CVE-2020-27633: In FNET 4.6.3, TCP ISNs are improperly random.

This month

CRITICAL 9.1REMOTEDOWNGRADE

CVE-2020-27633 is a critical-severity flaw (CVSS 9.1) affecting fnet. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

In FNET 4.6.3, TCP ISNs are improperly random.

Why this priority

Critical severity
Remotely exploitable, no login, no user action

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

Risk facts

CVSS score	9.1 (Critical)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	0.9% (top 41%)
CISA exploited list	No
Weakness	CWE-330
Published	2023-10-10
Last modified	2026-06-17

Publisher (CNA): MITRE · Vendor: butok

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

Am I affected?

Timeline

2023-10-10 17:15 UTCPublished CVE record published
2026-06-17 03:09 UTCUpdated Record last modified

butok fnet DOWNGRADE

Affected: 4.6.3

Fixed in: no fixed release known

Safe to downgrade to: a release before 4.6.3

1. Check installed version

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

2. No fixed release: DOWNGRADE to a release before 4.6.3 (Debian / Ubuntu)

apt-cache madison fnet sudo apt install fnet=<VERSION_BEFORE_4.6.3> sudo apt-mark hold fnet

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

dnf --showduplicates list fnet sudo dnf downgrade fnet-<VERSION_BEFORE_4.6.3> sudo dnf versionlock add fnet

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-2020-27633?

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

Is CVE-2020-27633 being exploited?

CVE-2020-27633 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-2020-27633 affect?

CVE-2020-27633 affects fnet. Severity is critical (CVSS 9.1). In FNET 4.6.3, TCP ISNs are improperly random.

How urgent is CVE-2020-27633?

Priority 52/100. Critical severity; Remotely exploitable, no login, no user action

References

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