CVE-2000-0453: XFree86 3.3.x and 4.0 allows a user to cause a denial of service via a negative counter value in a malformed T

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MEDIUM 5.0DOWNGRADE

CVE-2000-0453 is a medium-severity flaw (CVSS 5.0) affecting x11r6. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

XFree86 3.3.x and 4.0 allows a user to cause a denial of service via a negative counter value in a malformed TCP packet that is sent to port 6000.

Why this priority

Public exploit reference

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

Risk facts

CVSS score	5.0 (Medium)
EPSS exploit chance	3.0% (top 13%)
CISA exploited list	No
Published	2000-05-18
Last modified	2026-06-16

Publisher (CNA): MITRE · Vendor: xfree86 project

Last updated 2026-06-16 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2000-05-18 04:00 UTCPublished CVE record published
2026-06-16 21:51 UTCUpdated Record last modified

xfree86 project x11r6 DOWNGRADE

Affected: 3.3.5 · 3.3.6 · 4.0

Fixed in: no fixed release known

Safe to downgrade to: a release before 3.3.5

1. Check installed version

apt-cache policy x11r6 || rpm -q x11r6 || x11r6 --version

2. No fixed release: DOWNGRADE to a release before 3.3.5 (Debian / Ubuntu)

apt-cache madison x11r6 sudo apt install x11r6=<VERSION_BEFORE_3.3.5> sudo apt-mark hold x11r6

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

dnf --showduplicates list x11r6 sudo dnf downgrade x11r6-<VERSION_BEFORE_3.3.5> sudo dnf versionlock add x11r6

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: XFree86 X11R6 3.3.5/3.3.6/4.0 Xserver - Denial of Service

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-2000-0453?

x11r6: downgrade or apply the listed mitigation. Exact commands for each OS are on this page.

Is CVE-2000-0453 being exploited?

CVE-2000-0453 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-2000-0453 affect?

CVE-2000-0453 affects x11r6. Severity is medium (CVSS 5.0). XFree86 3.3.x and 4.0 allows a user to cause a denial of service via a negative counter value in a malformed TCP packet that is sent to port 6000.

How urgent is CVE-2000-0453?

Priority 44/100. Public exploit reference

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References

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