CVE-2021-37592Public exploitNetwork attack vector

TCP signature evasion via FIN-SYN stream desynchronization

Published Nov 19, 2021 · Updated Aug 4, 2024

TCP detection bypass in OISF Suricata before 5.0.8 and 6.0.0 through 6.0.3 allows remote attackers to evade signatures. StreamTcpHandleFin treats a FIN+SYN segment as a normal FIN, so a later invalid-sequence SYN replaces the live flow's sequence number and desynchronizes reassembly. Exploitation requires control of a client TCP/IP stack and a server that keeps the connection alive, allowing matching traffic to pass without alerts.

CVSS severity9.8
Critical
EPSS probability1.60%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

TCP detection bypass in OISF Suricata before 5.0.8 and 6.0.0 through 6.0.3 allows remote attackers to evade signatures. StreamTcpHandleFin treats a FIN+SYN segment as a normal FIN, so a later invalid-sequence SYN replaces the live flow's sequence number and desynchronizes reassembly. Exploitation requires control of a client TCP/IP stack and a server that keeps the connection alive, allowing matching traffic to pass without alerts.

The record

CVE
CVE-2021-37592
Published
Nov 19, 2021
Updated
Aug 4, 2024
Vendor
Unknown vendor
Product
Unknown product
Classifications
CWE-787, T1562.001
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Nov 19, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 4, 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

7.5NIST NVDAV:N/AC:L/Au:N/C:P/I:P/A:P
9.8NIST NVDCVSS: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.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 9, 2026Sep 15, 2026
Latest reporting daySep 15, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitPublished

Weakness, pattern, technique

CWE-787Out-of-bounds Write
T1562.001Impair Defenses: Disable or Modify Tools

Public exploit references

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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