.826 on Windows allow local privilege escalation vi

References

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