CVE-2021-34557: XScreenSaver 5.45 can be bypassed if the machine has more than ten disconnectable video outputs

Routine

MEDIUM 4.6DOWNGRADEMITIGATE

CVE-2021-34557 is a medium-severity flaw (CVSS 4.6) affecting xscreensaver, fedora. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

XScreenSaver 5.45 can be bypassed if the machine has more than ten disconnectable video outputs. A buffer overflow in update_screen_layout() allows an attacker to bypass the standard screen lock authentication mechanism by crashing XScreenSaver. The attacker must physically disconnect many video outputs.

Why this priority

Public exploit reference

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

Risk facts

CVSS score	4.6 (Medium)
Attack vector	Physical
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	0.5% (top 59%)
CISA exploited list	No
Weakness	CWE-120
Published	2021-06-10
Last modified	2026-06-17

Publisher (CNA): MITRE · Vendor: fedoraproject · xscreensaver project

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

Am I affected?

Timeline

2021-06-10 16:15 UTCPublished CVE record published
2026-06-17 03:56 UTCUpdated Record last modified

xscreensaver project xscreensaver DOWNGRADE

Affected: 5.45

Fixed in: no fixed release known

Safe to downgrade to: a release before 5.45

1. Check installed version

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

2. No fixed release: DOWNGRADE to a release before 5.45 (Debian / Ubuntu)

apt-cache madison xscreensaver sudo apt install xscreensaver=<VERSION_BEFORE_5.45> sudo apt-mark hold xscreensaver

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

dnf --showduplicates list xscreensaver sudo dnf downgrade xscreensaver-<VERSION_BEFORE_5.45> sudo dnf versionlock add xscreensaver

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.

fedoraproject fedora MITIGATE

Affected: 33

Fixed in: no fixed release known

Safe to downgrade to: a release before 33

Check your OS release

cat /etc/os-release; uname -r

Debian / Ubuntu

sudo apt update && sudo apt full-upgrade

RHEL / Amazon Linux / Fedora

sudo dnf upgrade --refresh

Windows

Install the latest cumulative update from Windows Update

If no patch exists yet

OS downgrades are not safe. Apply the vendor mitigation from the advisory and restrict exposure.

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-2021-34557?

xscreensaver: downgrade or apply the listed mitigation; fedora: mitigate or apply the listed mitigation. Exact commands for each OS are on this page.

Is CVE-2021-34557 being exploited?

CVE-2021-34557 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-2021-34557 affect?

CVE-2021-34557 affects xscreensaver, fedora. Severity is medium (CVSS 4.6). XScreenSaver 5.45 can be bypassed if the machine has more than ten disconnectable video outputs. A buffer overflow in update_screen_layout() allows an attacker to bypass the standard screen lock authentication mechanism by crashing XScreenS

How urgent is CVE-2021-34557?

Priority 31/100. Public exploit reference

Related CVEs

CVE-2021-31523 HIGH 7.8 MITIGATE The Debian xscreensaver 5.42+dfsg1-1 package for XScreenSaver has cap_net_raw enabled for the /usr/libexec/xsc
CVE-2015-8025 LOW 2.1 MITIGATE DOWNGRADE driver/subprocs.c in XScreenSaver before 5.34 does not properly perform an internal consistency check, which a
CVE-2011-2187 HIGH 7.8 UPGRADE MITIGATE xscreensaver before 5.14 crashes during activation and leaves the screen unlocked when in Blank Only Mode and

References

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