CVE-2025-39955

TCP retransmission failure via stale Fast Open state

Published Oct 9, 2025 · Updated Oct 9, 2025

State-management failure in affected Linux kernels allows local users to suppress a TCP retransmission through Fast Open socket reuse. tcp_disconnect() closes an accepted server-side Fast Open socket without removing tcp_sk(sk)->fastopen_rsk, so a later retransmit timer encounters stale request state. Triggering the flaw requires disconnecting the accepted socket before its three-way handshake completes and reconnecting it elsewhere; the documented result is a kernel warning and one intended packet not being retransmitted.

CVSS severity7.8
High
EPSS probability0.16%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

State-management failure in affected Linux kernels allows local users to suppress a TCP retransmission through Fast Open socket reuse. tcp_disconnect() closes an accepted server-side Fast Open socket without removing tcp_sk(sk)->fastopen_rsk, so a later retransmit timer encounters stale request state. Triggering the flaw requires disconnecting the accepted socket before its three-way handshake completes and reconnecting it elsewhere; the documented result is a kernel warning and one intended packet not being retransmitted.

The record

CVE
CVE-2025-39955
Published
Oct 9, 2025
Updated
Oct 9, 2025
Vendor
The Linux Kernel Organization
Product
Linux
Classifications
Unavailable
Attack vector
local
Privileges
authenticated

Timeline

How it unfolded

  1. Oct 9, 2025CVE publishedPublication date reported by the CVE source.
  2. Oct 9, 2025Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

Compare your product and version with the public record. A matching version still requires validation against your environment.

Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=3.7
  2. Affected versionversion=8336886f786fdacbc19b719c1f7ea91eb70706d4 <17d699727577814198d744d6afe54735c6b54c99
  3. Affected versionversion=8336886f786fdacbc19b719c1f7ea91eb70706d4 <33a4fdf0b4a25f8ce65380c3b0136b407ca57609
  4. Affected versionversion=8336886f786fdacbc19b719c1f7ea91eb70706d4 <45c8a6cc2bcd780e634a6ba8e46bffbdf1fc5c01
  5. Affected versionversion=8336886f786fdacbc19b719c1f7ea91eb70706d4 <7ec092a91ff351dcde89c23e795b73a328274db6
  6. Affected versionversion=8336886f786fdacbc19b719c1f7ea91eb70706d4 <a4378dedd6e07e62f2fccb17d78c9665718763d0
  7. Affected versionversion=8336886f786fdacbc19b719c1f7ea91eb70706d4 <ae313d14b45eca7a6bb29cb9bf396d977e7d28fb
  8. Affected versionversion=8336886f786fdacbc19b719c1f7ea91eb70706d4 <dfd06131107e7b699ef1e2a24ed2f7d17c917753
  9. Affected versionversion=8336886f786fdacbc19b719c1f7ea91eb70706d4 <fa4749c065644af4db496b338452a69a3e5147d9

What conditions does exploitation require?

Attack vectorlocal
Required privilegesauthenticated

What is affected?

The Linux Kernel Organization · Linuxversion=3.7; version=8336886f786fdacbc19b719c1f7ea91eb70706d4 <17d699727577814198d744d6afe54735c6b54c99; version=8336886f786fdacbc19b719c1f7ea91eb70706d4 <33a4fdf0b4a25f8ce65380c3b0136b407ca57609; version=8336886f786fdacbc19b719c1f7ea91eb70706d4 <45c8a6cc2bcd780e634a6ba8e46bffbdf1fc5c01; version=8336886f786fdacbc19b719c1f7ea91eb70706d4 <7ec092a91ff351dcde89c23e795b73a328274db6; version=8336886f786fdacbc19b719c1f7ea91eb70706d4 <a4378dedd6e07e62f2fccb17d78c9665718763d0; version=8336886f786fdacbc19b719c1f7ea91eb70706d4 <ae313d14b45eca7a6bb29cb9bf396d977e7d28fb; version=8336886f786fdacbc19b719c1f7ea91eb70706d4 <dfd06131107e7b699ef1e2a24ed2f7d17c917753; version=8336886f786fdacbc19b719c1f7ea91eb70706d4 <fa4749c065644af4db496b338452a69a3e5147d9

Published CVSS scores

7.8UbuntuCVSS:3.1/AV:L/AC:L/PR:L/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)
Vectorlocal
Privilegesauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

No sourced classifications are available.

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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