CVE-2020-27634Network attack vector

TCP session hijacking via constant-seeded ISNs

Published Oct 10, 2023 · Updated Sep 19, 2024

Insufficiently random TCP ISNs in Contiki-NG 4.5 allow remote attackers to hijack TCP connections via spoofed packets. The uIP TCP stack initializes its sequence-number generator with a constant, making subsequent ISNs predictable across connections. An attacker able to reach the TCP endpoint and predict a live connection's sequence space can read or inject connection data without authentication.

CVSS severity9.1
Critical
EPSS probability1.72%
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 Contiki-NG 4.5 allow remote attackers to hijack TCP connections via spoofed packets. The uIP TCP stack initializes its sequence-number generator with a constant, making subsequent ISNs predictable across connections. An attacker able to reach the TCP endpoint and predict a live connection's sequence space can read or inject connection data without authentication.

The record

CVE
CVE-2020-27634
Published
Oct 10, 2023
Updated
Sep 19, 2024
Vendor
Unknown vendor
Product
Unknown product
Classifications
CWE-330, T1659
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

9.1NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N

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
T1659Content Injection

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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