CVE-2020-27635: In PicoTCP 1.7.0, TCP ISNs are improperly random.

This month

CRITICAL 9.1REMOTEDOWNGRADE

CVE-2020-27635 is a critical-severity flaw (CVSS 9.1) affecting picotcp. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

In PicoTCP 1.7.0, 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: capgemini

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

capgemini picotcp DOWNGRADE

Affected: 1.7.0

Fixed in: no fixed release known

Safe to downgrade to: a release before 1.7.0

1. Check installed version

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

2. No fixed release: DOWNGRADE to a release before 1.7.0 (Debian / Ubuntu)

apt-cache madison picotcp sudo apt install picotcp=<VERSION_BEFORE_1.7.0> sudo apt-mark hold picotcp

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

dnf --showduplicates list picotcp sudo dnf downgrade picotcp-<VERSION_BEFORE_1.7.0> sudo dnf versionlock add picotcp

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-2020-27635?

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

Is CVE-2020-27635 being exploited?

CVE-2020-27635 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-2020-27635 affect?

CVE-2020-27635 affects picotcp. Severity is critical (CVSS 9.1). In PicoTCP 1.7.0, TCP ISNs are improperly random.

How urgent is CVE-2020-27635?

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

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References

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