CVE-2020-27631Network attack vector

TCP session hijacking via reversible ISN generator

Published Oct 10, 2023 · Updated Sep 19, 2024

Insufficiently random TCP ISNs in Oryx CycloneTCP 1.9.6 allow remote attackers to hijack or spoof TCP connections. The TCP initial-sequence-number generator uses a linear congruential generator that is reversible from observed outputs and seeded with publicly retrievable information. Exploitation requires enough observable sequence output to predict later numbers, inject session data, spoof a connection, or interrupt one.

CVSS severity9.8
Critical
EPSS probability1.08%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Insufficiently random TCP ISNs in Oryx CycloneTCP 1.9.6 allow remote attackers to hijack or spoof TCP connections. The TCP initial-sequence-number generator uses a linear congruential generator that is reversible from observed outputs and seeded with publicly retrievable information. Exploitation requires enough observable sequence output to predict later numbers, inject session data, spoof a connection, or interrupt one.

The record

CVE
CVE-2020-27631
Published
Oct 10, 2023
Updated
Sep 19, 2024
Vendor
Unknown vendor
Product
Unknown product
Classifications
CWE-330, T1565.002
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Oct 10, 2023CVE publishedPublication date reported by the CVE source.
  2. Sep 19, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Affected buildAffected product and version details are unavailable.

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Affected products and versions are unavailable in this record.

Published CVSS scores

9.8NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

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No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-330Use of Insufficiently Random Values
T1565.002Transmitted Data Manipulation

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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