CVE-2001-0851: Linux kernel 2.0, 2.2 and 2.4 with syncookies enabled allows remote attackers to bypass firewall rules by brut

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MEDIUM 5.0WATCHLISTDOWNGRADEDOWNGRADEMITIGATE

CVE-2001-0851 is a medium-severity flaw (CVSS 5.0) affecting openlinux server, openlinux workstation, openlinux. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

Linux kernel 2.0, 2.2 and 2.4 with syncookies enabled allows remote attackers to bypass firewall rules by brute force guessing the cookie.

Why this priority

On your watchlist

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	2001-12-06
Last modified	2026-06-16

Publisher (CNA): MITRE · Vendor: caldera · linux

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

Am I affected?

Timeline

2001-12-06 05:00 UTCPublished CVE record published
2026-06-16 21:55 UTCUpdated Record last modified

caldera openlinux server DOWNGRADE

Affected: 3.1

Fixed in: no fixed release known

Safe to downgrade to: a release before 3.1

1. Check installed version

apt-cache policy openlinux-server || rpm -q openlinux-server || openlinux-server --version

2. No fixed release: DOWNGRADE to a release before 3.1 (Debian / Ubuntu)

apt-cache madison openlinux-server sudo apt install openlinux-server=<VERSION_BEFORE_3.1> sudo apt-mark hold openlinux-server

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

dnf --showduplicates list openlinux-server sudo dnf downgrade openlinux-server-<VERSION_BEFORE_3.1> sudo dnf versionlock add openlinux-server

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.

caldera openlinux workstation DOWNGRADE

Affected: 3.1

Fixed in: no fixed release known

Safe to downgrade to: a release before 3.1

1. Check installed version

apt-cache policy openlinux-workstation || rpm -q openlinux-workstation || openlinux-workstation --version

2. No fixed release: DOWNGRADE to a release before 3.1 (Debian / Ubuntu)

apt-cache madison openlinux-workstation sudo apt install openlinux-workstation=<VERSION_BEFORE_3.1> sudo apt-mark hold openlinux-workstation

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

dnf --showduplicates list openlinux-workstation sudo dnf downgrade openlinux-workstation-<VERSION_BEFORE_3.1> sudo dnf versionlock add openlinux-workstation

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.

caldera openlinux MITIGATE

Affected: 2.3

Fixed in: no fixed release known

Safe to downgrade to: a release before 2.3

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.

caldera openlinux edesktop MITIGATE

Affected: 2.4

Fixed in: no fixed release known

Safe to downgrade to: a release before 2.4

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.

caldera openlinux eserver MITIGATE

Affected: 2.3.1

Fixed in: no fixed release known

Safe to downgrade to: a release before 2.3.1

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.

linux linux kernel MITIGATE

Affected: 2.0 · 2.2.0 · 2.4.0

Fixed in: no fixed release known

Safe to downgrade to: no unaffected older release

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.

Downgrade not recommended

This is a core system component: downgrading can break the OS. Apply the distro update or the vendor mitigation.

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-2001-0851?

openlinux server: downgrade or apply the listed mitigation; openlinux workstation: downgrade or apply the listed mitigation; openlinux: mitigate or apply the listed mitigation. Exact commands for each OS are on this page.

Is CVE-2001-0851 being exploited?

CVE-2001-0851 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-2001-0851 affect?

CVE-2001-0851 affects openlinux server, openlinux workstation, openlinux. Severity is medium (CVSS 5.0). Linux kernel 2.0, 2.2 and 2.4 with syncookies enabled allows remote attackers to bypass firewall rules by brute force guessing the cookie.

How urgent is CVE-2001-0851?

Priority 44/100. On your watchlist

Software pages

Linux kernel vulnerabilities

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References

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